BIOLOGICAL OVERVIEW

The spindle checkpoint transiently prevents cell cycle progression of cells that have incurred errors or have failed to complete steps during mitosis, including those involving kinetochore function. Bub1 is an evolutionarily conserved mitotic checkpoint control protein that is present in organisms as diverse as yeast and humans. Bub1 inhibits ubiquitin ligase activity of anaphase promoting complex (APC) preventing mitosis until all chromosomes are correctly attached to the mitotic spindle. Drosophila Bub1 (see below: The gene described in this section of the Interactive Fly is more properly termed BudR1) localizes strongly to the centromere/kinetochore of mitotic and meiotic chromosomes that have not yet reached the metaphase plate. Animals homozygous for P-element-induced, near-null mutations of bub1 die during late larval/pupal stages due to severe mitotic abnormalities indicative of a bypass of checkpoint function. These abnormalities include accelerated exit from metaphase and chromosome missegregation and fragmentation. Chromosome fragmentation possibly leads to the significantly elevated levels of apoptosis seen in mutants (Basu, 1999).

The relationship between Bub1 and other kinetochore components was investigated. Bub1 kinase activity is not required for phosphorylation of 3F3/2 epitopes (detected by anti 3F3/2 antibody) at prophase/prometaphase, but is needed for 3F3/2 dephosphorylation at metaphase. Neither 3F3/2 dephosphorylation nor loss of Bub1 from the kinetochore is a prerequisite for anaphase entry. Bub1's localization to the kinetochore does not depend on the products of the genes zw10, rod, polo, or fizzy, indicating that the kinetochore is constructed from several independent subassemblies (Basu, 1999).

Mammalian BubR1, a protein that is functionally similar, but not equivalent to mammalian Bud1 (Taylor, 2001), inhibits APC activity (Sudakin, 2001; Tang, 2001; Fang, 2002). BubR1 is a protein kinase that associates with Cdc20 (the homolog of Drosophila Fizzy in yeast and vertebrates) and the APC (Chan, 1999; Wu, 2000; Skoufias, 2001). Recombinant BubR1 has been shown to directly inhibit the ubiquitin ligase activity of the APC; the kinase activity of BubR1 is not required for this inhibition (Tang, 2001). In addition, a checkpoint complex has been purified (Tang, 2001) from HeLa cells that contains BubR1, Bub3 (see Drosophila Bub3) and substoichiometric amounts of Cdc20. Independently, Sudakin (2001) purified a mitotic checkpoint complex (MCC) that contains nearly stoichiometric amounts of BubR1, Bub3, Mad2 (see Drosophila ) and Cdc20 (Sudakin, 2001). The isolated MCC was shown to be about 3000-fold more potent than purified Mad2 alone at inhibiting the ubiquitin ligase activity of the APC. In experiments consistent with these studies, Fang found that BubR1 binds to Cdc20 with a high affinity and is more efficient than Mad2 as an inhibitor of Cdc20-APC in vitro (Fang, 2002). Moreover, BubR1 functions synergistically with Mad2 at physiological concentrations to inhibit APC activity. Interestingly, studies in fission yeast demonstrate that Mad3 also associates with Bub3, Cdc20 and Mad2 and that Mad3 is required for metaphase arrest caused by Mad2 overexpression (Millband, 2002). Collectively, these studies suggest that BubR1 and Mad2 cooperate in transducing the anaphase-delaying signal by inhibiting APC activity (Zhou, 2002 and references therein).

Both human Bub1 and Drosophila Bud1 associate strongly with the kinetochores of chromosomes unattached to the spindle prior to anaphase onset of normal mitosis, and with all the kinetochores in cells treated with microtubule depolymerizing drugs. Reduced amounts of both proteins are also found at the kinetochores of chromosomes either at the metaphase plate or being pulled toward the poles at anaphase. Near null mutations in the gene encoding this Drosophila protein cause phenotypes indicating an abrogation of the spindle checkpoint. These same mutations abolish the ability of another checkpoint component, Drosophila Bub3, to localize to the kinetochores (Basu, 1998). This latter finding fits well with a wealth of data substantiating an intimate relationship between Bub1 and Bub3. Taken together, these observations in Drosophila provide strong evidence for the conservation of Bub1 function throughout evolution (Basu, 1999).

In S. cerevisiae, bub and mad genes are nonessential in the absence of microtubule depolymerizing agents, though the growth of mutant cells is slowed (Roberts, 1994). In S. pombe, bub1 null mutants are viable, though some abnormalities in chromosome segregation are observable during mitosis (Bernard, 1998). In marked contrast, loss of bub1 function in Drosophila leads to lethality at the larval/pupal transition. Lethality at this stage has been observed for many mutations affecting essential cell cycle components, presumably because maternally supplied stores of protein obtained from a nonmutant mother are exhausted by this point in development. Examination of brain neuroblasts dissected from dying third instar bub1 homozygous mutant larvae has thus facilitated an analysis of how loss of checkpoint function affects cell division in a multicellular organism. In the description below, the possibility cannot be excluded that some aspects of the phenotype reported are indirect consequences of problems encountered in earlier cell divisions. However, it should be noted that all embryonic divisions appear to be normal, and essentially all bub1 mutant animals hatch into larvae that survive until the third instar. Since there is very little cell division in the larval brain before the third instar, the number of cell divisions that could take place between the exhaustion of maternal stores of Bub1 protein and the time of analysis is limited. Moreover, these phenotypes are quite specific to bub1 mutants, and have not been observed in analysis of many other mitotic mutants in Drosophila (Basu, 1999).

Treatment of bub1 mutant neuroblasts with colchicine causes precocious sister chromatid separation, instead of the prometaphase arrest with attached sister chromatids typical of wild-type neuroblasts. This phenotype is a predictable property of mutations affecting the operation of the spindle checkpoint, since it indicates that bub1 mutant neuroblasts attempt to enter anaphase despite the absence of a functional spindle (Basu, 1999).

More interesting are the effects of bub1 mutations on normal cell division in neuroblasts that have not been treated with microtubule depolymerizing drugs. Observations suggest that bub1 mutant neuroblasts enter anaphase prematurely even in these untreated cells. In particular, the ratio of metaphase figures to anaphase figures is decreased 5-10-fold in bub1 brains relative to wild-type brains. This result is consistent with studies showing that microinjection of Mad2 antibodies into mammalian cells causes premature sister chromatid separation and entry into anaphase. Interestingly, loss of Bub1 in Drosophila generates a sharp decrease in mitotic index. This finding could be explained by an accelerated transit through mitosis as has been suggested for mammalian cell cultures expressing dominant negative forms of mouse Bub1 (Taylor, 1997). However, it is also possible that the lowered mitotic index reflects the assumption of an apoptotic fate by many neuroblasts in the brain (Basu, 1999).

A high proportion of anaphases in untreated bub1 mutant brains show a variety of aberrations, including extensive chromatin bridging, lagging chromosomes (most likely leading to aneuploidy), and chromosome fragmentation. These aberrations are interpreted as further evidence for the precocious entry into anaphase. In this view, the proper synchronization of different aspects of sister chromatid separation at the metaphase/anaphase transition does not occurred. It is well known that the forces holding sister chromatids together along their arms are separable from the forces joining sister chromatids at their centromeres. For example, acentric chromosome fragments in irradiated grasshopper neuroblasts remain associated until the onset of anaphase. In addition, sister chromatid cohesion along the arms can also be disrupted independently of centromeric cohesion through treatment with hypotonic solutions. It is hypothesized that absence of bub1 function leads to loss of cohesive forces at the centromere before the separation of sister chromatids along their arms is completed. Thus, the chromatin bridging and fragmentation seen in bub1 mutant anaphases most likely reflect a failure to resolve concatenated sister chromatid DNAs along the arms at a time at which the centromeres have already separated and are being pulled toward the poles. In support of this interpretation, mutations in the Drosophila gene barren, which encodes a chromosome-associated protein that interacts with topoisomerase II, cause substantial chromatin bridging during anaphase of late embryonic divisions (Basu, 1999).

A striking feature of Drosophila bub1 mutants is the occurrence of significantly elevated frequencies of apoptotic nuclei in larval brains. This result was unexpected, since expression of a dominant negative form of mouse Bub1 has been reported to reduce the frequency of apoptotic nuclei in nocodazole-treated cells (Taylor, 1997), implying that loss of checkpoint function prevents the apoptotic response. The reasons for the apparent dichotomy between the results in Drosophila and those from mouse tissue culture cells are not clear. These effects could be organism or cell type-specific, or the differences could reflect unusual consequences of the dominant negative forms of Bub1 utilized in the mouse study (Basu, 1999).

A strong possibility for the high level of apoptotic cells seen in bub1 mutant brains emerges from findings that the chromosomes in many mutant anaphase figures are extensively fragmented. It has been well documented that chromosome breakage in Drosophila is normally a cell lethal event preventing entry into the next round of mitosis. The larval brains were examined of a number of new, relatively uncharacterized mitotic mutants that cause massive chromosome fragmentation, and these uniformly have high levels of apoptotic cells. Moreover, the induction of chromosome breakage with the FLP/FRT system is also associated with apoptosis. Apoptosis (or in fact any aspect of the bub1 mutant phenotype) cannot be an indirect consequence of aneuploidy, because brains from zw10 and rod mutants, which have many aneuploid cells, do not show the massive apoptotic response (or any of the cell cycle defects) generated by bub1 mutants. Regardless of the mechanism underlying the induction of apoptosis in bub1 mutant brains, it is clear that loss of spindle checkpoint function does not prevent a cell's entry into the apoptotic pathway (Basu, 1999).

In yeast, Bub1 acts as a kinase that can phosphorylate both itself and Bub3 (Roberts, 1994). Because Bub1's cell cycle distribution parallels that of 3F3/2 phosphoepitopes that appear to be intimately involved in the metaphase-anaphase transition, Bub1 has been suggested as a possible source of the kinase activity that generates these phosphoepitopes. This is not the case in Drosophila. F3/2 epitopes are strongly phosphorylated in a bub1 mutant, showing that Bub1 cannot be a significant source of 3F3/2 kinase activity in vivo. In addition, Bub3 fails to associate with the kinetochore in bub1 mutants, ruling out Bub3 as a major 3F3/2 phosphoepitope (Basu, 1999).

If Bub1 does not phosphorylate 3F3/2 phosphoepitopes, what kinase(s) can supply such an activity? A recent report indicates that ERK and MKK (extracellular signal-regulated protein kinase and mitogen-activated protein kinase kinase) localize to the kinetochore and can phosphorylate 3F3/2 phosphoepitopes. It is not clear whether this activity is direct or indirect; in any event, results indicate that Bub1 does not participate in the same 3F3/2 phosphorylation pathway (Basu, 1999 and references therein).

It was surprising to find that 3F3/2 epitopes at the kinetochores remain phosphorylated in anaphase figures from bub1 mutants. In contrast, 3F3/2 phosphoepitopes at the kinetochores are normally lost completely at the start of anaphase. The implications of this result are twofold. First, dephosphorylation of kinetochore-associated 3F3/2 epitopes is not required for the metaphase/anaphase transition, at least in a bub1 mutant background. One possibility is that 3F3/2 dephosphorylation is not, as commonly suggested, a part of the signaling pathway for anaphase onset, but is instead a downstream response to the signal. Alternatively, Bub1 may function downstream of 3F3/2 dephosphorylation in the pathway governing the metaphase–anaphase transition. A second implication of the observation is that Bub1 kinase activity is required, presumably indirectly, for the dephosphorylation of 3F3/2 epitopes at the metaphase-anaphase transition. A possible explanation for the continued phosphorylation of kinetochore-based 3F3/2 epitopes is that the accelerated transit through mitosis in bub1 mutants may not allow enough time for action of the relevant phosphatase(s) (Basu, 1999).

The localization of Bub1 to the kinetochore is not abolished by mutations in several genes encoding other kinetochore components, nor do mutations in bub1 affect the association of ZW10 or Polo proteins with the kinetochore. These findings suggest that the kinetochore may be assembled in at least two independent pathways. In one pathway, interaction between Bub1 and Bub3 is required for the kinetochore targeting of either protein (Roberts, 1994; Basu, 1998; Taylor, 1998). In a second subassembly, ZW10 and Rod proteins form a complex needed for the recruitment of the microtubule motor dynein to the kinetochore. The fact that polo mutations do not disrupt the kinetochore localization of Bub1, Bub3, or ZW10 suggests either a third independent pathway or that the kinetochore binding of Polo protein is subsequent to the recruitment of one of the two subassemblies (Basu, 1999 and references therein).

In colchicine-treated larval neuroblasts from zw10 mutants where the sister chromatids have separated prematurely, high levels of Bub1 protein remain at the kinetochores. This phenomenon is not restricted to a zw10 mutant background, since prolonged treatment of wild-type larval neuroblasts with hypotonic solution after colchicine incubation also generates precocious sister chromatid separation with continued strong Bub1 staining at the kinetochores. These observations indicate that it is possible to initiate anaphase despite the presence of the Bub1 'wait-anaphase' signal at the kinetochores. It is thus conceivable that the relative loss of Bub1 from kinetochores at metaphase and anaphase is not normally a prerequisite for anaphase onset (Basu, 1999).

Although the existence of a tension-dependent 'wait-anaphase' checkpoint in meiotic grasshopper spermatocytes has been well established, several observations suggest that such a checkpoint may not play a major role in Drosophila spermatogenesis. The presence of univalents (chromosomes without a pairing partner) does not obviously affect meiotic progression. Mutations in mei-S322 and ord, which lead to sister chromatid separation before the start of the second meiotic division, do not affect entry into anaphase II. Finally, colchicine-treated spermatocytes that cannot segregate their chromosome still exit meiosis and differentiate into spermatids (Basu, 1999).

Nevertheless, Drosophila Bub1 and Bub3 both associate strongly with the kinetochores of primary spermatocytes before metaphase of both meiotic divisions, and kinetochore staining cas be observed with antibodies against Xenopus Mad2 in prometaphase primary spermatocytes. Bub1 responds differentially to the presence and absence of tension across chromosomes during meiosis exactly as would be predicted were it acting as part of a functional spindle checkpoint. In addition, bub1 mutations have a dramatic effect on Drosophila spermatogenesis. Though it is difficult to distinguish aberrations introduced during mitotic germ line cell proliferation from those occurring during meiosis, the appearance of disrupted meiotic spindles and of multiple nuclei of uneven volume within 'onion-stage' spermatids are suggestive of defects specifically affecting meiosis (Basu, 1999).

On the basis of these observations, it is believed that a spindle checkpoint does exist in Drosophila meiotic spermatocytes, but that it operates with significantly reduced efficiency or according to different signals. The reasons underlying this apparent inefficiency remain unclear, but very likely involve part of the checkpoint pathway downstream of Bub1. One prediction of this viewpoint is that conditions that should enable the checkpoint would delay, but not completely block, cell cycle progression past the metaphase of either meiotic division. It will thus be of importance in the near future to verify this prediction through real-time observations of male meiosis in cultured spermatocytes (Basu, 1999).

Sister centromere fusion during meiosis I depends on maintaining cohesins and destabilizing microtubule attachments

Sister centromere fusion is a process unique to meiosis that promotes co-orientation of the sister kinetochores, ensuring they attach to microtubules from the same pole during metaphase I. This study found that the kinetochore protein SPC105R/KNL1 and Protein Phosphatase 1 (PP1-87B) regulate sister centromere fusion in Drosophila oocytes. The analysis of these two proteins, however, has shown that two independent mechanisms maintain sister centromere fusion. Maintenance of sister centromere fusion by SPC105R depends on Separase, suggesting cohesin proteins must be maintained at the core centromeres. In contrast, maintenance of sister centromere fusion by PP1-87B does not depend on either Separase or WAPL. Instead, PP1-87B maintains sister centromeres fusion by regulating microtubule dynamics. This study has demonstrated that this regulation is through antagonizing Polo kinase and BubR1, two proteins known to promote stability of kinetochore-microtubule (KT-MT) attachments, suggesting that PP1-87B maintains sister centromere fusion by inhibiting stable KT-MT attachments. Surprisingly, C(3)G, the transverse element of the synaptonemal complex (SC), is also required for centromere separation in Pp1-87B RNAi oocytes. This is evidence for a functional role of centromeric SC in the meiotic divisions, that might involve regulating microtubule dynamics. Together, this study proposes that two mechanisms maintain co-orientation in Drosophila oocytes: one involves SPC105R to protect cohesins at sister centromeres and another involves PP1-87B to regulate spindle forces at end-on attachments (Wang, 2019).

The necessity of sister kinetochores to co-orient toward the same pole for co-segregation at anaphase I differentiates the first meiotic division from the second division. A meiosis-specific mechanism exists that ensures sister chromatid co-segregation by rearranging sister kinetochores, aligning them next to each other and facilitating microtubule attachments to the same pole]. This process is referred to as co-orientation, in contrast to mono-orientation, when homologous kinetochores orient to the same pole. Given the importance of co-orientation in meiosis the mechanism underlying this process is still poorly understood, maybe because many of the essential proteins are not conserved across phyla (Wang, 2019).

Most studies of co-orientation have focused on how fusion of the centromeres and kinetochores is established. In budding yeast, centromere fusion occurs independently of cohesins: Spo13 and the Polo kinase homolog Cdc5 recruit a meiosis-specific protein complex, monopolin (Csm1, Lrs4, Mam1, CK1) to the kinetochore. Lrs4 and Csm1 form a V-shaped structure that interacts with the N-terminal domain of Dsn1 in the Mis12 complex to fuse sister kinetochores. While the monopolin complex is not widely conserved, cohesin-independent mechanisms may exist in other organisms. A bridge between the kinetochore proteins MIS12 and NDC80 is required for co-orientation in maize. In contrast, cohesins are required for co-orientation in several organisms. The meiosis-specific cohesin Rec8 is indispensable for sister centromere fusion in fission yeast and Arabidopsis. Cohesin is localized to the core-centromere in fission yeast and mice. In Drosophila melanogaster oocytes, cohesins (SMC1/SMC3/SOLO/SUNN) establish cohesion in meiotic S-phase and show an enrichment that colocalizes with centromere protein CID/CENP-A. Like fission yeast and mouse, Drosophila may require high concentrations of cohesins to fuse sister centromeres together for co-orientation during meiosis (Wang, 2019).

In mice, a novel kinetochore protein, Meikin, recruits Plk1 to protect Rec8 at centromeres. Although poorly conserved, Meikin is proposed to be a functional homolog of Spo13 in budding yeast and Moa1 in fission yeast. They all contain Polo-box domains that recruit Polo kinase to centromeres. Loss of Polo in both fission yeast (Plo1) and mice results in kinetochore separation, suggesting a conserved role for Polo in co-orientation. In fission yeast, Moa1-Plo1 phosphorylates Spc7 (KNL1) to recruit Bub1 and Sgo1 for the protection of centromere cohesion in meiosis I. These results suggest the mechanism for maintaining sister centromere fusion involves kinetochore proteins recruiting proteins that protect cohesion. However, how centromere cohesion is established prior to metaphase I, and how sister centromere fusion is released during meiosis II, still needs to be investigated (Wang, 2019).

Previous work has found that depletion of the kinetochore protein SPC105R (KNL1) in Drosophila oocytes results in separated centromeres at metaphase I, suggesting a defect in sister centromere fusion. Thus, Drosophila SPC105R and fission yeast Spc7 may have conserved functions in co-orientation (Radford, 2015). This study has identified a second Drosophila protein required for sister-centromere fusion, Protein Phosphatase 1 isoform 87B (PP1-87B). However, sister centromere separation in SPC105R and PP1-87B depleted Drosophila oocytes occurs by different mechanisms, the former is Separase dependent and the latter is Separase independent. Based on these results, a model is proposed for the establishment, protection and release of co-orientation. Sister centromere fusion necessary for co-orientation is established through cohesins that are protected by SPC105R. Subsequently, PP1-87B maintains co-orientation in a cohesin-independent manner by antagonizing stable kinetochore-microtubule (KT-MT) interactions. The implication is that the release of co-orientation during meiosis II is cohesin-independent and MT dependent. A surprising interaction was found between PP1-87B and C(3)G, the transverse element of the synaptonemal complex (SC), in regulating sister centromere separation. Overall, these results suggest a new mechanism where KT-MT interactions and centromeric SC regulate sister kinetochore co-orientation during female meiosis (Wang, 2019).

Female meiosis II and pronuclear fusion require the microtubule transport factor Bicaudal-D

Bicaudal-D (BicD) is a dynein adaptor that transports different cargoes along microtubules. Reducing the activity of BicD specifically in freshly laid Drosophila eggs by acute protein degradation revealed that BicD is needed to produce normal female meiosis II products, to prevent female meiotic products from re-entering the cell cycle, and for pronuclear fusion. As BicD is required to localize the spindle assembly checkpoint (SAC) components Mad2 and BubR1 to the female meiotic products, it appears that BicD functions to localize them to control metaphase arrest of polar bodies. BicD interacts with Clathrin heavy chain (Chc), and both proteins localize to centrosomes, mitotic spindles, and the tandem spindles during female meiosis II. Furthermore, BicD is required to correctly localize clathrin and the microtubule-stabilizing factors, D-TACC and Msps, to the meiosis II spindles, suggesting that failure to localize these proteins may perturb SAC function. Furthermore, right after the establishment of the female pronucleus, D-TACC and C. elegans BicD, tacc, and Chc are also needed for pronuclear fusion, pointing to the possibility that the underlying mechanism might be more widely used (Vazquez-Pianzola, 2022).

Encoded by a single gene, the Drosophila Bicaudal D (BicD) protein is part of a family of evolutionarily conserved dynein adaptors responsible for the transport of different cargoes along microtubules (MTs). The founding member of this protein family, Drosophila BicD, was identified because of its essential role during oogenesis and embryo development, in which it transports mRNAs that control polarity and cell fate. This process is mediated by its binding to the RNA-binding protein Egalitarian (Egl). Since its initial discovery, BicD and its orthologs have been shown to control a diverse group of MT transport processes through binding to different cargoes or adaptor proteins (Vazquez-Pianzola, 2022).

BicD can alternatively bind to Clathrin heavy chain (Chc) and this interaction facilitates Chc transport of recycling vesicles at the neuromuscular junctions and regulates endocytosis and the assembly of the pole plasm during oogenesis. The best-known function of Chc is in receptor-mediated endocytosis, in which it forms part of clathrin, a trimeric scaffold protein (called a triskelion), composed of three Chc and three Clathrin light chains (Clc). Aside from this, clathrin was shown to localize to mitotic spindles in mammalian and Xenopus cells and to have non-canonical activity by stabilizing the spindle MTs during mitosis. This function depends on clathrin trimerization and its interaction with Aurora A-phosphorylated Transforming Acidic Coiled-Coil protein 3 (TACC3) and the protein product of the colonic hepatic Tumor Overexpressed Gene (ch-TOG). This heterotrimer forms intermicrotubule bridges between kinetochore fibers (K-fibers), stabilizing these fibers and promoting chromosome congression. Recently, TACC3 and a mammalian homolog of Chc (CHC17) were shown to control the formation of a new liquid-like spindle domain (LISD) that promotes the assembly of acentrosomal mammalian oocyte spindles (Vazquez-Pianzola, 2022).

In order to transport its cargos along MTs, BicD interacts with the dynein/dynactin motor complex, a minus-end-directed MT motor. This complex is involved in different cellular processes, including intracellular trafficking of proteins and RNAs, organelle positioning and microtubule organization, some of which also require BicD. The dynein/dynactin complex also plays essential roles during cell division, in which it is required for centrosome separation, chromosome movements, spindle organization and positioning and mitotic checkpoint silencing (Vazquez-Pianzola, 2022).

Given that Drosophila BicD forms complexes with Chc and Dynein, both of which, as described above, perform essential activities during mitosis, this study set out to investigate possible BicD functions during cell division. Reducing BicD levels by specific protein-targeted degradation in freshly laid eggs revealed that BicD is essential for pronuclear fusion. In addition, it is required for metaphase arrest of female meiotic products after meiosis II completion. This activity appears to be mediated by the role of BicD in localizing the spindle assembly checkpoint (SAC) components. Furthermore, BicD interacts with its cargo protein, Chc, and they both localize to the mitotic spindles and centrosomes and the female tandem meiotic II spindles. In addition, BicD localizes D-TACC, clathrin, and Mini spindles (Msps; ch-TOG homolog) to the meiosis II spindles. The failure to localize these proteins accurately might also contribute to the SAC function defects observed in embryos with reduced BicD levels. D-TACC and Caenorhabditis elegans bicd-1, tac-1 and chc-1 are also needed after fertilization for pronuclear fusion, revealing an evolutionary conserved and essential role of these proteins in early zygote formation and suggesting that their mechanism of action on MTs might be widely used across species (Vazquez-Pianzola, 2022).

This study found that BicD localizes to the female tandem spindles and the central aster during MII. After fertilization, BicD also localizes to the mitotic spindles and the centrosomes. BicD^null mutants rarely survive and are sterile, but this study generated embryos with reduced levels of BicD at the beginning of embryogenesis (BicDhb-deGradFP embryos) by setting up a strategy based on the deGradFP technique. Consistent with BicD localization at the female MII spindles, it was discovered that BicDhb-deGradFP embryos arrest development, displaying aberrant meiotic products and no pronuclear fusion. Especially if combined with the CRIPSP-Cas9 strategy first to produce functional GFP-tagged proteins of interest, the construct designed in this study could be helpful for studying the role of female-sterile and lethal mutations during very early embryonic development (Vazquez-Pianzola, 2022).

In unfertilized BicDhb-deGradFP eggs, the female meiotic products were not arrested in metaphase as normally happens. Instead, they underwent additional rounds of replication. They failed to recruit or maintain the recruitment of the SAC pathway components BubR1 and Mad2, which are normally present at the kinetochores in the wild-type female meiotic polar bodies. Interestingly, in Drosophila eggs mutated for Rod, mps1 and BubR1, well-conserved orthologs of the SAC pathway, the polar bodies also cannot remain in a SAC-dependent metaphase-like state and decondense their chromatin. Furthermore, in these mutants, the polar bodies cycle in and out of M-phase, replicating their chromosomes similarly to those in BicDhb-deGradFP eggs. Thus, it appears that BicD functions to localize the SAC components to induce and/or maintain metaphase arrest of the polar bodies. Several mechanisms could explain the failure to maintain SAC activation observed in BicDhb-deGradFP embryos. BicD might be needed to recruit the SAC components to kinetochores directly. By contrast, during mitosis, the Rod-Zw10-Zwilch (RZZ) complex binds to the outer kinetochore region and recruits Mad2, Spindly and the dynactin complex. Spindly and dynactin act cooperatively to recruit dynein, which then transports the SAC components along the MTs away from kinetochores as a mechanism to trigger checkpoint silencing and anaphase onset. Given that the BicD N-terminal domain binds dynein and dynactin and promotes their interaction, it is also possible that BicD helps to move the SAC components away from the kinetochores. If this does not happen, the SAC remains persistently activated. It was also found that BicD activity in BicDhb-deGradFP embryos is insufficient to localize clathrin, TACC and Msps efficiently along the MTs of the spindle. During mitosis, impairment of MT motors, such as dynein, and treatments that prevent the TACC/clathrin complex from binding to the mitotic spindles and affecting K-fiber stability, also persistently activate the SAC. Thus, reduced levels of BicD in BicDhb-deGradFP embryos could additionally trigger SAC hyperactivation through its role in stabilizing the K-fibers. Although these data strongly suggest that the lack of BicD contributes to SAC defects through its role in localizing clathrin, D-TACC and Msps, further work is needed to elucidate whether BicD also acts more directly by binding to, and localizing, the SAC components, or indirectly by affecting the function of dynein (Vazquez-Pianzola, 2022).

Whereas persistent SAC activation leads to metaphase arrest and delayed meiosis (D-meiosis), this delay is known to be rarely permanent, at least during mitosis. Most cells that cannot satisfy the SAC ultimately escape delayed mitosis (D-mitosis) and enter G1 as tetraploid cells by a currently poorly understood mechanism. It is possible that, in BicDhb-deGradFP, the SAC pathway is constantly activated, delaying meiosis. However, at one point, the nuclei might escape metaphase II arrest, cycling in and out of M-phase, thereby replicating their chromosomes and decondensing their chromatin. The fact that female meiotic products over-replicate in BicDhb-deGradFP eggs and show no or only pericentromeric PH3 staining supports the notion that these nuclei are on an in-out metaphase arrest phase. That meiotic products in about half of the BicDhb-deGradFP embryos failed to stain for the SAC components BubR1 and Mad2 supports this hypothesis (Vazquez-Pianzola, 2022).

Chc, its partner Clc and BicD are enriched at mitotic spindles and centrosomes. Furthermore, these proteins and the clathrin-interacting partners D-TACC and Msps localize to the tandem spindles and the central aster of the female MII apparatus. The interaction of Drosophila Chc with D-TACC is conserved, and Chc interacts through the same protein domain directly with D-TACC and BicD. Moreover, BicD is needed for localizing D-TACC, Msps and clathrin throughout the MII tandem spindles. The TACC3/Chc interaction was proposed to form a domain in tandem to bind spindle MTs. It is hypothesize that BicD could help recruit Chc to the MTs by association with dynein. Given that Chc usually acts as a trimer with Clc (triskelion), each Chc might interact with either BicD or D-TACC. Thus, a mixed complex could be formed, and BicD might help to move, recruit or stabilize Chc and D-TACC along the spindles via the interaction of BicD/Chc in the same trimer. The fact that expression of D-TACC enhances the Chc/BicD interaction and that overexpression of Chc and D-TACC (tacc) arrested early development in a background in which BicD is reduced to a level that does not produce visible phenotypes on its own, supports this model. These results suggest that, with respect to BicD, the levels of D-TACC and Chc should be tightly balanced for these proteins to perform their normal function during early development, as has been shown previously for other BicD transport processes (Vazquez-Pianzola, 2022).

BicD has a role in pronuclear fusion that is conserved during evolution given that C. elegans eggs depleted for bicd-1 also failed to undergo pronuclear fusion. Moreover, Drosophila D-TACC and C. elegans chc-1 and tac-1 are also needed for pronuclear fusion. These genes might be required indirectly through their role in meiosis because preliminary data suggest that MII is also compromised in bicd-1 and chc-1 dsRNA-fed worms. Alternatively, they might play a more direct role in pronuclear migration, which depends on dynein and MTs in bovine, primate and C. elegans embryos. This would then suggest that the underlying mechanism may be used to build correctly or stabilize different types of MT. Determining their precise mechanistic involvement in pronuclear fusion is an interesting question for further studies (Vazquez-Pianzola, 2022).

GENE STRUCTURE

cDNA clone length - 4615 bp

Bases in 5' UTR - 56

Exons - 5

Bases in 3' UTR - 159

PROTEIN STRUCTURE

Amino Acids - 1460

Structural Domains

Two Drosophila EST sequences were identified through a BLAST search of the Berkeley Drosophila Genome Project (BDGP) database with mammalian Bub1. The cDNA sequence contains an open reading frame predicting a 165-kD protein closely related to Bub1. This protein shows 24.6% identity to human Bub1, 23.8% identity to mouse Bub1, and 14.5% identity to budding yeast Bub1p; it also displays 17.2% amino acid sequence identity to the human Bub1-related protein BubR1. The size of this Drosophila protein is somewhat larger than that of previously characterized human, mouse, and yeast members of the Bub1 family, whose predicted sizes range from 117-122 kD. The COOH-terminal third of the fly protein contains the strongly conserved kinase domain characteristic of Bub1. The NH₂-terminal third of the fly protein, in common with the other members of the Bub1 family, shares significant sequence similarity with the yeast checkpoint control component Mad3p. Most of the additional residues resulting in the relatively larger size of the fly protein are located in its middle third (Basu, 1999).

The gene described in this section of the Interactive Fly is more properly termed BudR1

Previous molecular and genetic analysis in Drosophila identified a gene (CG7838) that encodes a 165 kDa protein with significant homology to Bub1 proteins from other organisms and was reported as the Drosophila Bub1 homologue (Basu, 1999). However, completion of the Drosophila genome sequence revealed the existence of another gene (CG14030) that encodes a 125 kDa protein closely related to Bub1 proteins. Higher eukaryotes also have two genes that encode proteins closely related to Bub1, one of which has retained the name Bub1, while the other is called BubR1. The N-terminal domains of both Bub1 and BubR1 are highly homologous to yeast Mad3, but both Bub1 and BubR1 contain a C-terminal Ser/Thr kinase domain not found in Mad3. Neither Drosophila nor vertebrate genomes appear to encode a Mad3-like protein without a kinase domain. Since Bub1 and BubR1 appear to respond to different checkpoint signals, it became essential to determine which of the two Drosophila genes corresponds to Bub1 and BubR1. Phylogenetic analysis, including both Bub1-like Drosophila proteins, as well as Bub1 and BubR1 proteins from other species, did not provide a simple solution to this problem. The results show that the proteins encoded by the CG7838 and CG14030 genes are more closely related to each other and to the Anopheles homologues than to either Bub1 or BubR1 from other species, whereas the human, mouse and Xenopus proteins do fall into defined clusters. More recently, it has been suggested that a distinctive feature of BubR1 proteins is the presence of a conserved sequence, the KEN motif, in the N-terminal domain. The KEN motif is also present in yeast Mad3 and other Mad3 homologues, but not in Bub1. Sequence comparisons indicated that the previously described CG7838-encoded protein (165 kDa) contains a N-terminal KEN motif, while the new Bub1-like CG14030 protein (125 kDa) does not. This suggests that CG7838 encodes the Drosophila homologue of BubR1 while CG14030 encodes the bona fide Drosophila homologue of Bub1. However, if the sequence alignment is restricted to the C-terminal kinase domain, CG7838 is found to encode a protein, which is significantly more similar to Bub1 than to BubR1 proteins. In vertebrates, BubR1 is clearly distinct from Bub1 because the kinase domain of the former is significantly less conserved. Surprisingly, in Drosophila, the kinase domain of the KEN-box-containing protein (CG7838: 165 kDa) has almost as many conserved amino acids as the newly identified Bub1 protein (CG14030: 125 kDa) explaining why the protein encoded by the CG7838 gene was originally classified as Bub1. Even though from sequence analysis it seems difficult to determine unequivocally which Drosophila gene codes for Bub1 and BubR1, functional analysis of these proteins presented in this study clearly suggests that the newly identified CG14030 gene encodes the Drosophila homologue of bub1. Therefore, in contrast with previous usage (Basu, 1998; Basu, 1999), CG7838 is more accurately described as the Drosophila bubR1 gene, so the previously described mutant alleles l(2)K06109 and l(2)K03113 of CG7838 should be renamed bubR1¹ and bubR1². The use of this classification is suggested for future investigations and it will be adopted in this study (Logarinho, 2004).

date revised: 25 August 2023

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