InteractiveFly: GeneBrief

Tropomyosin 1: Biological Overview | References

BIOLOGICAL OVERVIEW
Tm1 orthoTm1-I/C, a tropomyosin-1 isoform, links kinesin-1 in a strongly inhibited state to DDBE-associated oskar mRNA. Nuclear magnetic resonance spectroscopy, small-angle X-ray scattering and structural modeling indicate that Tm1-I/C suppresses kinesin-1 activity by stabilizing its autoinhibited conformation, thus preventing competition with dynein until kinesin-1 is activated in the oocyte. Thus work reveals a new strategy for ensuring sequential activity of microtubule motors (Heber, 2024).

oskar (osk) mRNA localization in the Drosophila egg chamber is an attractive system for studying dual motor transport. Delivery of osk to the posterior pole of the developing oocyte, which drives abdominal patterning and germline formation in the embryo, is driven by the successive activities of dynein and kinesin-1. In early oogenesis, osk mRNA that is synthesized in the nurse cells is transported into the interconnected oocyte by dynein in complex with dynactin and the activating adaptor Bicaudal D (BicD), which is linked to double-stranded mRNA localization signals by the RNA-binding protein Egalitarian (Egl). Association of Egl with BicD and consequent dynein activation are enhanced by binding of Egl to RNA, indicating a role for the cargo in promoting dynein activity. In early oogenesis, microtubule minus ends are nucleated in the oocyte, consistent with the dynein-based delivery of mRNAs into this cell. During mid-oogenesis, the polarity of the microtubule network shifts dramatically, with plus ends pointing towards the oocyte posterior. At this stage, Khc translocates osk to the posterior pole. This process is independent of Klc, raising the question of how Khc is linked to osk and how its motor activity is regulated (Heber, 2024).

Transport of osk RNA by Khc requires the unique I/C isoform of tropomyosin-1, Tm1-I/C (hereafter Tm1). Tm1 binds to a noncanonical but conserved cargo-binding region in the Khc tail and stabilizes interaction of the motor with RNA, suggesting a function as an adaptor. Both Khc and Tm1 are loaded onto osk ribonucleoprotein particles (RNPs) shortly after their export from the nurse cell nuclei, although the motor only appears to become active in the mid-oogenesis oocyte. Similarly, dynein remains associated with osk RNPs during Khc-mediated transport within the oocyte, but is inactivated by displacement of Egl by Staufen. How the two motors are linked simultaneously to osk RNPs, and how Khc is inhibited during dynein-mediated transport into the oocyte, is not known (Heber, 2024).

This study shows that Tm1 inhibits Khc by stabilizing its autoinhibited conformation through a new mechanism involving the motor’s regulatory tail domain and stalk. Tm1 also links Khc to the dynein-transported osk RNP, thereby allowing cotransport of inactive Khc on osk RNA by dynein. In vivo, such a mechanism would avoid competition between the two motors during delivery of osk RNPs to the oocyte by dynein, while ensuring that Khc is available on these structures to mediate their delivery to the oocyte posterior in mid-oogenesis. With its cargo-binding and motor regulatory functions, it is proposed that Tm1 is a noncanonical light chain for kinesin-1 (Heber, 2024).

Tm1 was recently implicated as an RNA adaptor for kinesin-1. The current study reveals a previously unknown role of Tm1 in osk transport. Tm1 is shown to negatively regulates Khc activity, which is proposed to occurs a conformational change in the Khc stalk that stabilizes the Khc motor-tail interaction and thereby enhances autoinhibition (Heber, 2024).

With its functions in cargo binding and motor regulation, it is speculated that Tm1 is an alternative Klc in the Drosophila female germline. Because precise osk RNA localization during oogenesis is critical for development, positive regulators of Khc function such as PAT1 and negative regulators such as Tm1 may have replaced Klc to provide nuanced control of Khc activity. If this hypothesis is correct, Klc should also be dispensable for Tm1-dependent RNA localization in somatic tissues (Heber, 2024).

Tm1 also stimulates association of Khc in a strongly inhibited state with dynein-associated osk RNPs. This mechanism would allow dynein-mediated transport of osk RNPs from the nurse cells to the oocyte to proceed without competition with Khc, while positioning the plus-end directed motor on the RNPs for their posteriorward transport within the ooplasm in mid-oogenesis (Heber, 2024).

Kinesin-1 autoinhibition is not fully understood, in part because of a paucity of structural information for the full-length molecule. It has been proposed that interaction of the tail’s IAK motif with the motor domain plays a key role in kinesin-1 autoinhibition. Consistent with this notion, strongly enhanced motility was obserced of Drosophila Khc when the IAK motif was deleted. However, it was recently shown that Klc inhibits Khc activity independently of the IAK-motor interaction. It was also found that the IAK motif is not needed for inhibition of Khc by Tm1. Instead, it was found that Tm1-mediated inhibition occurs via the Khc AMB domain, a region adjacent to the IAK motif that is essential for osk localization (Heber, 2024).

Structural analyses suggest that Tm1 stabilizes the autoinhibited folded conformation of Khc by inducing rearrangement of the Khc coiled-coil stalk. Therefore, a model is proposed in which both the motor-IAK interaction and interactions within the Tm1-bound stalk that include the AMB domain act synergistically to achieve the stable inhibited conformation of Khc (Heber, 2024).

Two recent studies have proposed compact structural arrangements for mammalian kinesin-1. Those studies used chemical crosslinking and cryo-EM, which are likely to enrich for a homogeneous population of compact conformations of Khc and Khc–Klc tetramers, to provide detailed static structural information. By contrast, in the current study of Drosophila Khc and Khc–Tm1 complexes, NMR and SAXS were employed providing insight into the different conformational states, and thus flexibility, of kinesin-1 by enabling analysis of structures in solution. A previous study observed the compact, inhibited conformation in the isolated Khc, showing that Klc-binding does not induce a new fold of Khc but rather stabilizes the inhibited conformation. This is in agreement with the current model, in which Tm1, a putative alternative Klc, shifts the structural equilibrium of Khc towards the autoinhibited state by stabilizing its compact conformation (Heber, 2024).

Although it is known that many cargo types are transported by the concerted action of dynein and kinesins, the underlying regulatory mechanisms have been elusive. This study has identified one of very few examples of a factor that not only links dynein and kinesin-mediated transport, but also modulates transport through differential motor regulation. Linkage of dynein and kinesin-3 by the dynein-activating adaptor Hook3 has been demonstrated, but the cellular events for which this is relevant are still emerging. Recently, reconstituted coupling of dynein and kinesin-1 by TRAK1 and TRAK2 has provided insight into how the motors are recruited and regulated for mitochondrial transport. However, unlike this study's integration of DDBE and Khc in reconstituted osk RNPs, these systems lacked intact native cargoes and thereby excluded potential cargo-directed positioning and modulation of motor complexes (Heber, 2024).

Several studies have reported that active dynein and kinesin motors engage in a tug-of-war when artificially coupled. This study also observe motor opposition in reconstituted RNPs containing DDBE and Khc, presumably because of the stochastic engagement of autoinhibited Khc with microtubules. However, the observation that Tm1 supports efficient dynein-mediated RNA transport through robust inhibition of Khc highlights the importance of regulatory factors in addition to mechanical coupling in native transport complexes. Supporting the in vivo relevance of negative regulation of Khc during bidirectional transport, kinesin-1-activating IAK mutations were recently shown to impair dynein-mediated transport processes in Aspergillus nidulans Collectively, these observations point to complex interplay between opposite-polarity motors that are bound simultaneously to cargoes. Further reconstitutions of dual motor systems on native cargoes should reveal generalities of dynein-kinesin crosstalk, as well as any cargo-specific regulatory mechanisms (Heber, 2024).

This study provides mechanistic insight into two critical aspects of osk mRNA transport—assembly of the dual motor complex and how Khc activity is suppressed during dynein-mediated delivery of the transcript from the nurse cells to the oocyte. However, it is not understood how Khc takes over from dynein after osk RNPs arrive in the oocyte. Although recent work has shown that inactivation of dynein by the RNA-binding protein Stau is part of this process, how Tm1- and IAK-mediated inhibition of Khc is alleviated to allow delivery of the mRNA to the oocyte posterior is an open question. One candidate to fulfill this role is Ensconsin, which is required for posterior osk localization and is enriched in the oocyte relative to the nurse cells. Strikingly, the human counterpart of Ensconsin (MAP7) was recently shown to stimulate activity of mammalian kinesin-1 in vitro. Other candidate Khc activators include the exon junction complex, which, together with the SOLE RNA structure, is essential for transport of osk to the oocyte posterior. Because Tm1 needs to remain bound to the osk RNP throughout its posterior translocation, it is likely that the activating factor(s) induces a conformational change in the Khc–Tm1 complex rather than dissociation of Tm1. Future investigations of these regulatory mechanisms are likely to elucidate how kinesin-1 activity is orchestrated in other systems (Heber, 2024).

Molecular basis of mRNA transport by a kinesin-1-atypical tropomyosin complex

Kinesin-1 carries cargos including proteins, RNAs, vesicles, and pathogens over long distances within cells. The mechanochemical cycle of kinesins is well described, but how they establish cargo specificity is not fully understood. Transport of oskar mRNA to the posterior pole of the Drosophila oocyte is mediated by Drosophila kinesin-1, also called kinesin heavy chain (Khc), and a putative cargo adaptor, the atypical tropomyosin, aTm1. How the proteins cooperate in mRNA transport is unknown. This study presents the high-resolution crystal structure of a Khc-aTm1 complex. The proteins form a tripartite coiled coil comprising two in-register Khc chains and one aTm1 chain, in antiparallel orientation. aTm1 binds to an evolutionarily conserved cargo binding site on Khc, and mutational analysis confirms the importance of this interaction for mRNA transport in vivo. Furthermore, this study demonstrates that Khc binds RNA directly and that it does so via its alternative cargo binding domain, which forms a positively charged joint surface with aTm1, as well as through its adjacent auxiliary microtubule binding domain. Finally, aTm1 was shown to plays a stabilizing role in the interaction of Khc with RNA, which distinguishes aTm1 from classical motor adaptors (Dimitrova-Paternoga, 2021).

Dynamic structural order of a low-complexity domain facilitates assembly of intermediate filaments

The coiled-coil domains of intermediate filament (IF) proteins are flanked by regions of low sequence complexity (LC). Whereas IF coiled-coil domains assume dimeric and tetrameric conformations on their own, maturation of eight tetramers into cylindrical IFs is dependent on either "head" or "tail" domains of low sequence complexity. This study confirms that the tail domain required for assembly of Drosophila Tropomyosin 1 (Tm1-I/C) IFs functions by forming labile cross-β interactions. These interactions are seen in polymers made from the tail domain alone, as well as in assembled IFs formed by the intact Tm1-I/C protein. The ability to visualize such interactions in situ within the context of a discrete cellular assembly lends support to the concept that equivalent interactions may be used in organizing other dynamic aspects of cell morphology (Sysoev, 2020).

Deposition of germ granules at the posterior tip of Drosophila melanogaster oocytes specifies formation of cells of the germinal lineage. Forward genetic studies have illuminated the importance of a number of genes essential to germ cell formation. Many of these genes encode RNA-binding proteins, of which RNA granules are themselves composed. Perplexingly, mutations proximal to the locus encoding the fly tropomyosin gene also impede the deposition of germ granules and subsequent formation of germ cells. These mutations interfere with the formation of a nonmuscle isoform of tropomyosin, designated Tm1-I/C, that is somehow required for germ cell specification (Sysoev, 2020).

A program of alternative pre-mRNA splicing allows fly oocytes to produce an isoform of tropomyosin that replaces the domains necessary for interaction with troponin and the resulting sensitivity to regulation of muscle contraction by free calcium with protein segments of low sequence complexity. Previously reported experiments have shown that the oocyte-specific Tm1-I/C isoform assembles into intermediate filaments, and that its C-terminal low-complexity (LC) domain is essential for filament assembly. This study presents a combination of biochemical and biophysical studies providing evidence that the C-terminal LC domain of Tm1-I/C facilitates the assembly of intermediate filaments by forming structurally specific cross-β interactions that are unusually dynamic. These studies of the fly Tm1-I/C isoform of tropomyosin may be relevant to the behavior of prototypic intermediate filaments long studied in a variety of more complex organisms. They may further be instructive as to the behavior of LC domains in many other aspects of cell morphology (Sysoev, 2020).

Other than nuclear lamins, the Tm1-I/C protein is the sole intermediate filament protein found in fruit flies. The central, coiled-coil domain of the Tm1-I/C isoform of tropomyosin is flanked by LC sequences. Instead of interacting with troponin in a manner allowing for calcium-mediated regulation of muscle contraction, the C-terminal LC domain directs the Tm1-I/C protein to assemble into intermediate filaments (Sysoev, 2020).

Why is proper deposition of germ granules at the posterior pole of Drosophila oocytes dependent on Tm1-I/C-specified intermediate filaments? Visualization of a GFP-tagged form of Tm1-I/C in living oocytes has shown that the protein becomes precisely restricted to the posterior tip of oocytes well before the completion of polar granule deposition. It is speculated that assembled Tm1-I/C intermediate filaments localized to the posterior pole of fly oocytes might constitute a Velcro-like landing pad for one or more of the constituent RNA-binding proteins specifying polar granules (Sysoev, 2020).

In an architectural sense, assembled intermediate filaments contain repeating collars of LC domain sequences circumferentially displayed along their axial length. In the case of vimentin intermediate filaments, these LC domain collars represent repetitively organized binding sites for a GFP:FUS fusion protein, such that the latter protein can be iteratively bound to the filaments at 45 nm intervals as viewed by transmission electron microscopy. It is perhaps of importance that binding of the GFP:FUS protein to vimentin intermediate filaments is dependent upon the integrity of the LC tail domain of vimentin. As such, it is speculated that Tm1-I/C intermediate filaments, on localization to the posterior tip of fly oocytes, may allow for the subsequent binding and organization of the germ granules themselves (Sysoev, 2020).

The experiments described in this paper were focused on the C-terminal LC domain of the Tm1-I/C protein, herein referred to as the tail domain. The Tm1-I/C tail domain has already been shown to be required for Tm1-I/C to assemble into intermediate filaments, and the isolated Tm1-I/C tail domain is known to form labile cross-β polymers. The primary focus of the present study was to determine whether the same structural forces leading to the formation of cross-β polymers observed in studies of the isolated Tm1-I/C tail domain might also be involved in the assembly of Tm1-I/C intermediate filaments (Sysoev, 2020).

This central question was approached via the use of ss-NMR spectroscopy. ss-NMR spectra observed from isotopically-labeled tail domain-only polymers was compared with spectra derived from fully assembled intermediate filaments. In the latter case, intein chemistry was used to restrict isotopic labeling to only the tail domain of the intact Tm1-I/C protein. The 13C/15N isotopic labels segmentally introduced into the intact Tm1-I/C protein were identical to those introduced into tail domain-only polymers (Sysoev, 2020).

Four observations from these studies. are highlighted. First, highly similar spectra diagnostic of structural order were observed in both tail domain-only cross-β polymers and segmentally labeled Tm1-I/C intermediate filaments. The portion of the Tm1-I/C tail domain specifying these cross-β interactions was mapped to a region of 28 amino acids spanning residues 384 to 411 of the Tm1-I/C polypeptide. Second, evidence of robust molecular order within the tail domain was observed only upon cooling of the Tm1-I/C intermediate filaments. Third, roughly 30 amino acid residues were observed to exist in a state of distinct structural disorder in both the tail domain-only cross-β polymers and the fully assembled, segmentally labeled intermediate filaments. Importantly, there was almost perfect overlap of the spectra reporting on these disordered residues in the two structures. Fourth, the observed Φ/Φ torsion angles based on 13C and 15N chemical shifts for almost all residues within the structurally ordered tail domain of Tm1-I/C were predictive of either β-strand secondary structure or an extended β-strand–like conformation. Many of these sites have 4- to 5-Å intermolecular distances appropriate for a parallel, in-register, cross-β structure (Sysoev, 2020).

A parsimonious interpretation of these observations is that Tm1-I/C intermediate filaments assemble via the formation of dynamically ordered cross-β interactions specified by the amino acid sequence of the C-terminal tail domain. When studied in the context of isolated, tail domain-only polymers, these cross-β interactions remain labile to disassembly but are far more stable than those formed in the context of biologically relevant Tm1-I/C intermediate filaments (Sysoev, 2020).

This interpretation posits an unusual form of protein structure. It is therefore suggested that what is being observed in this study is a form of protein structure that retains an ordered ground state sufficiently precise to dictate both molecular order and functional specificity. Curiously, however, the amino acid residues specifying molecular order are apparently in a state of continuous flux in and out of the structurally ordered state. This behavior should not be conflated with a transition of the dynamically ordered residues into disordered residues. The results presented in this study illustrate that these regions of differential order and disorder arise from entirely different, mutually exclusive parts of the C-terminal tail domain of Tm1-I/C. It is speculated that what was have observed in studies of the LC domain-specifying function of the C-terminal tail domain of the Tm1-I/C protein may be instructive for the thousands of LC domains operative in many other aspects of cell biology (Sysoev, 2020).

Given the temperature-dependent behavior of the dynamically ordered region of the Tm1-I/C C-terminal tail domain, it is of interest to consider the behavior of the protein at more physiological temperatures for Drosophila (>16 °C). The data presented in this study for temperatures ranging from −19 °C to 16 °C indicate that the same conformation of the dynamically ordered segments is maintained as the temperature increases, with the protein acquiring increasingly rapid small-amplitude backbone and sidechain motions rather than a complete disassembly of the structured state. In the context of studies on protein dynamics at elevated temperatures, these motions would be expected to become increasingly more rapid above 16 °C. As would occur for most structured proteins, at some temperature a transition to a thermally denatured state is expected, but that temperature is likely higher than the physiological temperatures relevant for Drosophila (Sysoev, 2020).

It is also important to consider that the in vitro intermediate filaments are in an environment that is relatively dilute compared to the inside of a cell. The crowded environment within the Drosophila embryo would likely favor the formation of the dynamically ordered state over a disordered or denatured one. In support of this concept, molecular crowding agents favor the formation of condensed structures in vitro for both LC domain proteins and amyloid forming proteins. Therefore, it is considered reasonable to conclude that the dynamically ordered structure observed in this study would be present in the physiological environment of a living fly egg or embryo (Sysoev, 2020).

The fact is stressed that no more than half of the amino acids of the Tm1-I/C tail domain achieve any state of molecular order whether studied in either tail domain-only polymers or assembled intermediate filaments. Precisely the same 30+ amino acid residues exist in a state of molecular disorder in both tail domain-only polymers and segmentally labeled intermediate filaments. That these disordered residues are likely to be of functional importance is supported by studies of systematically deleted variants of the Tm1-I/C polypeptide. Deletion of only 20 residues from the C terminus of the Tm1-I/C protein, although not impinging whatsoever on the structurally ordered region of the tail domain, significantly impedes the assembly of intermediate filaments and yields tail domain-only polymers with a distinctly twisted morphology. That LC domains use no more than a modest fraction of their amino acid sequences to achieve transient structural order has also been reported for both the hnRNPA2 and FUS RNA-binding proteins. Why LC domains are reliant on regions specifying both structural order and disorder remains an intriguing mystery (Sysoev, 2020).

Distortion of the Actin A-triad results in contractile disinhibition and cardiomyopathy

Striated muscle contraction is regulated by the movement of tropomyosin over the filament surface, which blocks or exposes myosin binding sites on actin. Thin filaments consist of actin, the troponin complex, and tropomyosin. Findings suggest that electrostatic contacts, particularly those between K326, K328, and R147 on actin and tropomyosin, establish an energetically favorable F-actin-tropomyosin configuration, with tropomyosin positioned in a location that impedes actomyosin associations and promotes relaxation. This study provides data that directly support a vital role for these actin residues, termed the A-triad, in tropomyosin positioning in intact functioning muscle. By examining the effects of an A295S alpha-cardiac actin hypertrophic cardiomyopathy-causing mutation, over a range of increasingly complex in silico, in vitro, and in vivo Drosophila muscle models, it is proposed that subtle A-triad-tropomyosin perturbation can destabilize thin filament regulation, which leads to hypercontractility and triggers disease. These efforts increase understanding of basic thin filament biology and help unravel the mechanistic basis of a complex cardiac disorder (Viswanathan, 2017).

An atypical tropomyosin in Drosophila with intermediate filament-like properties

A longstanding mystery has been the absence of cytoplasmic intermediate filaments (IFs) from Drosophila despite their importance in other organisms. In the course of characterizing the in vivo expression and functions of Drosophila Tropomyosin (Tm) isoforms, this study discovered an essential but unusual product of the Tm1 locus, Tm1-I/C, which resembles an IF protein in some respects. Like IFs, Tm1-I/C spontaneously forms filaments in vitro that are intermediate in diameter between F-actin and microtubules. Like IFs but unlike canonical Tms, Tm1-I/C contains N- and C-terminal low-complexity domains flanking a central coiled coil. In vivo, Tm1-I/C forms cytoplasmic filaments that do not associate with F-actin or canonical Tms. Tm1-I/C is essential for collective border cell migration, in epithelial cells for proper cytoarchitecture, and in the germline for the formation of germ plasm. These results suggest that flies have evolved a distinctive type of cytoskeletal filament from Tm (Cho, 2016).

A new isoform of Drosophila non-muscle Tropomyosin 1 interacts with Kinesin-1 and functions in oskar mRNA localization

Recent studies have revealed that diverse cell types use mRNA localization as a means to establish polarity. Despite the prevalence of this phenomenon, much less is known regarding the mechanism by which mRNAs are localized. The Drosophila melanogaster oocyte provides a useful model for examining the process of mRNA localization. oskar (osk) mRNA is localized at the posterior of the oocyte, thus restricting the expression of Oskar protein to this site. The localization of osk mRNA is microtubule dependent and requires the plus-end-directed motor Kinesin-1. Unlike most Kinesin-1 cargoes, localization of osk mRNA requires the Kinesin heavy chain (Khc) motor subunit, but not the Kinesin light chain (Klc) adaptor. This report, demonstrates that a newly discovered isoform of Tropomyosin 1, referred to as Tm1C, directly interacts with Khc and functions in concert with this microtubule motor to localize osk mRNA. Apart from osk mRNA localization, several additional Khc-dependent processes in the oocyte are unaffected upon loss of Tm1C. These results therefore suggest that the Tm1C-Khc interaction is specific for the osk localization pathway (Veeranan-Karmegam, 2016).

An RNA-binding atypical tropomyosin recruits kinesin-1 dynamically to oskar mRNPs

Localization and local translation of oskar mRNA at the posterior pole of the Drosophila oocyte directs abdominal patterning and germline formation in the embryo. The process requires recruitment and precise regulation of motor proteins to form transport-competent mRNPs. The posterior-targeting kinesin-1 is loaded upon nuclear export of oskar mRNPs, prior to their dynein-dependent transport from the nurse cells into the oocyte. Kinesin-1 recruitment requires the DmTropomyosin1-I/C isoform, an atypical RNA-binding tropomyosin that binds directly to dimerizing oskar 3'UTRs. The isoform is one of 17 predicted mRNA isoforms and 13 distinct polypeptides encoded by the TM1 gene. Finally, a small but dynamically changing subset of oskar mRNPs gets loaded with inactive kinesin-1, and the motor is activated during mid-oogenesis by the functionalized spliced oskar RNA localization element. This inefficient, dynamic recruitment of Khc decoupled from cargo-dependent motor activation constitutes an optimized, coordinated mechanism of mRNP transport, by minimizing interference with other cargo-transport processes and between the cargo-associated dynein and kinesin-1 (Gaspar, 2016).

Noncanonical roles for Tropomyosin during myogenesis

For skeletal muscle to produce movement, individual myofibers must form stable contacts with tendon cells and then assemble sarcomeres. The myofiber precursor is the nascent myotube, and during myogenesis the myotube completes guided elongation to reach its target tendons. Unlike the well-studied events of myogenesis, such as myoblast specification and myoblast fusion, the molecules that regulate myotube elongation are largely unknown. In Drosophila, hoi polloi (hoip) encodes a highly-conserved RNA binding protein and hoip mutant embryos are largely paralytic due to defects in myotube elongation and sarcomeric protein expression. The hoip mutant background was used as a platform to identify novel regulators of myogenesis, and surprising developmental functions were uncovered for the sarcomeric protein Tropomyosin 2. Hoip responsive sequences were identified in the coding region of the Tm2 mRNA that are essential for Tm2 protein expression in developing myotubes. Tm2 overexpression rescued the hoip myogenic phenotype by promoting F-actin assembly at the myotube leading edge, by restoring the expression of additional sarcomeric RNAs, and by promoting myoblast fusion. Embryos that lack Tm2 also showed reduced sarcomeric protein expression, and embryos that expressed a gain-of-function Tm2 allele showed both fusion and elongation defects. Tropomyosin therefore dictates fundamental steps of myogenesis prior to regulating contraction in the sarcomere (Williams, 2015).

A novel tropomyosin isoform functions at the mitotic spindle and Golgi in Drosophila

Most eukaryotic cells express multiple isoforms of the actin-binding protein tropomyosin that help construct a variety of cytoskeletal networks. Using biochemical and molecular genetic approaches, this study identified three tropomyosins expressed in Drosophila S2 cells: Tm1A, Tm1J (both coded for by the Tm1 gene), and Tm2A (coded for by Tm2). The Tm1A isoform localizes to the cell cortex, lamellar actin networks, and the cleavage furrow of dividing cells- always together with myosin-II. Isoforms Tm1J and Tm2A colocalize around the Golgi apparatus with the formin-family protein Diaphanous and loss of either isoform perturbs cell cycle progression. During mitosis, Tm1J localizes to the mitotic spindle where it promotes chromosome segregation. Using chimeras, this study identified the determinants of tropomyosin localization near the C-terminus. This work: (1) identifies and characterizes previously unknown non-muscle tropomyosins in Drosophila; (2) reveals a function for tropomyosin in the mitotic spindle; and (3) uncovers sequence elements that specify isoform-specific localizations and functions of tropomyosin (Goins, 2015).

Arp2/3 complex and cofilin modulate binding of tropomyosin to branched actin networks

Tropomyosins are coiled-coil proteins that bind actin filaments and regulate multiple cytoskeletal functions, including actin network dynamics near the leading edge of motile cells. Tropomyosins inhibit actin nucleation by the Arp2/3 complex and prevent filament disassembly by cofilin. This study finds that the Arp2/3 complex and cofilin, in turn, regulate the binding of tropomyosin to actin filaments. Using fluorescence microscopy, this study showed that tropomyosin (non-muscle Drosophila Tm1A) polymerizes along actin filaments, starting from "nuclei" that appear preferentially on ADP-bound regions of the filament, near the pointed end. Tropomyosin fails to bind dendritic actin networks created in vitro by the Arp2/3 complex, in part because the Arp2/3 complex blocks pointed ends. Cofilin promotes phosphate dissociation and severs filaments, generating new pointed ends and rendering Arp2/3-generated networks competent to bind tropomyosin. Tropomyosin's attraction to pointed ends reveals a basic molecular mechanism by which lamellipodial actin networks are insulated from the effects of tropomyosin (Hsiao, 2015).

Staufen targets coracle mRNA to Drosophila neuromuscular junctions and regulates GluRIIA synaptic accumulation and bouton number

The post-synaptic translation of localised mRNAs has been postulated to underlie several forms of plasticity at vertebrate synapses, but the mechanisms that target mRNAs to these postsynaptic sites are not well understood. This study shows that the evolutionary conserved dsRNA binding protein, Staufen, localises to the postsynaptic side of the Drosophila neuromuscular junction (NMJ), where it is required for the localisation of coracle mRNA and protein. Staufen plays a well-characterised role in the localisation of oskar mRNA to the oocyte posterior, where Staufen dsRNA-binding domain 5 is specifically required for its translation. Removal of Staufen dsRNA-binding domain 5, disrupts the postsynaptic accumulation of Coracle protein without affecting the localisation of cora mRNA, suggesting that Staufen similarly regulates Coracle translation. Tropomyosin II, which functions with Staufen in oskar mRNA localisation, is also required for coracle mRNA localisation, suggesting that similar mechanisms target mRNAs to the NMJ and the oocyte posterior. Coracle, the orthologue of vertebrate band 4.1, functions in the anchoring of the glutamate receptor IIA subunit (GluRIIA) at the synapse. Consistent with this, staufen mutant larvae show reduced accumulation of GluRIIA at synapses. The NMJs of staufen mutant larvae have also a reduced number of synaptic boutons. Altogether, this suggests that this novel Staufen-dependent mRNA localisation and local translation pathway may play a role in the developmentally regulated growth of the NMJ (Gardiol, 2014).

Tropomyosin is an interaction partner of the Drosophila coiled coil protein Yuri Gagarin

The Drosophila gene yuri gagarin is a complex locus encoding three protein isoform classes that are ubiquitously expressed in the organism. Mutations to the gene affect processes as diverse as gravitactic behavior and spermatogenesis. The larger Yuri isoforms contain extensive coiled-coil regions. Previous studies indicate that one of the large isoform classes (Yuri-65) is required for formation of specialized F-actin-containing structures generated during spermatogenesis, including the so-called actin "cones" that mediate spermatid individualization. The tandem affinity purification of a tagged version of Yuri-65 (the TAP-tagging technique) was used to identify proteins associated with Yuri-65 in the intact organism. Tropomyosin, primarily as the 284-residue isoform derived from the ubiquitously expressed Tropomyosin 1 gene was thus identified as a major Yuri interaction partner. Co-immunoprecipitation experiments confirmed this interaction. The stable F-actin cones of spermatogenesis contain Tropomyosin 1 (Tm1) and in mutant yuri^F64, failure of F-actin cone formation is associated with failure of Tm1 to accumulate at the cone initiation sites. In investigating possible interactions of Tm1 and Yuri in other tissues, it was discovered that Tm1 and Yuri frequently colocalize with the endoplasmic reticulum. Tropomyosin has been implicated in actin-mediated membrane trafficking activity in other systems. These findings suggest that Yuri-Tm1 complexes participate in related functions (Texada, 2011).

Troponin I and Tropomyosin regulate chromosomal stability and cell polarity

The Troponin-Tropomyosin (Tn-Tm) complex regulates muscle contraction through a series of Ca(2+)-dependent conformational changes that control actin-myosin interactions (see video). Members of this complex in Drosophila include the actin-binding protein Troponin I (TnI), and two Tropomyosins (Tm1 and Tm2), which are thought to form heterodimers. Troponin itself contains three subunits, Ca2+ binding (TnC; TpnC47D and TpnC73F in Drosophila), inhibitory (TnI; Wings up A in Drosophila), and tropomyosin binding (TnT; Upheld in Drosophila) (Sahota, 2009).

Pre-cellular embryos of TnI, Tm1 and Tm2 mutants exhibit abnormal nuclear divisions with frequent loss of chromosome fragments. During cellularization, apico-basal polarity is also disrupted as revealed by the defective location of Discs large (Dlg) and its ligand Rapsynoid (Raps; also known as Partner of Inscuteable, Pins). In agreement with these phenotypes in early development, on the basis of RT-PCR assays of unfertilized eggs and germ line mosaics of TnI mutants, it was also shown that TnI is part of the maternal deposit during oogenesis. In cultures of the S2 cell line, native TnI is immunodetected within the nucleus and immunoprecipitated from nuclear extracts. SUMOylation at an identified site (see SUMO) is required for the nuclear translocation. These data illustrate, for the first time, a role for TnI in the nucleus and/or the cytoskeleton of non-muscle cells. It is proposed that the Tn-Tm complex plays a novel function as regulator of motor systems required to maintain nuclear integrity and apico-basal polarity during early Drosophila embryogenesis (Sahota, 2009).

Troponin I (TnI) and Tropomyosin (Tm) are actin-binding proteins that regulate muscle sarcomere contraction. The Tn-Tm complex contains three different Troponin polypeptides, C, T and I, and it regulates acto-myosin interactions in response to the rise of free calcium (Clark, 2002). Mammals have three genes expressing TnI known as slow twitch (TNNI1), fast twitch (TNNI2) and cardiac (TNNI3). In humans, mutations in TNNI2 and TNNI3 cause distal arthrogryposis type 2B (Sung, 2003) and familial hypertrophic cardiomyopathy (Kimura, 1997), respectively. In Drosophila, viable mutations in the single gene expressing TnI, wings up A (wupA) [also known as held up (hdp)], result in hypercontraction and degeneration of the indirect flight muscles of the thorax due to recessive hypomorphic point mutations (Prado, 1995). However, studies on lack of function mutations for this gene have been hampered by the fact that null alleles are dominant lethals (Prado, 1999). Mammals contain four tropomyosin genes, TPM1-4, while Drosophila has two, Tm1 and Tm2. In humans, mutant TPM1 is thought to be responsible for type 3 familial hypertrophic cardiomyopathy, whereas TPM2 is involved in nemaline myopathy and TPM3 has been linked to dominant nemaline myopathy. TPM1 has also been identified as a suppressor of malignant transformation as it is downregulated in mammalian transformed cells, and its expression is abolished in human breast tumors. Indeed, it is widely accepted that actin regulation plays a crucial role in cell motility, which is a key feature in metastatic cancers (Sahota, 2009).

Although some of these pathological phenotypes appear unrelated to muscle biology, several lines of evidence indicate that these muscle-specific proteins could have a role in other cell types and processes. For instance, Tm1 is part of the maternal deposit during Drosophila oogenesis, it is required to localize the oskar mRNA at the posterior pole of the oocyte (Erdelyi, 1995), and later in development it localizes to various cell types including the gut, brain and epidermis (Hales, 1994). Also, this study demonstrates that TnI RNA is detected in mature unfertilized eggs, which suggests a role in early embryogenesis. Thus, this study set out to analyze early development phenotypes and their mechanisms in TnI and Tm mutants (Sahota, 2009).

This study shows a novel function for the Tn-Tm complex in regulating nuclear divisions during early embryogenesis in Drosophila. Evidence is provided that TnI is required for maintaining stable chromosomal integrity, which was also show for Tm1 and Tm2. Importantly, the three genes seem required for correct epithelial apico-basal polarity; mutant phenotypes include cellularization defects that mislocalize the polarity markers Discs large (Dlg) and its ligand Rapsynoid (Raps) [also known as Partner of Inscuteable (Pins)]. Consistent with the function of these genes in cellularization and spindle integrity, defects in mitosis and chromosome segregation are observed. In a stable cell line, S2, TnI can be detected within the nucleus. Furthermore, the translocation of TnI to the nucleus is dependent upon a mechanism involving SUMOylation. Taken together, these data implicate the Tn-Tm complex in regulating nuclear functions. Moreover, the results suggest that the Tn-Tm complex is required to maintain correct segregation of chromosomes, as disruption of this complex leads to aberrations including chromosome fragment losses. This is the first evidence that the Tn-Tm complex can regulate both nuclear divisions and cell polarity in Drosophila. This is likely to have important implications in cancer progression since chromosomal instability and the generation of aneuploidies are characteristic hallmarks of many cancers (Sahota, 2009).

Embryonic lethal insertion lines located near the wupA locus called PL87 and PG31 are lacZ reporter and Gal4 lines, respectively. Both insertion sites were located upstream of the promoter region by means of plasmid rescue experiments (Marin, 2004). A third mutant, Df(1)23437, deletes 2 Kb of the promoter region and is also an embryonic lethal. Quantitative RT-PCR data had shown severely reduced levels of TnI RNA expression in these three mutants, with 23437 showing the most reduction, followed by PL87 and then PG31 (Marin, 2004). This study confirmed that their lethal mutant phenotypes were caused by the TnI gene, as opposed to another gene putatively affected by these chromosomal rearrangements. To this end transgenic lines were generated using the embryonic L9/wupRA isoform of wupA cDNA, under the control of upstream activating sequences (UAS). This isoform was sufficient to completely rescue the embryonic lethality of all three alleles when driven by the general Gal4 driver LL7 [inserted at the αtubulin84B (tub) gene]. Thus, the PL87 and PG31 alleles represent bona fide mutants for TnI, and the TnI L9/wupRA isoform encodes all functions required for correct embryogenesis. Rescued adults were fertile and able to fly, although 40% of pupae failed to emerge and showed a cryptocephalic phenotyp. This phenotype is consistent with the reported downregulation of the TnI gene during metamorphosis (Furlong, 2001). Given that there are adult isoforms of the TnI gene that contain an additional exon, it is unlikely that the L9/wupRA isoform is able to completely replace the functionality of the adult isoforms. This was confirmed by the failed attempt to rescue the wings held up phenotype of viable wupA alleles when driving the L9/wupRA isoform in the adult indirect flight muscles. Indeed, the wupA gene can produce a repertoire of cDNA isoforms, several of which are embryo-specific whereas others are adult-specific. Because embryonic TnI gene expression has been detected before the onset of myogenesis (Prado, 1999), the embryonic lethal mutants were used to look for defects during early embryogenesis. These data demonstrate that the embryonic lethality of the TnI mutants can be rescued to viability using the earliest expressed TnI isoform, and that the phenotypes associated with the three TnI alleles are due to the absence of the TnI gene products (Sahota, 2009).

The Tn-Tm complex has been well studied in the context of muscle contraction (Boussouf, 2007). This study shows that members of the complex also play an earlier role in development to maintain nuclear integrity. The nuclear defects observed in mutants for the three proteins TnI, Tm1 and Tm2 suggest that the whole Tn-Tm complex is required to maintain nuclear integrity. Embryonic lethal mutants are currently not available for the remaining components, mainly TnC and TnT (Sahota, 2009).

Several sarcomere proteins have been reported to play nuclear functions. Titin, a large protein spanning the sarcomere length between Z bands is required for chromosome integrity and control of gene expression through one of its kinase domains. However, the chromosomal effects are not likely to result from a direct binding of titin to chromosomes because a thorough search for proteins associated to metaphase chromosomes of HeLa cells failed to identify it. The issue, however, seems to be still controversial since a titin domain has been identified within the cell nucleus playing a role in proliferation. Zyxin, another actin-associated protein, acts as a tumor suppressor gene in Ewing tumor cells on the basis of its DNA-binding LIM domain and localizes to the nucleus to regulate gene transcription (Sahota, 2009).

This study has immunolocalized TnI to the nucleus and shown nuclear phenotypes in the mutants. It should be noted, however, that the nuclear localization, either in the syncitial embryo or the regular S2 cells, seems dependent on the physiological state of the cell and nucleus. Also, with the techniques used in this study, it cannot be determined whether TnI is bound directly to the chromosomes or through intervening proteins. Because the repertoire of HeLa metaphase chromosome-associated proteins does not include TnI, nor other muscle proteins, the observed effects on chromosome integrity might be produced through indirect links. Nevertheless, one should realize that the referred repertoire is also subject to the technical constrains of the purification methods used in the study of HeLa cells (Sahota, 2009).

This study has also shown that the required nuclear translocation is achieved by SUMOylation, at least in the case of TnI. The putative SUMOylation sequence in exon 10 is required for nuclear import. This site, VKEE, is found in the C-termini of all TnI isoforms because it can be incorporated into the protein sequence, either from exon 9 or exon 10. Thus, all TnI isoforms could be tagged for their function. Other putative SUMOylation sites, if actually used for SUMOylation, could provide further functional diversity for TnI. This mechanism for tagging TnI in Drosophila is likely to be conserved in mammals since the VKEE motif is present in the three TnI gene types (slow twitch, fast twitch and cardiac). Although not addressed in this study, it is possible that a similar mechanism might be used to import Tm1 and Tm2 into the nucleus since they contain suitable motifs in the three isoforms of Tm2 and in one of the two isoforms of Tm1 (Sahota, 2009).

This work on the Tn-Tm complex provides an insight into how DNA aberrations and cellularization defects can be linked, and how this complex is crucially required for both DNA and cellular stability. Given that the Tn-Tm complex is also involved in muscle contraction, it appears likely that there may be other processes where disruption of this complex may be detrimental to the development of the organism. In support of this, it has been shown that mutant TnI allele 23437 displays severe defects in axon guidance and fasciculation and that the TnI L9/wupRA isoform rescues these defects. Considering the role of the Tn-Tm complex in sarcomere contraction and the range of phenotypes described in this study, it seems reasonable to propose that TnI, Tm1 and Tm2 are components of a force-generating complex within the nucleus and in the cytoplasm. However, this remains to be determined since the TnI-associated partners have not being investigated in this study (Sahota, 2009).

Being an actin-binding protein, TnI should perform its nuclear functions in association with actin. This protein is known to help RNA polymerase to move during gene transcription (Ye, 2008). It is currently a matter of debate whether this function requires actin in a globular or a filament structure. However, a recent study reports the interaction of vertebrate fast skeletal TnI with the estrogen receptor during transcription (Li, 2008). By analogy to the role that TnI plays in the sarcomere, where the Tn-Tm complex interacts with the actin filaments, it seems likely that during transcription actin has a filament structure, as in the sarcomere thin filament. Actin is also important for morphogenesis of cells and organs in the early embryo, ranging from nuclear divisions and chromosomal segregation in conjunction with myosin, to the regulation of cell shape and movements. All these processes are also relevant to the formation and progression of tumors. In addition, chromosomal instability, mitotic defects and cell polarity defects are characteristic features of many cancers. The fact that TnI, Tm1 and Tm2 all regulate actin strengthens the argument that they execute this regulation as a complex. Defects in all three genes give rise to similar DNA defects, and also to similar defects in apico-basal cell polarity. These common features provide the basis for a mechanism leading to aneuploidy and aberrant cell signaling. That is, molecules that ensure proper actin function during nuclear divisions also ensure that actin correctly regulates cell polarity, which, in turn, is important in proliferation and growth. The tubulin spindle was also affected in the three mutants, indicating that the integrity of the cytoskeletal network may be compromised when any of these molecules are depleted (Sahota, 2009).

In addition to the cytoskeletal network, the localization of Dlg and Pins were also shown to be disrupted in TnI-Tm mutants. Dlg has been described as a neoplastic tumor suppressor and disruption of polarity is a hallmark of cancer progression. The Pins protein is involved in orientation of asymmetric cell divisions, which is important for specifying cell fate. Consistent with the altered Pins expression, spindle orientation defects are observed in the three mutants. Also, spindle orientation is particularly important for specifying neuronal identity in Drosophila neuroblasts. The recycling of molecules for distinct processes is a recurrent theme in development. Indeed, many actin-binding proteins were first identified for their effects on axon guidance and growth, and were subsequently shown to play important roles during cellularization. Also, Dlg was associated with synaptogenesis before its role in cellularization was determined. The novel function for the Tn-Tm complex uncovered in this study might have opened the way to reveal requirements in other actin-associated events. It was observed that TnI, as well as Tm1 and Tm2, are crucial for the correct development of the central nervous system. Further studies on the role of the Tn-Tm complex during nuclear divisions seem appropriate towards understanding how these proteins affect cell proliferation, and might provide novel targets for controlling cell divisions (Sahota, 2009).

Myosin's powerstroke transitions define atomic scale movement of cardiac thin filament tropomyosin

Dynamic interactions between the myosin motor head on thick filaments and the actin molecular track on thin filaments drive the myosin-crossbridge cycle that powers muscle contraction. The process is initiated by Ca2+ and the opening of troponin-tropomyosin-blocked myosin-binding sites on actin. The ensuing recruitment of myosin heads and their transformation from pre-powerstroke to post-powerstroke conformation on actin produce the force required for contraction. Cryo-EM-based atomic models confirm that during this process, tropomyosin occupies three different average positions on actin. Tropomyosin pivoting on actin away from a TnI-imposed myosin-blocking position accounts for part of the Ca2+ activation observed. However, the structure of tropomyosin on thin filaments that follows pre-powerstroke myosin binding and its translocation during myosin's pre-powerstroke to post-powerstroke transition remains unresolved. This study approach this transition computationally in silico. The myosin helix-loop-helix motif was used as an anchor to dock models of pre-powerstroke cardiac myosin to the cleft between neighboring actin subunits along cardiac thin filaments. Targeted molecular dynamics simulations were performed of the transition between pre- and post-powerstroke conformations on actin in the presence of cardiac troponin-tropomyosin. These simulations show Arg 369 and Glu 370 on the tip of myosin Loop-4 encountering identically charged residues on tropomyosin. The charge repulsion between residues causes tropomyosin translocation across actin, thus accounting for the final regulatory step in the activation of the thin filament, and, in turn, facilitating myosin movement along the filament. It is suggested that during muscle activity, myosin-induced tropomyosin movement is likely to result in unencumbered myosin head interactions on actin at low-energy cost (Rynkiewicz, 2024).

Troponin-I-induced tropomyosin pivoting defines thin-filament function in relaxed and active muscle

Regulation of the crossbridge cycle that drives muscle contraction involves a reconfiguration of the troponin-tropomyosin complex on actin filaments. By comparing atomic models of troponin-tropomyosin fitted to cryo-EM structures of inhibited and Ca2+-activated thin filaments, this study found that tropomyosin pivots rather than rolls or slides across actin as generally thought. It is proposed that pivoting can account for the Ca2+ activation that initiates muscle contraction and then relaxation influenced by troponin-I (TnI). Tropomyosin is well-known to occupy either of three meta-stable configurations on actin, regulating access of myosin motorheads to their actin-binding sites and thus the crossbridge cycle. At low Ca2+ concentrations, tropomyosin is trapped by TnI in an inhibitory B-state that sterically blocks myosin binding to actin, leading to muscle relaxation. Ca2+ binding to TnC draws TnI away from tropomyosin, while tropomyosin moves to a C-state location over actin. This partially relieves the steric inhibition and allows weak binding of myosin heads to actin, which then transition to strong actin-bound configurations, fully activating the thin filament. Nevertheless, the reconfiguration that accompanies the initial Ca2+-sensitive B-state/C-state shift in troponin-tropomyosin on actin remains uncertain and at best is described by moderate-resolution cryo-EM reconstructions. Recent computational studies indicate that intermolecular residue-to-residue salt-bridge linkage between actin and tropomyosin is indistinguishable in B- and C-state thin filament configurations. This study shows that tropomyosin can pivot about relatively fixed points on actin to accompany B-state/C-state structural transitions. It is argued that at low Ca2+ concentrations C-terminal TnI domains attract tropomyosin, causing it to bend and then pivot toward the TnI, thus blocking myosin binding and contraction (Lehman, 2023).

A pathogenic mechanism associated with myopathies and structural birth defects involves TPM2-directed myogenesis

Nemaline myopathy (NM) is the most common congenital myopathy, characterized by extreme weakness of the respiratory, limb, and facial muscles. Pathogenic variants in Tropomyosin 2 (TPM2), which encodes a skeletal muscle-specific actin binding protein essential for sarcomere function, cause a spectrum of musculoskeletal disorders that include NM as well as cap myopathy, congenital fiber type disproportion, and distal arthrogryposis (DA). The in vivo pathomechanisms underlying TPM2-related disorders are unknown, so this study expressed a series of dominant, pathogenic TPM2 variants in Drosophila embryos and found 4 variants significantly affected muscle development and muscle function. Transient overexpression of the 4 variants also disrupted the morphogenesis of mouse myotubes in vitro and negatively affected zebrafish muscle development in vivo. This study used transient overexpression assays in zebrafish to characterize two potentially novel TPM2 variants and one recurring variant that was identified in patients with DA (V129A, E139K, A155T, respectively) and found these variants caused musculoskeletal defects similar to those of known pathogenic variants. The consistency of musculoskeletal phenotypes in these assays correlated with the severity of clinical phenotypes observed in patients with DA, suggesting disrupted myogenesis is a potentially novel pathomechanism of TPM2 disorders and that myogenic assays can predict the clinical severity of TPM2 variants (McAdow, 2022).

cGMP interacts with tropomyosin and downregulates actin-tropomyosin-myosin complex interaction

The nitric oxide-soluble guanylate cyclase-cyclic guanosine monophosphate (NO-sGC-cGMP) signaling pathway, plays a critical role in the pathogenesis of pulmonary arterial hypertension (PAH); however, its exact molecular mechanism remains undefined. Biotin-cGMP pull-down assay was performed to search for proteins regulated by cGMP. The interaction between cGMP and tropomyosin was analyzed with antibody dependent pull-down in vivo. Tropomyosin fragments were constructed to explore the tropomyosin-cGMP binding sites. The expression level and subcellular localization of tropomyosin were detected with Real-time PCR, Western blot and immunofluorescence assay after the 8-Br-cGMP treatment. Finally, isothermal titration calorimetry (ITC) was utilized to detect the binding affinity of actin-tropomyosin-myosin in the existence of cGMP-tropomyosin interaction. cGMP interacted with tropomyosin. Isoform 4 of TPM1 gene was identified as the only isoform expressed in the human pulmonary artery smooth muscle cells (HPASMCs). The region of 68-208aa of tropomyosin was necessary for the interaction between tropomyosin and cGMP. The expression level and subcellular localization of tropomyosin showed no change after the stimulation of NO-sGC-cGMP pathway. However, cGMP-tropomyosin interaction decreased the affinity of tropomyosin (Zou, 2018).

Search PubMed for articles about Drosophila Tropomyosin

Cho, A., Kato, M., Whitwam, T., Kim, J.H. and Montell, D.J. (2016). An atypical tropomyosin in Drosophila with intermediate filament-like properties. Cell Rep [Epub ahead of print]. PubMed ID: 27396338

Dimitrova-Paternoga, L., Jagtap, P. K. A., Cyrklaff, A., Vaishali, Lapouge, K., Sehr, P., Perez, K., Heber, S., Low, C., Hennig, J. and Ephrussi, A. (2021). Molecular basis of mRNA transport by a kinesin-1-atypical tropomyosin complex. Genes Dev 35(13-14): 976-991. PubMed ID: 34140355

Gardiol, A. and St Johnston, D. (2014). Staufen targets coracle mRNA to Drosophila neuromuscular junctions and regulates GluRIIA synaptic accumulation and bouton number. Dev Biol. PubMed ID: 24951879

Gaspar, I., Sysoev, V., Komissarov, A. and Ephrussi, A. (2016). An RNA-binding atypical tropomyosin recruits kinesin-1 dynamically to oskar mRNPs. Embo J [Epub ahead of print]. PubMed ID: 28028052

Goins, L. M. and Mullins, R. D. (2015). A novel tropomyosin isoform functions at the mitotic spindle and Golgi in Drosophila. Mol Biol Cell [Epub ahead of print]. PubMed ID: 25971803

Heber, S., McClintock, M. A., Simon, B., Mehtab, E., Lapouge, K., Hennig, J., Bullock, S. L., Ephrussi, A. (2024). Tropomyosin 1-I/C coordinates kinesin-1 and dynein motors during oskar mRNA transport. Nat Struct Mol Biol, 31(3):476-488 PubMed ID: 38297086

Hsiao, J. Y., Goins, L. M., Petek, N. A. and Mullins, R. D. (2015). Arp2/3 complex and cofilin modulate binding of tropomyosin to branched actin networks. Curr Biol 25: 1573-1582. PubMed ID: 26028436

Lehman, W., Rynkiewicz, M. J. (2023). Troponin-I-induced tropomyosin pivoting defines thin-filament function in relaxed and active muscle. J Gen Physiol, 155(7) PubMed ID: 37249525

McAdow, J., Yang, S., Ou, T., Huang, G., Dobbs, M. B., Gurnett, C. A., Greenberg, M. J. and Johnson, A. N. (2022). A pathogenic mechanism associated with myopathies and structural birth defects involves TPM2-directed myogenesis. JCI Insight 7(12). PubMed ID: 35579956

Rynkiewicz, M. J., Childers, M. C., Karpicheva, O. E., Regnier, M., Geeves, M. A., Lehman, W. (2024). Myosin's powerstroke transitions define atomic scale movement of cardiac thin filament tropomyosin. J Gen Physiol, 156(5) PubMed ID: 38607351

Sahota, V. K., Grau, B. F., Mansilla, A. and Ferrás, A. (2009). Troponin I and Tropomyosin regulate chromosomal stability and cell polarity. J. Cell Sci. 122(Pt 15): 2623-31. PubMed ID: 19567471

Sysoev, V. O., Kato, M., Sutherland, L., Hu, R., McKnight, S. L. and Murray, D. T. (2020). Dynamic structural order of a low-complexity domain facilitates assembly of intermediate filaments. Proc Natl Acad Sci U S A 117(38): 23510-23518. PubMed ID: 32907935

Texada, M. J., Simonette, R. A., Deery, W. J., Beckingham, K. M. (2011). Tropomyosin is an interaction partner of the Drosophila coiled coil protein yuri gagarin. Exp Cell Res, 317(4):474-487 PubMed ID: 21126519

Veeranan-Karmegam, R., Boggupalli, D. P., Liu, G. and Gonsalvez, G. B. (2016). A new isoform of Drosophila non-muscle Tropomyosin 1 interacts with Kinesin-1 and functions in oskar mRNA localization. J Cell Sci 129: 4252-4264. PubMed ID: 27802167

Viswanathan, M. C., Schmidt, W., Rynkiewicz, M. J., Agarwal, K., Gao, J., Katz, J., Lehman, W. and Cammarato, A. (2017). Distortion of the Actin A-triad results in contractile disinhibition and cardiomyopathy. Cell Rep 20(11): 2612-2625. PubMed ID: 28903042

Williams, J., Boin, N. G., Valera, J. M. and Johnson, A. N. (2015). Noncanonical roles for Tropomyosin during myogenesis. Development [Epub ahead of print]. PubMed ID: 26293307

Zou, L., Zhang, J., Han, J., Li, W., Su, F., Xu, X., Zhai, Z., Xiao, F. (2018). cGMP interacts with tropomyosin and downregulates actin-tropomyosin-myosin complex interaction. Respir Res, 19(1):201 PubMed ID: 30314482

date revised: 12 May, 2024

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