Sep1, another Drosophila septin homolog

Sep1, a Drosophila melanogaster septin, has been identifed based on its homology to the yeast septins. The predicted Sep1 amino acid sequence is 35-42% identical to the known S. cerevisiae septins; 52% identical to Pnut, a second D. melanogaster septin; and 53-73% identical to the known mammalian septins. Sep1-specific antibodies have been used to characterize its expression and localization. The protein is concentrated at the leading edge of the cleavage furrows of dividing cells and cellularizing embryos, suggesting a role in furrow formation. Other aspects of Sep1 localization suggest roles not directly related to cytokinesis. For example, Sep1 exhibits orderly, cell-cycle-coordinated rearrangements within the cortex of syncytial blastoderm embryos and in the cells of post-gastrulation embryos; Sep1 is also concentrated at the leading edge of the epithelium during dorsal closure in the embryo, in the neurons of the embryonic nervous system, and at the baso-lateral surfaces of ovarian follicle cells. The distribution of Sep1 typically overlaps (but remains distinct from) that of actin. Both immunolocalization and biochemical experiments show that Sep1 is intimately associated with Pnut, suggesting that the Drosophila septins, like those in yeast, function as part of a complex (Fares, 1995).

Fungal septin homologs

To investigate the role of septins in filamentous fungi, presumptive septin homologs have been identified in Aspergillus nidulans. One of these septins, aspB, is expressed during vegetative growth and asexual sporulation. Based on cDNA and genomic sequences, the predicted aspB protein shares a P-loop motif and coiled coil regions with septins from other organisms. Antibodies generated against an aspB fusion protein recognize an A. nidulans protein of approximately 50 kDa, but cannot localize the septin product in cells by immunofluorescence. Hybridization to a chromosome-specific ordered cosmid library places the aspB gene on the right arm of chromosome I. Disruption of aspB showed that it is an essential gene (Momany, 1997).

The yeast Ste20 protein kinase is involved in pheromone response. Mammalian homologs of Ste20 exist, but their function remains unknown. A novel yeast STE20 homolog, CLA4, has been identified in a screen for mutations lethal in the absence of the G1 cyclins Cln1 and Cln2. Cla4 is involved in budding and cytokinesis and interacts with Cdc42, a GTPase required for polarized cell growth. Despite a cytokinesis defect, cla4 mutants are viable. However, double cla4 ste20 mutants cannot maintain septin rings at the bud neck and cannot undergo cytokinesis. Mutations in CDC12, which encodes one of the septins, were found in the same screen. Cla4 and Ste20 kinases apparently share a function in localizing cell growth with respect to the septin ring. This work implies that the septin ring is subject to cell cycle regulation (Cvrckova, 1995).

The SPR3 gene is selectively activated only during the sporulation phase of the Saccharomyces cerevisiae (Sc) life cycle. The predicted amino acid sequence has homology to microfilament proteins that are involved in cytokinesis and other proteins of unknown function. These include the products of Sc cell division cycle (CDC) genes involved in bud formation (Cdc3p, Cdc10p, Cdc11p and Cdc12p), Candida albicans proteins that accumulate in the hyphal phase (CaCdc3p and CaCdc10p), mouse brain-specific (H5p) and lymphocyte (Diff6p) proteins, Drosophila protein Peanut (which is localized to the cleavage furrow of dividing cells), a Diff6p homolog (Drosophila Diff6p), and the Sc septin protein (Sep1hp), a homolog of the 10-nm filament proteins of Sc. One strongly conserved region of SPR3 contains a potential ATP-GTP-binding domain. Primer extension analysis reveals six major transcription start points (tsp) beginning at -142 relative to the ATG start codon. The sequence immediately upstream from the tsp contains consensus binding sites for the HAP2/3/4 and ABFI transcription factors, a T-rich sequence and two putative novel elements for mid to late sporulation, termed SPR3 and PAL. Electrophoretic mobility shift assay (EMSA) and footprint analyses demonstrate that the ABFI protein binds to a region containing the putative ABFI site in vitro, and site-directed mutagenesis shows that the ABFI motif is essential for expression of SPR3 at the appropriate stage in sporulating cells (Ozsarac, 1995).

The Saccharomyces cerevisiae CDC3, CDC10, CDC11, and CDC12 genes encode a family of related proteins, the septins, which are involved in cell division and the organization of the cell surface during vegetative growth. A search for additional S. cerevisiae septin genes using the polymerase chain reaction identified SPR3, a gene that had been identified previously on the basis of its sporulation-specific expression. The predicted SPR3 product shows 25-40% identity in amino acid sequence to the previously known septins from S. cerevisiae and other organisms. Immunoblots confirmed the sporulation-specific expression of Spr3p and show that other septins are also present at substantial levels in sporulating cells. Consistent with the expression data, deletion of SPR3 in either of two genetic backgrounds has no detectable effect on exponentially growing cells. In one genetic background, deletion of SPR3 produces a threefold reduction in sporulation efficiency, although meiosis appears to be completed normally. In this background, deletion of CDC10 has no detectable effect on sporulation. In the other genetic background tested, the consequences of the two deletions are reversed. Immunofluorescence observations suggest that Spr3p, Cdc3p, and Cdc11p are localized to the leading edges of the membrane sacs that form near the spindle-pole bodies and gradually extend to engulf the nuclear lobes that contain the haploid chromosome sets, thus forming the spores. Deletion of SPR3 does not prevent the localization of Cdc3p and Cdc11p, but these proteins appear to be less well organized, and the intensity of their staining is reduced. Taken together, the results suggest that the septins play important but partially redundant roles during the process of spore formation (Fares, 1996).

Saccharomyces cerevisiae mating pheromones induce production of Afr1p, a protein that negatively regulates pheromone receptor signaling and is required for normal formation of the projection of cell growth that becomes the site of cell fusion during conjugation. Afr1p interacts with Cdc12p, which belongs to a family of filament-forming proteins (termed septins) that have been studied primarily for their role in bud morphogenesis and cytokinesis. The significance of the interaction between Afr1p and Cdc12p was tested in this study by examining the effects of AFR1 mutations that destroy the Cdc12p-binding domain. The results demonstrate that sequences in the C-terminal half of Afr1p are required for interaction with Cdc12p and for proper localization of Afr1p to the base of the mating projection. However, the Cdc12p-binding domain is not required for regulation of receptor signaling or for mating projection formation. This result is surprising because cells carrying a temperature-sensitive cdc12-6 mutation are defective in projection formation, indicating a role for Cdc12p in this process. Although the Cdc12p-binding domain is not essential for Afr1p function, this domain improves the ability of Afr1p to promote morphogenesis, suggesting that the proper localization of Afr1p is important for its function (Giot, 1997).

Five previously described Saccharomyces cerevisiae septins are associated with the neck filaments of vegetative cells and/or with the developing prospore wall of sporulating cells. SPR28, a sixth member of the S. cerevisiae septin gene family is a new member of the group of 'late genes' that are expressed at high levels during the meiotic divisions and ascospore formation. The septin it encodes, Spr28p, exhibits specific two-hybrid interactions with itself and with three other septins that are expressed in sporulating cells. Consistent with these results, an Spr28p-green fluorescent protein fusion is induced during meiosis I and appears to be associated with the developing prospore walls. Deletion of SPR28 in either a wild-type or an spr3 delta background produces no obvious abnormalities in vegetative cells and has little or no effect on sporulation, suggesting that the septins have redundant roles during spore formation (De Virgilio, 1996).

In budding yeast, a protein kinase called Gin4 is specifically activated during mitosis and functions in a pathway initiated by the Clb2 cyclin to control bud growth. Genetics and biochemistry have been used to identify additional proteins that function with Gin4 in this pathway, and both of these approaches have identified members of the septin family. Loss of septin function produces a phenotype that is very similar to the phenotype caused by loss of Gin4 function: the septins are required early in mitosis to activate Gin4 kinase activity. Furthermore, septin mutants display a prolonged mitotic delay at the short spindle stage, consistent with a role for the septins in the control of mitotic events. Members of the septin family bind directly to Gin4, demonstrating that the functions of Gin4 and the septins must be closely linked within the cell. These results demonstrate that the septins in budding yeast play an integral role in the mitosis-specific regulation of the Gin4 kinase and that they carry out functions early in mitosis (Carroll, 1998).

The mechanisms that couple cell cycle progression with the organization of the peripheral cytoskeleton are poorly understood. In Saccharomyces cerevisiae, the Swe1 protein phosphorylates and inactivates the cyclin-dependent kinase, Cdc28, thereby delaying the onset of mitosis. The nim1-related protein kinase, Hsl1, induces entry into mitosis by negatively regulating Swe1. Hsl1 physically associates with the septin cytoskeleton in vivo and Hsl1 kinase activity depends on proper septin function. Genetic analysis indicates that two additional Hsl1-related kinases, Kcc4 and Gin4, act redundantly with Hsl1 to regulate Swe1. Kcc4, like Hsl1 and Gin4, localizes to the bud neck in a septin-dependent fashion. Interestingly, hsl1;kcc4;gin4 triple mutants develop a cellular morphology extremely similar to that of septin mutants. Consistent with the idea that Hsl1, Kcc4, and Gin4 link entry into mitosis to proper septin organization, septin mutants incubated at the restrictive temperature trigger a Swe1-dependent mitotic delay that is necessary to maintain cell viability. These results reveal for the first time how cells monitor the organization of their cytoskeleton and demonstrate the existence of a cell cycle checkpoint that responds to defects in the peripheral cytoskeleton. Moreover, Hsl1, Kcc4, and Gin4 have homologs in higher eukaryotes, suggesting that the regulation of Swe1/Wee1 by this class of kinases is highly conserved (Barral, 1999).

SMT3 of Saccharomyces cerevisiae is an essential gene encoding a ubiquitin-like protein similar to mammalian SUMO-1. When a tagged Smt3 or human SUMO-1 was expressed from GAL1 promoter, either gene rescued the lethality of the smt3 disruptant. By indirect-immunofluorescent microscopy, the HA-tagged Smt3 was detected mostly in nuclei and also at the mother-bud neck just like septin fibers. Indeed immunoprecipitation experiments revealed that Cdc3, one of septin components, was modified with Smt3. Furthermore, the protein level of the Cdc3-Smt3 conjugate was reduced and the septin rings disappeared in a ubc9-1 mutant at a restrictive temperature, where the Smt3 conjugation system should be defective. Thus, it is concluded that Smt3 is conjugated to Cdc3 in septin rings localized at the mother-bud neck. Around the time of cytokinesis, the Cdc3-Smt3 conjugate disappeared. The biological significance of this Smt3 conjugation to a septin component is discussed (Takahashi, 1999).

SUMO1/Smt3, a ubiquitin-like protein modifier, is known to be conjugated to other proteins and modulate their functions in various important processes. Similar to the ubiquitin system, SUMO1/Smt3 is activated in an ATP-dependent reaction by thioester bond formation with E1 (activating enzyme), transferred to E2 (conjugating enzyme), and passed to a substrate lysine. It remained unknown, however, whether any SUMO1/Smt3 ligases (E3s) are involved in the final transfer of this modifier. This study reports a novel factor Siz1 (YDR409w) required for septin-sumoylation of budding yeast, possibly acting as E3. Siz1 is a member of a new family (Miz1, PIAS3, etc.) containing a conserved domain with a similarity to a zinc-binding RING-domain, often found in ubiquitin ligases. In the siz1 mutant, septin-sumoylation was completely abolished. A conserved cysteine residue in the domain was essential for this conjugation. Furthermore, Siz1 is localized at the mother-bud neck in the M-phase and physically binds to both E2 and the target proteins (Takahashi, 2001).

The septins are a family of GTPases involved in cytokinesis in budding yeast, Drosophila, and vertebrates. Septins are associated with a system of 10 nm filaments at the S. cerevisiae bud neck, and heteromultimeric septin complexes have been isolated from cell extracts in a filamentous state. A number of septins have been shown to bind and hydrolyze guanine nucleotide. However, the role of GTP binding and hydrolysis in filament formation has not been elucidated. Furthermore, several lines of evidence suggest that not all the subunits of the septin complex are required for all aspects of septin function. To address these questions, filament assembly has been reconstituted in vitro by using a recombinant Xenopus septin, Xl Sept2. Filament assembly is GTP dependent; moreover, the coiled-coil domain common to most septins is not essential for filament formation. Septin polymerization is preceded by a lag phase, suggesting a cooperative assembly mechanism. The slowly hydrolyzable GTP analog, GTP-gamma-S, also induces polymerization, indicating that polymerization does not require GTP hydrolysis. If the properties of Xl Sept2 filaments reflect those of native septin complexes, these results imply that the growth or stability of septin filaments, or both, is regulated by the state of bound nucleotide (Mendoza, 2002).

In Saccharomyces cerevisiae, the spindle position checkpoint ensures that cells do not exit mitosis until the mitotic spindle moves into the mother/bud neck and thus guarantees that each cell receives one nucleus. Mitotic exit is controlled by the small G protein Tem1p. Tem1p and its GTPase activating protein (GAP) Bub2p/Bfa1p are located on the daughter-bound spindle pole body. The GEF Lte1p is located in the bud. This segregation helps keep Tem1p in its inactive GDP state until the spindle enters the neck. However, the checkpoint functions without Lte1p and apparently senses cytoplasmic microtubules in the mother/bud neck. To investigate this mechanism, mutants defective for septins, which compose a ring at the neck, were examined. The septin mutants sep7Δ and cdc10Δ are defective in the checkpoint. When movement of the spindle into the neck is delayed, mitotic exit occurs, inappropriately leaving both nuclei in the mother. In sep7Δ and cdc10Δ mutants, Lte1p is mislocalized to the mother. In sep7Δ, but not cdc10Δ mutants, inappropriate mitotic exit depends on Lte1p. These results suggest that septins serve as a diffusion barrier for Lte1p, and that Cdc10p is needed for the septin ring to serve as a scaffold for a putative microtubule sensor (Castillon, 2003).

Phosphorylation-dependent regulation of septin dynamics during the cell cycle in yeast

Septins are GTPases involved in cytokinesis. In yeast, they form a ring at the cleavage site. Using Fluorescence Recovery After Photobleaching (FRAP), it has been shown that septins are mobile within the ring at bud emergence and telophase and are immobile during S, G2, and M phases. Immobilization of the septins is dependent on both Cla4, a PAK-like kinase, and Gin4, a septin-dependent kinase that can phosphorylate the septin Shs1/Sep7. Induction of septin ring dynamics in telophase is triggered by the translocation of Rts1 (a kinetochore-associated regulatory subunit of PP2A phosphatase) to the bud neck and correlates with Rts1-dependent dephosphorylation of Shs1. In rts1-Delta cells, the actomyosin ring contracts properly but cytokinesis fails. Together these results implicate septins in a late step of cytokinesis and indicate that proper regulation of septin dynamics, possibly through the control of their phosphorylation state, is required for the completion of cytokinesis (Dobbelaere, 2003).

The observation that the septin ring spends most of its time in the frozen state suggests that this is its functional conformation. This is in contrast to tubulin- and actin-dependent structures, which need to be dynamic to be functional. Together with the findings that septins are able to form filaments in vitro and are part of the 10 nm neck filaments observed in vivo, the results are consistent with septins forming stable filaments in vivo. Furthermore, FRAP results indicate that there is no substantial pool of free septin in the cytoplasm. Together, these data suggest that polymerization is a central aspect of septin function. In yeast the main role of the septin ring is to form a spatial landmark at the bud neck. There, it acts as a scaffold for the recruitment of other neck components and as a diffusion barrier to prevent mixing of bud- and mother-specific membrane and membrane-associated factors. It is proposed that its ability to freeze is crucial for the maintenance of its position over an extended period of time. Probably, freezing is also crucial for the barrier and scaffold functions of the septin ring, although further studies will be required to clarify this point (Dobbelaere, 2003).

Thus, activation of septin dynamics at the onset of cytokinesis depends on PP2A and Rts1. Translocation of Rts1 to the bud neck is required for normal activation of septin dynamics. PP2A catalytic subunits have been shown to relocalize to the bud neck in a septin and Rts1-dependent manner in late mitosis. Thus, these results are consistent with PP2ARts1 inducing septin dynamics by dephosphorylating some factor(s) at the bud neck. Consequently, phosphorylation of the same factor(s) may be required to establish the frozen state. Cla4 and Gin4 clearly meet the criteria for being involved in the control of septin dynamics. Disruption of either of them is colethal with cdc12-6, both mutations are epistatic over rts1-Δ, and prevent proper freezing of the septin ring. One of the substrates of Gin4 is the septin Shs1/Sep7. In summary, the results are consistent with the following working model. It is proposed that septin phosphorylation regulates their dynamics. It would presumably do so by stabilizing interactions between septin complexes. Supporting this idea, genetic data suggest that Shs1 acts specifically in septin ring stabilization and not in septin ring formation. Moreover, the septin ring fails to properly freeze in cells lacking Shs1. However, it is by far not as fluid as during bud emergence or cytokinesis. Therefore, Shs1 is a relevant but not the only substrate of the Cla4/Gin4 pathway. Other septins, such as Cdc3 and Cdc11, are also phosphorylated in vivo, although their modification is not well characterized. Also, several mammalian septins are phosphorylated in vivo. Therefore, phosphorylation-dependent modulation of septin dynamics may be a general phenomenon (Dobbelaere, 2003).

Possibly, Cla4 first establishes ring freezing upon bud emergence. Subsequent recruitment and activation of Gin4 by septins would reinforce this signal and help to maintain the frozen state when Cdc42, and hence Cla4, are being shut down at the apical to isotropic growth transition. In the absence of Cla4, Gin4 may be able to reduce septin dynamics, although not as efficiently as in the presence of Cla4. Thus, this model would have the advantage to explain why cla4 and gin4 mutants have partial defects in septin dynamics and why the double mutant cla4-Δ gin4-Δ has a much stronger phenotype than either of the single mutants. It would also explain why gin4-Δ affects the dynamics of the ring only in large-budded cells. Activation of septin dynamics at the onset of cytokinesis depends on the relocalization of PP2ARts1 to the bud neck. It is proposed that Rts1 acts twice at cytokinesis: first to induce a transient increase in dynamics at ring splitting and second during ring disassembly. The first event is difficult to characterize at the biochemical level, presumably because of its transient nature. However, both microscopy and genetic data strongly argue for Rts1 acting already at this stage on septin dynamics. During cytokinesis, the rings were found to freeze again. Interestingly, Cla4 is found to relocalize briefly to the bud neck after ring splitting. Hence, Cla4 may trigger refreezing of the ring during cytokinesis. Three observations argue for Rts1 acting in ring disassembly in early G1: (1) the septin ring is found to be again dynamic upon cell separation, indicating that septin dephosphorylation occur again at the end of cytokinesis; (2) rts1-Δ cells show defects in septin ring disassembly; (3) Shs1 dephosphorylation prior to bud emergence is at least in part dependent on Rts1 function. Thus, Rts1 may act to induce septin dynamics at both the onset and the completion of cytokinesis (Dobbelaere, 2003).

Finally, the translocation of Rts1 to the bud neck depends on the completion of mitosis and activation of the MEN pathway. Thus, Rts1 links septin dynamics to exit of mitosis. In this regard, it is interesting to notice that Rts1 localizes to kinetochores very much like chromosomal passenger proteins in higher eukaryotes. Thus, it may serve to integrate septin dynamics with proper spindle function. Alternatively, the function of Rts1 at the kinetochore may be independent of its function at the bud neck. In order to distinguish between these two possibilities, further studies will have to establish the mechanism controlling Rts1 function (Dobbelaere, 2003).

Interaction of the actin based cytoskeleton and other budding components with the septin ring in yeast

A Saccharomyces cerevisiae protein, Cyk1p, exhibits sequence similarity to the mammalian IQGAPs. Gene disruption of Cyk1p results in a failure of cytokinesis without affecting other events in the cell cycle. Cyk1p is diffused throughout most of the cell cycle but localizes to a ring structure at the mother-bud junction after the initiation of anaphase. This ring contains filamentous actin and Myo1p, a myosin II homolog. In vivo observation with green fluorescent protein-tagged Myo1p shows that the ring decreases drastically in size during cell division and therefore may be contractile. These results indicate that cytokinesis in budding yeast is likely to involve an actomyosin-based contractile ring. The assembly of this ring occurs in temporally distinct steps: Myo1p localizes to a ring that overlaps the septins at the G1-S transition slightly before bud emergence; Cyk1p and actin then accumulate in this ring after the activation of the Cdc15 pathway late in mitosis. The localization of myosin is abolished by a mutation in Cdc12p, implying a role for the septin filaments in the assembly of the actomyosin ring. The accumulation of actin in the cytokinetic ring is not observed in cells depleted of Cyk1p, suggesting that Cyk1p plays a role in the recruitment of actin filaments, perhaps through a filament-binding activity similar to that demonstrated for mammalian IQGAPs (Lippincott, 1998).

Cells select bud sites according to one of two predetermined patterns. Mating type MATa and MAT alpha cells bud in an axial pattern, and MATa/alpha cells bud in a bipolar pattern. These budding patterns are thought to depend on the placement of spatial cues at specific sites in the cell cortex. Because cytoskeletal elements play a role in organizing the cytoplasm and establishing distinct plasma membrane domains, they are well suited for positioning bud-site selection cues. Indeed, the septin-containing neck filaments are crucial for establishing the axial budding pattern characteristic of MATa and MAT alpha cells. The budding patterns of cells carrying mutations in the actin gene or in genes encoding actin-associated proteins was studied: MATa/alpha cells are defective in the bipolar budding pattern, but MATa and MAT alpha cells still exhibit a normal axial budding pattern. MATa/alpha actin cytoskeleton mutant daughter cells correctly position their first bud at the distal pole of the cell, but mother cells position their buds randomly. The actin cytoskeleton therefore functions in the generation of the bipolar budding pattern and is required specifically for proper selection of bud sites in mother MATa/alpha cells. These observations and the results of double mutant studies support the conclusion that different rules govern bud-site selection in mother and daughter MATa/alpha cells. A defective bipolar budding pattern does not preclude an sla2-6 mutant from undergoing pseudohyphal growth, highlighting the central role of daughter cell bud-site selection cues in the formation of pseudohyphae. By examining the budding patterns of mad2-1 mitotic checkpoint mutants treated with benomyl to depolymerize their microtubules, previous evidence indicating that microtubules do not function in axial or bipolar bud-site selection has been confirmed and extended (Yang, 1997).

A search for Saccharomyces cerevisiae proteins that interact with actin in the two-hybrid system and a screen for mutants that affect the bipolar budding pattern both identified the same gene, AIP3/BUD6. This gene is not essential for mitotic growth but is necessary for normal morphogenesis. MATa/alpha daughter cells lacking Aip3p place their first buds normally, at their distal poles, but then choose random sites for budding in subsequent cell cycles. This suggests that actin and associated proteins are involved in placing the bipolar positional marker at the division site but not at the distal tip of the daughter cell. Although aip3 mutant cells are not obviously defective in the initial polarization of the cytoskeleton at the time of bud emergence, they appear to lose cytoskeletal polarity as the bud enlarges, resulting in the formation of cells that are larger and rounder than normal. aip3 mutant cells also show inefficient nuclear migration and nuclear division, defects in the organization of the secretory system, and abnormal septation, all defects that presumably reflect the involvement of Aip3p in the organization and/or function of the actin cytoskeleton. The sequence of Aip3p is novel but contains a predicted coiled-coil domain near its C terminus that may mediate the observed homo-oligomerization of the protein. Aip3p shows a distinctive localization pattern that correlates well with its likely sites of action: it appears at the presumptive bud site prior to bud emergence, remains near the tips of small buds, and forms a ring (or pair of rings) in the mother-bud neck that is detectable early in the cell cycle but becomes more prominent prior to cytokinesis. Surprisingly, the localization of Aip3p does not appear to require either polarized actin or the septin proteins of the neck filaments (Amberg, 1997).

The budding yeast Saccharomyces cerevisiae can bud in two spatially programmed patterns: axial or bipolar. In the axial budding pattern, cells polarize and divide adjacent to the previous site of cell separation, in response to a cell-division remnant, which includes Bud3p, Bud4p and septin proteins. This paper investigates the role of an additional component of the cell-division remnant, Bud10p, in axial budding. The sequence of Bud10p predicts a protein that contains a single trans-membrane domain but lacks similarity to known proteins. Subcellular fractionations confirm that Bud10p is associated with membranes. Bud10p accumulates as a patch at the bud site prior to bud formation, and then persists at the mother-bud neck as the bud grows. Toward the end of the cell cycle, the localization of Bud10p refines to a tight double ring which splits at cytokinesis into two single rings, one in each progeny cell. Each single ring remains until a new concentration of Bud10p forms at the developing axial bud site, immediately adjacent to the old ring. Certain aspects of Bud10p localization are dependent on BUD3, suggesting a close functional interaction between Bud10p and Bud3p. Bud10p is the first example of a transmembrane protein that controls cell polarization during budding. Because Bud10p contains a large extracellular domain, it is possible that Bud10p functions in a manner analogous to an extracellular matrix receptor. Clusters of Bud10p at the mother-bud neck formed in response to Bud3p (and possibly to an extracellular cue, such as a component of the cell wall), might facilitate the docking of downstream components that direct polarization of the cytoskeleton (Halme, 1996).

C. elegans septins

Caenorhabditis elegans has two genes, unc-59 and unc-61, encoding septin-family GTPases. Mutations in the septin genes cause defects in locomotory behavior that have been previously attributed to cytokinesis failures in postembryonic neuroblasts. Mutations in either septin gene frequently cause uncoordination in newly hatched larvae in the absence of cytokinesis failures. The septins exhibit developmentally regulated expression, including expression in various neurons at times when processes are extending and synapses are forming. Motor neurons in the mutant larvae display defects in multiple aspects of axonal migration and guidance that are likely to be responsible for the locomotory behavior defects. The septins are also expressed in migrating distal tip cells, which are leaders for gonad arm extension. Septin mutants affect morphology of the distal tip cells, as well as their migration and guidance during gonadogenesis. These results suggest that septins may be generally required for developmental migrations and pathfinding (Finger, 2003).

Mammalian septins

A Drosophila-related expressed sequence tag (DRES) with sequence similarity to the peanut gene has previously been localized to human chromosome 22q11. The cDNA corresponding to this DRES has been isolated. It is a novel member of the family of septin genes, which encodes proteins with GTPase activity thought to interact during cytokinesis. The predicted protein has P-loop nucleotide binding and GTPase motifs. The gene, which is called PNUTL1, maps to the region of 22q11.2 frequently deleted in DiGeorge and velo-cardio-facial syndromes and is particularly highly expressed in the brain. The mouse homolog, Pnutl1, maps to MMU16, adding to the growing number of genes from the DiGeorge syndrome region that map to this chromosome (McKie, 1997).

The mouse Nedd5 gene encodes a 41.5-kD GTPase similar to the Saccharomyces and Drosophila septins essential for cytokinesis. Nedd5 accumulates near the contractile ring from anaphase through telophase, and finally condenses into the midbody. Microinjection of anti-Nedd5 antibody interferes with cytokinesis, giving rise to binucleated cells. In interphase and postmitotic cells, Nedd5 localizes to fibrous or granular structures depending on the growth state of the cell. The Nedd5-containing fibers are disrupted by microinjection of GTPgammaS and by Nedd5 mutants lacking GTP-binding activity, implying that GTP hydrolysis is required for its assembly. The Nedd5-containing fibers also appear to physically contact actin bundles and focal adhesion complexes and are disrupted by cytochalasin D, C3 exoenzyme, and serum starvation, suggesting a functional interaction with the actin-based cytoskeletal systems in interphase cells (Kinoshita, 1997).

Both the sec6/8 complex and septin filaments have been implicated in directing vesicles and proteins to sites of active membrane addition in yeast. The rat brain sec6/8 complex coimmunoprecipitates with a filament composed of four mammalian septins, suggesting an interaction between these complexes. One of the septins, CDC10, displays broad subcellular and tissue distributions and is found in postmitotic neurons as well as dividing cells. Electron microscopic studies have shown that the purified rat brain septins form filaments of 8.25 nm in diameter; the lengths of the filaments are multiples of 25 nm. Glutaraldehyde-fixed rat brain sec6/8 complex adopts a conformation resembling the letter 'T' or 'Y'. The sec6/8 and septin complexes likely play an important role in trafficking vesicles and organizing proteins at the plasma membrane of neurons (Hsu, 1998).

Septins are GTPases required for the completion of cytokinesis in diverse organisms, yet their roles in cytokinesis or other cellular processes remain unknown. A newly identified septin, CDCrel-1, is predominantly expressed in the nervous system. This protein is associated with membrane fractions, and a significant fraction of the protein copurifies and coprecipitates with synaptic vesicles. In detergent extracts, CDCrel-1 and another septin, Nedd5, immunoprecipitates with the SNARE protein syntaxin by directly binding to syntaxin via the SNARE interaction domain. Transfection of HIT-T15 cells with wild-type CDCrel-1 inhibits secretion, whereas GTPase dominant-negative mutants enhance secretion. These data suggest that septins may regulate vesicle dynamics through interactions with syntaxin (Beites, 1999).

Mammalian septins are GTP-binding proteins the functions of which are not well understood. Knockdown of SEPT2, 6, and 7 causes stress fibers to disintegrate and cells to lose polarity. This phenotype is induced by nuclear accumulation of the adaptor protein NCK; the effects can be reversed or induced by cytoplasmic or nuclear NCK, respectively. NCK is carried into the nucleus by SOCS7 (suppressor of cytokine signaling 7), which possesses nuclear import/export signals. SOCS7 interacts with septins and NCK through distinct domains. DNA damage induces actin and septin rearrangement and rapid nuclear accumulation of NCK and SOCS7. Moreover, NCK expression is essential for cell-cycle arrest. The septin-SOCS7-NCK axis intersects with the canonical DNA damage cascade downstream of ATM/ATR and is essential for p53 Ser15 phosphorylation. These data illuminate an unanticipated connection between septins, SOCS7, NCK signaling, and the DNA damage response (Kremer, 2007).

Septin Function in Dendrites

Septins, a highly conserved family of GTP-binding proteins, were originally identified in a genetic screen for S. cerevisiae mutants defective in cytokinesis. In yeast, septins maintain the compartmentalization of the yeast plasma membrane during cell division by forming rings at the cortex of the bud neck, and these rings establish a lateral diffusion barrier. In contrast, very little is known about the functions of septins in mammalian cells including postmitotic neurons. This study shows that Septin 7 (Sept7) localizes at the bases of filopodia and at branch points in developing hippocampal neurons. Upon downregulation of Sept7, dendritic branching is impaired. In mature neurons, Sept7 is found at the bases of dendritic spines where it associates with the plasma membrane. Mature Sept7-deficient neurons display elongated spines. Furthermore, Sept5 and Sept11 colocalize with and coimmunoprecipitate with Sept7, thereby arguing for the existence of a Septin5/7/11 complex. Taken together, these findings show an important role for Sept7 in regulating dendritic branching and dendritic-spine morphology. These observations concur with data from yeast, in which downregulation of septins yields elongated buds, suggesting a conserved function for septins from yeast to mammals (Xie, 2007).

Septins are GTP-binding proteins that polymerize into heteromeric filaments and form microscopic bundles or ring structures in vitro and in vivo. Because of these properties and their ability to associate with membrane, F-actin, and microtubules, septins have been generally regarded as cytoskeletal components. Septins are known to play roles in cytokinesis, in membrane trafficking, and as structural scaffolds; however, their function in neurons is poorly understood. Many members of the septin family, including Septin 7 (Sept7), were found by mass-spectrometry analysis of postsynaptic density (PSD) fractions of the brain, suggesting a possible postsynaptic function of septins in neurons. This study reports that rat Sept7 is localized at the base of dendritic protrusions and at dendritic branch points in cultured hippocampal neurons - a distribution reminiscent of septin localization in the bud neck of budding yeast. Overexpression of Sept7 increased dendrite branching and the density of dendritic protrusions, whereas RNA interference (RNAi)-mediated knockdown of Sept7 led to reduced dendrite arborization and a greater proportion of immature protrusions. These data suggest that Sept7 is critical for spine morphogenesis and dendrite development during neuronal maturation (Tada, 2007).

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