InteractiveFly: GeneBrief

glorund: Biological Overview | References

The Drosophila melanogaster protein Glorund (Glo) represses nanos (nos) translation and uses its quasi-RNA recognition motifs (qRRMs) to recognize both G-tract and structured UA-rich motifs within the nos translational control element (TCE). It has been shown previously that each of the three qRRMs is multifunctional, capable of binding to G-tract and UA-rich motifs, yet if and how the qRRMs combine to recognize the nos TCE remained unclear. This study determined solution structures of a nos TCEI_III RNA containing the G-tract and UA-rich motifs. The RNA structure demonstrated that a single qRRM is physically incapable of recognizing both RNA elements simultaneously. In vivo experiments further indicated that any two qRRMs are sufficient to repress nos translation. interactions of Glo qRRMs were probed with TCEI_III RNA using NMR paramagnetic relaxation experiments. The in vitro and in vivo data support a model whereby tandem Glo qRRMs are indeed multifunctional and interchangeable for recognition of TCE G-tract or UA-rich motifs. This study illustrates how multiple RNA recognition modules within an RNA-binding protein may combine to diversify the RNAs that are recognized and regulated (Warden, 2023).

Specific recognition of target RNAs by RNA-binding proteins is essential for gene control and often achieved by modular combination of multiple RNA-binding domains. Tandem RNA-binding domains may arise from duplication to allow recognition of longer linear RNA sequences or more complex structures than a single domain can accomplish. For example, the tandem zinc fingers of Tristetraprolin each recognize a UAUU sequence within the UUAUUUAUU AU-rich element (ARE). Duplicated domains also may diverge to recognize distinct sequence or structural elements. In the case of HuD protein, its two N-terminal RNA recognition motif (RRM) domains both recognize UU elements in AREs, but RRM1 also interacts with four additional 5' flanking nucleotides. Although amino acid sequences of RNA-binding domains can indicate whether they retain the characteristic RNA recognition features of that domain family, it remains impossible to predict most domain-specific variations. Hence, the specificity of RNA-binding domains must be determined experimentally. Each study expands knowledge of sequence features that drive specificity and improves prediction methods to identify target RNAs (Warden, 2023).

During Drosophila development, translational control of nanos (nos) mRNA is essential for the formation of the anterior-posterior body axis of the embryo. This process begins in the ovary when nos mRNA is transferred to oocytes from nurse cells during late oogenesis and translationally repressed until it is localized to the posterior. The translational repression of unlocalized nos mRNA is mediated by a translational control element (TCE) in the 3' UTR of nos mRNA. nos TCE RNA is composed of one stem (TCEI) and two stem loop structures (TCEII and TCEIII). During oogenesis, binding of TCEI and TCEIII by the protein Glorund (Glo) controls Nos expression. This protein:RNA interaction is essential to repress unlocalized nos mRNA during oogenesis, which ultimately ensures the formation of functional anterior structures in the fly embryo. Repression of nos initiated by Glo during oogenesis is also maintained in the early embryo by another RNA-binding protein, Smaug, which recognizes TCE stem loop II (Warden, 2023).

Glo is an RNA-binding protein belonging to the heterogenous nuclear ribonucleoprotein F/H (hnRNP F/H) family. Mammalian hnRNP F and H are predominantly nuclear and best known as regulators of alternative splicing. Additionally, hnRNP F and H1 are abundant in regenerating axons, where they bind to mRNAs involved in axonal growth and are thought to regulate their axonal transport and/or translation. Notably, mammalian hnRNP F and H proteins are upregulated in a variety of cancers and are thought to contribute to pathogenicity in part by altering translation. Like hnRNPs F/H, Glo contains three tandem quasi-RNA recognition motifs (qRRMs). It was previously demonstrated that the Glo qRRMs are multifunctional, each capable of binding to G-tract and structured UA-rich features of the nos TCE. The G-tract sequence is found in TCEI and the structured UA-rich motif in TCEIII. Both features are essential for TCE function and Glo recognition, yet the affinity for G-tract binding is substantially higher than UA-rich motif binding. Disruption of either RNA motif results in modestly increased Nos protein, but mutation of both RNA motifs dramatically increases Nos protein levels. Moreover, engineered mutations that preferentially disrupt Glo qRRM binding to either the G-tract or UA-rich motif demonstrate that nos repression requires both modes of Glo-TCE recognition in vivo. How the tandem Glo qRRMs combine to recognize the G-tract and UA-rich motifs of TCE RNA remained unanswered (Warden, 2023).

This study presents a high-resolution solution structure of an RNA containing the elements recognized by Glo: nos TCEI and TCEIII, which is called in this study TCEI_III. The divide-and-conquer method, a well-established and effective approach combining structural biology methods, was used to build a larger macromolecular model from high-resolution structures of its components. This divide-and-conquer method allowed this study to avoid the technical difficulties of crystallizing structured RNAs without engineered crystal contacts such as tetraloop/tetraloop insertions. First a solution structure of TCEIII RNA was determined by NMR. Then the TCEIII structure along with NMR data for TCEI_III RNA and overall shape information from small-angle X-ray scattering (SAXS) were used to produce the hybrid TCEI_III model. This model suggested that more than one Glo qRRM is necessary for TCE recognition, and in vivo experiments indicated that a minimum of two qRRMs is sufficient for nos repression. Furthermore, NMR paramagnetic relaxation experiments (PREs) were used to probe the interactions between the G-tract and UA-rich motifs of TCE RNA and individually-labeled Glo qRRMs. Our in vitro and in vivo data are consistent with a model where the Glo qRRMs are indeed multifunctional and interchangeable for recognition of nos TCE G-tract or UA-rich motifs (Warden, 2023).

Combinations of RNA recognition modules generate the diverse specificities of RNA-binding proteins needed to control gene expression. RRM or KH domains often occur in tandem and together generate distinct RNA recognition specificity to select target mRNAs. Although much information is available about the specificities of RNA-binding domains, including high-throughput screening of sequence specificity, there relatively little information on how multiple RNA-binding modules bring their individual specificities to RNA target selection. This study demonstrates that at least two of the three multifunctional Glo qRRMs are required to recognize the nos TCE RNA, one for the TCEIII UA-rich stem and one for the TCEI G-tract. Therefore, the efficiency of having two RNA-binding modes in one qRRM is not utilized: one domain does not simultaneously recognize both RNA features. It was also found that the three qRRMs are interchangeable: any two of the three Glo qRRMs are sufficient in vivo to repress nos translation and maintain viability. Although qRRMs 1 and 2 are closely spaced with 8 residues between the C-terminal α helix of qRRM1 and the N-terminal β strand of qRRM2, this length linker appears sufficient to bridge the distance between G-tract and UA-rich elements. Alternatively, the C-terminal α helix of qRRM1 could unfold upon RNA binding, as seen for the RRM of Nop15 when it binds to pre-rRNA. Finally, it does not matter which RNA feature is recognized by the two qRRMs. This striking redundancy of the Glo qRRMs seems excessive, as some specialization would have been expected to have evolved (Warden, 2023).

Several evolutionary benefits are suggested from having three redundant multifunctional qRRMs in a single protein. (i) Functional redundancy protects crucial gene regulation. Repression of nos translation during oogenesis is essential to ensure correct anterior patterning in the embryo, yet mutation of one qRRM avoids a biological disruption as nos expression is normal in embryos. Similarly, although the target RNA(s) are not known, Glo with two intact qRRMs supports viability to adulthood. (ii) Redundancy gives Glo the potential to recognize a variety of target RNAs with different combinations of G-tract and UA-rich features. In addition, the G-tracts may be either single-stranded or base-paired. Previous work has demonstrated that Glo qRRMs bind to short single-stranded G-tracts, and this study observed visible imino proton resonances for the G-tract sequence (G87, G88, and G89) in the TCEI_III RNA, indicating that the G-tract remains base paired in the presence of Glo. Therefore, Glo qRRMs appear to differ from hnRNP F qRRMs that disrupt RNA structures when binding to G-tracts. Although the in vivo and in vitro evidence indicates the importance of base pairing for TCE activity, it will require additional structural studies to determine how Glo qRRMs specifically recognize duplex RNAs. (iii) Functionally redundant qRRMs with different spacing between the qRRMs adds additional opportunities for distinct RNA recognition. The linker between Glo qRRMs 1 and 2 is short (∼8 aa) whereas qRRMs 2 and 3 are separated by a long linker (∼240 aa). Although the distance between qRRMs does not affect nos regulation as any two qRRMs are sufficient, the spacing may alter specificity for other RNAs. For example, qRRMs 1 and 2 would require RNAs with G-tract and/or UA-rich motifs near each other whereas the long linker between qRRMs 2 and 3 should make that two-domain combination less restrictive in arrangement of G-tract and/or UA-rich motifs in target RNAs. To appreciate the range of specificity and roles of its multifunctional qRRMs, additional RNAs that are controlled by Glo will need to be identified (Warden, 2023).

Adding to its multifunctionality, Glo also binds to other proteins. Glo was recently shown to interact with Drosophila Fragile X mental retardation protein (dFMRP) to stall ribosomes and decrease the translation elongation rate on nos mRNA. Although this study has not identified differences in RNA recognition, Glo qRRMs do have different protein binding specificity. Glo interacts with dFMRP via qRRM2, but qRRM1 and qRRM3 do not directly interact with dFMRP. It is tempting to speculate that interaction between qRRM2 and dFMRP alters RNA recognition, and that other protein interactions could modify RNA recognition by the other qRRMs to generate specialization (Warden, 2023).

The Drosophila hnRNP F/H homolog Glorund recruits dFMRP to inhibit nanos translation elongation

Translational control of maternal mRNAs generates spatial and temporal patterns of protein expression necessary to begin animal development. Translational repression of unlocalized nanos (nos) mRNA in late-stage Drosophila oocytes by the hnRNP F/H homolog, Glorund (Glo), is important for embryonic body patterning. While previous work has suggested that repression occurs at both the translation initiation and elongation phases, the molecular mechanism by which Glo regulates nos translation remains elusive. This study identified the Drosophila fragile X mental retardation protein, dFMRP, as a Glo interaction partner with links to the translational machinery. Using an oocyte-based in vitro translation system, it was confirmed that Glo regulates both initiation and elongation of a nos translational reporter and showed that dFMRP specifically represses translation elongation and promotes ribosome stalling. Furthermore, mutational analysis and in vivo and in vitro binding assays were combined to show that Glo's qRRM2 domain specifically and directly interacts with dFMRP. These findings suggest that Glo regulates nos translation elongation by recruiting dFMRP and that Glo's RNA-binding domains can also function as protein-protein interaction interfaces critical for its regulatory functions. Additionally, they reveal a mechanism for targeting dFMRP to specific transcripts (Peng, 2022).

The Drosophila hnRNP F/H homolog, Glo, represses the translation of unlocalized nos during late stages of oogenesis to ensure proper anterior-posterior patterning of the embryo. Previous work suggested that Glo imposes both an initiation and a post-initiation block on nos translation, but the molecular mechanism and the nature of the post-initiation block remained unknown. Moreover, how a single protein composed primarily of RNA-binding domains can confer repression by two different mechanisms is unclear. This study has begun to fill these gaps by identifying dFMRP as a Glo-interacting protein during late stages of oogenesis and biochemically dissecting its role in nos regulation. These results lead to a new model wherein Glo recruits dFMRP specifically through qRRM2 to repress elongating ribosomes on nos and suggest that Glo's distinct activities are dictated by different effector proteins recruited through different qRRMs (Peng, 2022).

Since the discovery of FMRP, numerous high-throughput analyses along with transcript-specific studies have uncovered a multiplicity of mechanisms by which it is thought to regulate translation in both neuronal and non-neuronal contexts. These include repression of cap-dependent translation initiation, modulation of microRNA activity, and stalling of elongating ribosomes. Among its large set of targets, FMRP is capable of binding to a wide variety of sequence motifs, further complicating the understanding of its target selectivity. This work shows that an FMRP-interacting protein recognizing an mRNA signature specific to an individual target mRNA can provide the required target selectivity (Peng, 2022).

A recent ribosome profiling study found that dFMRP activates translation of a collection of stored mRNAs, particularly those encoding large autism-related proteins, in Drosophila oocytes. The current work, however, suggests that dFMRP, recruited by specificity factors, can also function as translational repressor of individual targets in oocytes by blocking translation elongation. Indeed, dFMRP has been reported to interact with the 60S ribosomal protein L5, which may prevent tRNA binding during elongation. The Glo/TCE-dependent elongation-repressing activity of dFMRP together with the increased ribosome load following dFMRP KD leads to two non-exclusive hypotheses: 1) dFMRP can repress nos translation initiation independently of Glo/TCE and/or 2) in the absence of the dFMRP-mediated elongation block, the initiation block, regardless of its dFMRP dependency, is insufficient for complete shutdown of translation. Although it is not possible to distinguish among these possibilities, the latter is consistent with the relative magnitude of the initiation versus elongation-based repression (2.4-fold versus 3.6-fold) measured by a translation run-off assay (Peng, 2022).

Whereas Glo-RNA interactions have been characterized in some detail, much less is known about Glo-protein interactions and how they contribute to Glo's functional diversity. The lack of apparent functional domains apart from the qRRMs raises the possibility that these RNA-binding domains also mediate protein-protein interactions. Notably, the RRM and its variants have been implicated in RRM-RRM inter-domain packing and RRM-protein interactions. Among these protein-binding RRM family members, the U2AF homology motif (UHM) has emerged as a specialized protein-interacting platform rather than an RNA-interacting platform. Interestingly, the mammalian hnRNP F qRRM2 was recently shown to physically interact with FOXP3, which modulates the activity of hnRNP F in regulating alternative splicing. The demonstration that the interaction between Glo and dFMRP also involves qRRM2 suggests that qRRM2 might be an important protein-interaction platform as well as an RNA-binding platform for the hnRNP F/H family more generally. This finding also indicates that despite their structural similarity, the 3 Glo qRRMs differ in their protein binding specificity. Followup studies of other high-confidence Glo-interacting proteins identified by our IP-MS analysis may provide further evidence for differential protein-protein interaction capacity among Glo's qRRMs (Peng, 2022).

The incorporation of protein-binding and RNA-binding platforms into a single qRRM may enable specific protein interactors to influence RNA target selectivity. Depending on which part of qRRM is used for interaction, a qRRM-interacting protein could affect the accessibility of RNA-binding residues to RNA targets. While the FOXP3-hnRNP F qRRM2 interaction prevents hnRNP F from binding to BCL-X pre-mRNA, the interaction of SF3b155 to p14, a human RRM-containing U2 and U11/12 snRNP component, exposes secondary RNA-binding residues on the latter. Previously, it was shown that each Glo qRRM harbors 2 distinct RNA-binding interfaces that allow Glo to recognize two different sequence motifs: a single-stranded G-tract and a double-stranded UA-rich motif. The integration of both binding modes into a single qRRM and the combination of 3 qRRMs are thought to diversify the RNA target repertoire of Glo. Both the G-tract and UA-rich motif binding modes are required for Glo-TCE interaction, and hence nos regulation, as well as for regulation of as yet unknown targets required for viability. By contrast, only the G-tract binding mode is required for Glo's function as a putative splicing regulator in dorsal-ventral patterning and ovarian nurse cell chromatin structure remodeling. It remains to be determined if and how binding of dFMRP affects the RNA-binding affinity and specificity of qRRM2. Regardless, the choice of protein interactor could influence the combinatorial use of the 6 RNA-binding interfaces of Glo. For example, binding of Glo to dFMRP or other nos regulatory factors may leave both RNA-binding surfaces exposed, favoring nos TCE recognition and translational repression, whereas binding of Glo to Hrp48 and Hfp may leave only the G-tract binding residues exposed, favoring a different subset of targets for alternative splicing (Peng, 2022).

Using nos regulation as a model, this study has shown that Glo represses translation elongation by interacting with a polysome-associated protein, dFMRP, and that Glo's RNA-binding qRRM domains also mediate protein-protein interaction. Since Glo is a multi-functional post-transcriptional regulator, the same principles could also be applied to its other activities where integration of multiple regulatory modes into a single node is required. Identification of such targets and additional regulatory factors involved will forward our understanding of how qRRMs of Glo contribute to its functional diversity and, more generally, how RNA-binding proteins confer post-transcriptional gene regulation (Peng, 2022).

The heterogeneous nuclear ribonucleoprotein (hnRNP) Glorund functions in the Drosophila fat body to regulate lipid storage and transport

The availability of excess nutrients in Western diets has led to the overaccumulation of these nutrients as triglycerides, a condition known as obesity. The full complement of genes important for regulating triglyceride storage is not completely understood. Genome-wide RNAi screens in Drosophila cells have identified genes involved in mRNA splicing as important lipid storage regulators. Previous work showed that a group of splicing factors called heterogeneous nuclear ribonucleoproteins (hnRNPs) regulate lipid metabolism in the fly fat body; however, the identities of all the hnRNPs that function to control triglyceride storage are not known. This study used the GAL4/UAS system to induce RNAi to the hnRNP glorund (glo) in the Drosophila fat body to assess whether this hnRNP has any metabolic functions. Decreasing glo levels resulted in less triglycerides being stored throughout the fly. Interestingly, decreasing fat body glo expression resulted in increased triglyceride storage in the fat body, but blunted triglyceride storage in non-fat body tissues, suggesting a defect in lipid transport. Consistent with this hypothesis, the expression of apolipophorin (apolpp), microsomal triglyceride transfer protein (mtp), and apolipoprotein lipid transfer particle (apoltp), apolipoprotein genes important for lipid transport through the fly hemolymph, was decreased in glo-RNAi flies, suggesting that glo regulates the transport of lipids from the fly fat body to surrounding tissues. Together, these results indicate that glorund plays a role in controlling lipid transport and storage and provide additional evidence of the link between gene expression and the regulation of lipid metabolism (Kolasa, 2021).

The Drosophila hnRNP F/H homolog Glorund uses two distinct RNA-binding modes to diversify target recognition

The Drosophila hnRNP F/H homolog, Glorund (Glo), regulates nanos mRNA translation by interacting with a structured UA-rich motif in the nanos 3' untranslated region. Glo regulates additional RNAs, however, and mammalian homologs bind G-tract sequences to regulate alternative splicing, suggesting that Glo also recognizes G-tract RNA. To gain insight into how Glo recognizes both structured UA-rich and G-tract RNAs, mutational analysis was used guided by crystal structures of Glo's RNA-binding domains, and two discrete RNA-binding surfaces were identified that allow Glo to recognize both RNA motifs. By engineering Glo variants that favor a single RNA-binding mode, it was shown that a subset of Glo's functions in vivo is mediated solely by the G-tract binding mode, whereas regulation of nanos requires both recognition modes. These findings suggest a molecular mechanism for the evolution of dual RNA motif recognition in Glo that may be applied to understanding the functional diversity of other RNA-binding proteins (Tamayo, 2017).

Glorund, a Drosophila hnRNP F/H homolog, is an ovarian repressor of nanos translation

Patterning of the anterior-posterior body axis of the Drosophila embryo requires production of Nanos protein selectively in the posterior. Spatially restricted Nanos synthesis is accomplished by translational repression of unlocalized nanos mRNA together with translational activation of posteriorly localized nanos. Repression of unlocalized nanos mRNA is mediated by a bipartite translational control element (TCE) in its 3' untranslated region. TCE stem-loop II functions during embryogenesis, through its interaction with the Smaug repressor. Stem-loop III represses unlocalized nanos mRNA during oogenesis, but trans-acting factors that carry out this function have remained elusive. This study identifies a Drosophila hnRNP, Glorund, that interacts specifically with stem-loop III. The ability of the TCE to repress translation in vivo reflects its ability to bind Glorund in vitro. These data, together with the analysis of a glorund null mutant, reveal a specific role for an hnRNP in repression of nanos translation during oogenesis (Kalifa, 2006).

Thus far, a single protein, Smaug (Smg), has been shown to interact with the TCE. Smg, which is present only after fertilization, binds to unpaired nucleotides in stem-loop II (the Smg recognition element; SRE), and is required for repression of nos in the early embryo. In contrast, stem-loop III function in the ovary depends on the double-stranded sequence and structure of the helical stem. The essential, Smg-independent function of stem-loop III during oogenesis suggests that this motif is the binding site for an ovarian repressor of nos translation. A factor that recognizes stem-loop III has yet to be identified, however (Kalifa, 2006).

By using a biochemical approach to isolate TCE binding proteins, a previously uncharacterized Drosophila protein, Glorund, has been identified. Members of the hnRNP family participate in all aspects of nuclear and cytoplasmic RNA metabolism, including mRNA processing, nuclear export, localization, translation, and stability. Although all mRNAs are associated with hnRNPs and many hnRNPs recognize numerous mRNAs, recent studies have identified roles for several hnRNPs in localization and/or translational control of specific mRNAs. hnRNPs K and E1, which repress translation of 15-lipoxygenase (LOX) mRNA during erythrocyte differentiation, and hnRNP I, which participates in localization of Vg1 and VegT mRNAs in Xenopus oocytes, recognize specific primary sequence motifs in the 3'UTRs of their target mRNAs that are required for regulation. In Drosophila oocytes, two hnRNP A/B family members, Squid and Hrp48/ Hrb27C, play roles in localization and translational control of gurken (grk) and osk mRNAs. The sequence motifs in the grk and osk mRNAs that are targeted by these hnRNPs are not well defined, however. Glo is most closely related to mammalian hnRNPs F and H, whose RNA binding domains contain RNA recognition motifs (RRM) that deviate from RRMs found in the more common hnRNP A/B class. Glo is the first hnRNP F/H family member to be implicated in translational repression. Glo interacts specifically with a double-stranded motif in TCE stem-loop III, and the ability of the TCE to bind Glo correlates with its translational regulatory function. Furthermore, through the analysis of a glo null mutation, evidence is provided that Glo is required for translational repression of unlocalized nos mRNA in late oocytes. Thus, Glo acts prior to Smg to establish the repressed state of nos during oogenesis through its interaction with TCE stem-loop III (Kalifa, 2006).

Several lines of evidence establish Glo as a repressor of nos translation. (1) Binding of Glo to the double-stranded UA-rich motif of TCE stem-loop III in vitro correlates with the ability of this element to repress translation in vivo. (2) Analysis of a GFP-Nos reporter in glo mutant ovaries shows increased accumulation of GFP-Nos in late oocytes. (3) Loss of glo results in derepression of unlocalized, translationally silent nos RNA in late oocytes. The translational activity of nos RNA during oogenesis is both spatially and temporally dynamic. nos is translated first in the nurse cells, but becomes repressed in the oocyte after nurse cell dumping. The rapid inactivation of nos translation upon entry into the oocyte has been proposed to occur by a mechanism that blocks translation downstream of the initiation step. The identification of Glo will now facilitate investigation of this mechanism. Since Glo is present in both the nurse cells and oocyte, the ability of Glo to interact with nos or repress its translation must be largely restricted to the oocyte. Evidence that Nos protein produced in nurse cells is targeted for degradation in the oocyte suggests that there are significant physiologic differences between the nurse cells and oocyte that can affect protein behavior. Thus, Glo could be negatively regulated by a nurse cell-specific cofactor or modification event, or positively regulated by an oocyte-specific factor or modification (Kalifa, 2006).

Following fertilization, repression of nos translation is mediated primarily by the interaction of Smg with stemloop II. Because misexpression of Smg in the female germline severely disrupts oogenesis, Smg cannot fulfill the role of an ovarian repressor of nos translation. At the same time, although Glo is normally present in the early embryo, derepression of nos RNA in smg mutants indicates that Glo cannot substitute for Smg during embryogenesis. Thus, Glo and Smg fulfill temporally distinct roles in nos regulation. The minor requirement observed for stem-loop III in embryonic repression may, however, reflect a role for Glo at the beginning of embryogenesis while Smg is accumulating. The ability of Glo and Smg to bind to the TCE simultaneously would therefore ensure that repression is maintained across the transition from oogenesis to embryogenesis (Kalifa, 2006).

The translational activity of posteriorly localized nos RNA requires that repression by Glo and Smg is alleviated at the posterior of the oocyte and embryo, respectively. However, Glo, like Smg, is uniformly distributed. Thus, both proteins must be prevented from functioning at the posterior pole. Genetic evidence that binding of localization factors and translational repressors to nos RNA is mutually exclusive suggests that localization factors at the posterior may compete with Glo and Smg for binding to the nos 3'UTR. Alternatively, factors at the posterior pole may inactivate the repressors locally by posttranslational modification (Kalifa, 2006).

The effect of eliminating glo is less severe than anticipated from analysis of the effect of TCE stem-loop III mutations that eliminate Glo binding (Crucs, 2000). For example, approximately 25% of glo mutant embryos develop and hatch as larvae, whereas all embryos produced by females carrying the nos-tub:TCEIIIA transgene die with anterior defects. Thus, it is possible that a second ovarian factor, which recognizes TCE stem-loop III similarly to Glo, can partially compensate for loss of glo function. Given that the nos-tub:TCEIIIA transgene lacks all nos 3'UTR sequences outside of the TCE (Crucs, 2000), an alternative explanation is favored that additional factors binding to other regions of the nos 3'UTR can contribute to repression during oogenesis. Attempts to identify such regulatory sequences and factors are currently in progress (Kalifa, 2006).

Glo is the closest Drosophila homolog to mammalian hnRNPs F and H. A second Drosophila protein, Fusilli (Fus), contains RNA binding domains that are more distantly related to hnRNPs F and H and even more distantly related to Glo, but show greatest similarity to a human protein of unknown function. Thus, hnRNPs F and H appear to be represented in Drosophila by a single protein. Members of the F/H family have been implicated as general splicing factors through their interaction with the nuclear cap binding proteins and as regulators of alternative splicing in mammalian neuronal cells. An hnRNP F/H-related protein, guanine-rich sequence factor 1 (GRSF-1), stimulates translation of influenza virus-encoded mRNAs by binding to sequences in their 5'UTRs. Glo is the first hnRNP F/H family member to be identified as a translational repressor, however. The ability to recognize the double-stranded UA motif in TCE stem-loop III sets Glo apart from its mammalian splicing counterparts, which bind preferentially to poly(rG) sequences. In addition, Glo differs from the mammalian proteins by the insertion of a glycine- and asparagine- rich domain between the second and third q-RRMs. These differences may reflect the unique acquisition by Glo of functions such as translational repression. It will therefore be of interest to determine whether the mammalian proteins participate in translational control in addition to splicing and, likewise, whether Glo also functions as a splicing factor (Kalifa, 2006).

Like Hrp48/Hrb27C, which was first identified as a splicing regulator and subsequently shown to regulate both localization and translation of grk and osk mRNAs, Glo may serve multiple functions. The pleiotropy of the glo mutant phenotype, including its zygotic lethality, suggests that glo acts at different developmental stages to regulate RNAs in addition to nos. Expression of Glo in the central nervous system (CNS) at late stages of embryogenesis is particularly intriguing in light of increasing evidence for translational control in neuronal development and synaptic function. Furthermore, the TCE can mediate translational repression in subsets of cells in the CNS and this repression is Smg independent. Glo is therefore a good candidate to mediate repression of RNAs with TCElike motifs in the CNS (Kalifa, 2006).

Search PubMed for articles about Drosophila Glorund

Kalifa, Y., Huang, T., Rosen, L. N., Chatterjee, S., Gavis, E. R. (2006). Glorund, a Drosophila hnRNP F/H homolog, is an ovarian repressor of nanos translation. Dev Cell, 10(3):291-301 PubMed ID: 16516833

Kolasa, A. M., Bhogal, J. K. and DiAngelo, J. R. (2021). The heterogeneous nuclear ribonucleoprotein (hnRNP) glorund functions in the Drosophila fat body to regulate lipid storage and transport. Biochem Biophys Rep 25: 100919. PubMed ID: 33537463

Peng, Y. and Gavis, E. R. (2022). The Drosophila hnRNP F/H homolog Glorund recruits dFMRP to inhibit nanos translation elongation. Nucleic Acids Res. PubMed ID: 35699205

Tamayo, J. V., Teramoto, T., Chatterjee, S., Hall, T. M. and Gavis, E. R. (2017). The Drosophila hnRNP F/H homolog Glorund uses two distinct RNA-binding modes to diversify target recognition. Cell Rep 19(1): 150-161. PubMed ID: 28380354

Warden, M. S., DeRose, E. F., Tamayo, J. V., Mueller, G. A., Gavis, E. R. and Hall, T. M. T. (2023). The translational repressor Glorund uses interchangeable RNA recognition domains to recognize Drosophila nanos. Nucleic Acids Res. PubMed ID: 37427795

date revised: 3 May 2024

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