Septin filament formation is essential in budding yeast

Septins are GTP-binding proteins that form ordered, rod-like multimeric complexes and polymerize into filaments, but how such supramolecular structure is related to septin function was unclear. In Saccharomyces cerevisiae, four septins form an apolar hetero-octamer (Cdc11-Cdc12-Cdc3-Cdc10-Cdc10-Cdc3-Cdc12-Cdc11) that associates end-to-end to form filaments. We show that septin filament assembly displays previously unanticipated plasticity. Cells lacking Cdc10 or Cdc11 are able to divide because the now-exposed subunits (Cdc3 or Cdc12, respectively) retain an ability to homodimerize via their so-called G interface, thereby allowing for filament assembly. In such cdc10Δ and cdc11Δ cells, the remaining septins, like wild-type complexes, localize to the cortex at the bud neck and compartmentalize nonseptin factors, consistent with a diffusion barrier composed of continuous filaments in intimate contact with the plasma membrane. Conversely, Cdc10 or Cdc11 mutants that cannot self-associate, but "cap" Cdc3 or Cdc12, respectively, prevent filament formation, block cortical localization, and kill cells.     

Protein-protein interactions governing septin heteropentamer assembly and septin filament organization in Saccharomyces cerevisiae

Mitotic yeast (Saccharomyces cerevisiae) cells express five related septins (Cdc3, Cdc10, Cdc11, Cdc12, and Shs1) that form a cortical filamentous collar at the mother-bud neck necessary for normal morphogenesis and cytokinesis. All five possess an N-terminal GTPase domain and, except for Cdc10, a C-terminal extension (CTE) containing a predicted coiled coil. Here, we show that the CTEs of Cdc3 and Cdc12 are essential for their association and for the function of both septins in vivo. Cdc10 interacts with a Cdc3-Cdc12 complex independently of the CTE of either protein. In contrast to Cdc3 and Cdc12, the Cdc11 CTE, which recruits the nonessential septin Shs1, is dispensable for its function in vivo. In addition, Cdc11 forms a stoichiometric complex with Cdc12, independent of its CTE. Reconstitution of various multiseptin complexes and electron microscopic analysis reveal that Cdc3, Cdc11, and Cdc12 are all necessary and sufficient for septin filament formation, and presence of Cdc10 causes filament pairing. These data provide novel insights about the connectivity among the five individual septins in functional septin heteropentamers and the organization of septin filaments.     

Polymerization of Purified Yeast Septins: Evidence That Organized Filament Arrays May Not Be Required for Septin Function

The septins are a family of proteins required for cytokinesis in a number of eukaryotic cell types. In budding yeast, these proteins are thought to be the structural components of a filament system present at the mother–bud neck, called the neck filaments. In this study, we report the isolation of a protein complex containing the yeast septins Cdc3p, Cdc10p, Cdc11p, and Cdc12p that is capable of forming long filaments in vitro. To investigate the relationship between these filaments and the neck filaments, we purified septin complexes from cells deleted for CDC10 or CDC11. These complexes were not capable of the polymerization exhibited by wild-type preparations, and analysis of the neck region by electron microscopy revealed that the cdc10Δ and cdc11Δ cells did not contain detectable neck filaments. These results strengthen the hypothesis that the septins are the major structural components of the neck filaments. Surprisingly, we found that septin dependent processes like cytokinesis and the localization of Bud4p to the neck still occurred in cdc10Δ cells. This suggests that the septins may be able to function in the absence of normal polymerization and the formation of a higher order filament structure.     

Characterization of the CDC10 product and the timing of events of the budding site of Saccharomyces cerevisiae

Budding cells of the yeast Saccharomyces cerevisiae possess a ring of septin filaments of unknown biochemical nature that lies under the inner surface of the plasma membrane in the neck that connects the mother cell to its bud. Mutants, defective in any of the four genes (CDC3, CDC10, CDC11, CDC12), lack these septin filaments and display a pleiotropic phenotype that involves abnormal bud growth and an inability to complete cytokinesis. The cloned CDC10 was fused to bacterial genes to generate antibodies specific for the CDC10 product was a constituent of the septin filaments. Cdc10p-specific antibodies for septin staining and actin-specific rhodamine-phalloidine were used to investigate the timing of the localization of septin and actin at the budding site using the immunofluorescence microscopic technique. In wild-type cells, the timing of the appearance and disappearance of these proteins was indistinguishable. In addition, the cdc10 mutant did not prevent actin localization at the budding site. The mutant that was blocked in the actin function also did not prevent the septin localization of the Cdc10p. This result may suggest an organizational independence between these proteins in the bud formation. Finally, the localization of septin and actin in the cdc24 mutant cell was examined. It was found that the CDC24 function was necessary for the organization of septin and actin at the budding site.     

Septin collar formation in budding yeast requires GTP binding and direct phosphorylation by the PAK, Cla4

Assembly at the mother-bud neck of a filamentous collar containing five septins (Cdc3, Cdc10, Cdc11, Cdc12, and Shs1) is necessary for proper morphogenesis and cytokinesis. We show that Cdc10 and Cdc12 possess GTPase activity and appropriate mutations in conserved nucleotide-binding residues abrogate GTP binding and/or hydrolysis in vitro. In vivo, mutants unable to bind GTP prevent septin collar formation, whereas mutants that block GTP hydrolysis do not. GTP binding-defective Cdc10 and Cdc12 form soluble heteromeric complexes with other septins both in yeast and in bacteria; yet, unlike wild-type, mutant complexes do not bind GTP and do not assemble into filaments in vitro. Absence of a p21-activated protein kinase (Cla4) perturbs septin collar formation. This defect is greatly exacerbated when combined with GTP binding-defective septins; conversely, the septin collar assembly defect of such mutants is suppressed efficiently by CLA4 overexpression. Cla4 interacts directly with and phosphorylates certain septins in vitro and in vivo. Thus, septin collar formation may correspond to septin filament assembly, and requires both GTP binding and Cla4-mediated phosphorylation of septins.     

Assembly,molecular organization,and membrane-binding properties of development-specific septins

Septin complexes display remarkable plasticity in subunit composition, yet how a new subunit assembled into higher-order structures confers different functions is not fully understood. Here, this question is addressed in budding yeast, where during meiosis Spr3 and Spr28 replace the mitotic septin subunits Cdc12 and Cdc11 (and Shs1), respectively. In vitro, the sole stable complex that contains both meiosis-specific septins is a linear Spr28–Spr3–Cdc3–Cdc10–Cdc10–Cdc3–Spr3–Spr28 hetero-octamer. Only coexpressed Spr3 and Spr28 colocalize with Cdc3 and Cdc10 in mitotic cells, indicating that incorporation requires a Spr28-Spr3 protomer. Unlike their mitotic counterparts, Spr28-Spr3–capped rods are unable to form higher-order structures in solution but assemble to form long paired filaments on lipid monolayers containing phosphatidylinositol-4,5-bisphosphate, mimicking presence of this phosphoinositide in the prospore membrane. Spr28 and Spr3 fail to rescue the lethality of a cdc11Δ cdc12Δ mutant, and Cdc11 and Cdc12 fail to restore sporulation proficiency to spr3Δ/spr3Δ spr28Δ/spr28Δ diploids. Thus, specific meiotic and mitotic subunits endow septin complexes with functionally distinct properties.     

Septin stability and recycling during dynamic structural transitions in cell division and development

Septins are conserved proteins found in hetero-oligomeric complexes that are incorporated into distinct structures during cell division and differentiation; yeast septins Cdc3, Cdc10, Cdc11, and Cdc12 form hetero-octamers and polymerize into filaments, which form a "collar" at the mother-bud neck [1]. Posttranslational modifications, nucleotide binding, and protein-protein and protein-lipid interactions influence assembly and disassembly of septin structures [2], but whether individual septins are used repeatedly to build higher-order assemblies was not known. We used fluorescence-based pulse-chase methods to visualize the fate of pre-existing (old) and newly synthesized (new) molecules of two septins, Cdc10 and Cdc12. They were recycled through multiple mitotic divisions, and old and new molecules were incorporated indistinguishably into the collar. Likewise, old and new subunits intermixed within hetero-octamers, indicating that exchange occurs at this organizational level. Remarkably, in meiosis, Cdc10 made during vegetative growth was reutilized to build sporulation-specific structures and reused again during spore germination for budding and during subsequent mitotic divisions. Although Cdc12 also persisted during sporulation, it was excluded from septin structures and replaced by another subunit, Spr3; only new Cdc12 populated the collar of germinating spores. Thus, mechanisms governing septin incorporation are specific to each subunit and to the developmental state of the cell.     

Nucleotide binding and filament assembly of recombinant yeast septin complexes

Septins are filament-forming GTPases involved in cytokinesis and cortical organization. In the yeast Saccharomyces cerevisiae, the septins encoded by CDC3, CDC10, CDC11, and CDC12 form a high-molecular-weight complex, localized at the cytoplasmic face of the plasma membrane in the mother-bud neck. While septin function at the cellular level is fairly well understood, progress on structure-function analysis of these proteins has been slow and limited by the lack of large amounts of pure complex. While monomeric septins form apparently non-native aggregates, stable recombinant complexes of two, three, or four yeast septins can be produced by co-expression from bi-cistronic vectors in E. coli. The septin polypeptides show various degrees of saturation with guanine nucleotides in different complexes. The binary core Cdc3p-Cdc12p complex contains no bound nucleotide. While ternary complexes are partially saturated and can bind extraneously added nucleotide with micromolar affinity, only the complete four-component septin complex is fully coordinated with tightly bound GDP/GTP after chromatographic purification. We show here that the nucleotide-binding sites of the septins show drastic changes on formation of higher oligomers. Although the binary core Cdc3p-Cdc12p complex does not form filaments, the ternary and quaternary complexes form bundles of paired filaments. In the case of ternary complexes, filament formation is stimulated by guanine nucleotide, but is not dependent on the presence or absence of the gamma-phosphate.     

A novel septin-associated protein, Syp1p, is required for normal cell cycle-dependent septin cytoskeleton dynamics in yeast

Septins are a family of GTP-binding proteins whose heterooligomeric complex is the basic structural element of the septin filaments found in many eukaryotic organisms. In budding yeast, septins are mainly confined at the mother–daughter junction and are required for cell morphogenesis and division. Septins undergo assembly and disassembly in accordance with the progression of the cell cycle. In this report, we identified the yeast protein Syp1p as a new regulator of septin dynamics. Syp1p colocalizes with septins throughout most of the cell cycle. Syp1p interacts with the septin subunit Cdc10p and can be precipitated by Cdc10p and Cdc12p. In the syp1Δ mutant, both formation of a complete septin ring at the incipient bud site and disassembly of the septin ring in later stages of cell division are significantly delayed. In addition, overexpression of Syp1p causes marked acceleration of septin disassembly. The fluorescence recovery after photobleaching (FRAP) assay further showed that Syp1p promotes septin turnover in different cell cycle stages. These results suggest that Syp1p is involved in the regulation of cell cycle-dependent dynamics of the septin cytoskeleton in yeast.     

Some assembly required: yeast septins provide the instruction manual

Septins are a family of conserved proteins that form hetero-oligomeric complexes that assemble into filaments. The filaments can be organized into linear arrays, coils, rings and gauzes. They serve as membrane-associated scaffolds and as barriers to demarcate local compartments, especially for the establishment of the septation site for cytokinesis. Studies in budding and fission yeast have revealed many of the protein-protein interactions that govern the formation of multi-septin complexes. GTP binding and phosphorylation direct the polymerization of filaments that is required for septin-collar assembly in budding yeast, whereas a homolog of anillin instructs timely formation of the ring of septin filaments at the medial cortex in fission yeast. These insights should aid understanding of the organization and function of the diverse septin structures in animal cells.     

Essential roles for the nuclear receptor coactivator Ncoa3 in pluripotency

Septins are guanine nucleotide-binding proteins that form hetero-oligomeric complexes, which assemble into filaments and higher-order structures at sites of cell division and morphogenesis in eukaryotes. Dynamic changes in the organization of septin-containing structures occur concomitantly with progression through the mitotic cell cycle and during cell differentiation. Septins also undergo stage-specific post-translational modifications, which have been implicated in regulating their dynamics, in some cases via purported effects on septin turnover. In our recent study, the fate of two of the five septins expressed in mitotic cells of budding yeast (Saccharomyces cerevisiae) was tracked using two complementary fluorescence-based methods for pulse-chase analysis. During mitotic growth, previously-made molecules of both septins (Cdc10 and Cdc12) persisted through multiple successive divisions and were incorporated equivalently with newly synthesized molecules into hetero-oligomers and higher-order structures. Similarly, in cells undergoing meiosis and the developmental program of sporulation, pre-existing copies of Cdc10 were incorporated into new structures. In marked contrast, Cdc12 was irreversibly excluded from septin complexes and replaced by another septin, Spr3. Here, we discuss the broader implications of these results and related findings with regard to how septin dynamics is coordinated with the mitotic cell cycle and in the yeast life cycle, and how these observations may relate to control of the dynamics of other complex multi-subunit assemblies.     

Phosphorylation-dependent septin interaction of Bni5 is important for cytokinesis

In budding yeast, septin plays as a scaffold to recruits protein components and regulates crucial cellular events including bud site selection, bud morphogenesis, Cdc28 activation pathway, and cytokinesis. Phosphorylation of Bni5 isolated as a suppressor for septin defect is essential to Swe1-dependent regulation of bud morphogenesis and mitotic entry. The mechanism by which Bni5 regulates normal septin function is not completely understood. Here, we provide evidence that Bni5 phosphorylation is important for interaction with septin component Cdc11 and for timely delocalization from septin filament at late mitosis. Phosphorylation-deficient bni5-4A was synthetically lethal with hof1Delta. bni5-4A cells had defective structure of septin ring and connected cell morphology, indicative of defects in cytokinesis. Two-hybrid analysis revealed that bni5-4A has a defect in direct interaction with Cdc11 and Cdc12. GFP-tagged bni5-4A was normally localized at mother-bud neck of budded cells before middle of mitosis. In contrast, at large-budded telophase cells, bni5-4A-GFP was defective in localization and disappeared from the neck approximately 2 min earlier than that of wild type, as evidenced by time-lapse analysis. Therefore, earlier delocalization of bni5-4A from septin filament is consistent with phosphorylation-dependent interaction with the septin component. These results suggest that timely delocalization of Bni5 by phosphorylation is important for septin function and regulation of cytokinesis.     

Casein kinase 1 delta functions at the centrosome to mediate Wnt-3a-dependent neurite outgrowth

Septins are conserved guanosine triphosphate-binding cytoskeletal proteins involved in membrane remodeling. In budding yeast, five mitotic septins (Cdc3, Cdc10, Cdc11, Cdc12, and Shs1), which are essential for cytokinesis, transition during bud growth from a patch to a collar, which splits into two rings in cytokinesis and is disassembled before the next cell cycle. Cdc3, Cdc10, Cdc11, and Cdc12 form an apolar octameric rod with Cdc11 at each tip, which polymerizes into straight paired filaments. We show that Shs1 substitutes for Cdc11, resulting in octameric rods that do not polymerize into filaments but associate laterally, forming curved bundles that close into rings. In vivo, half of shs1Δ mutant cells exhibit incomplete collars and disrupted neck filaments. Importantly, different phosphomimetic mutations in Shs1 can either prevent ring formation or promote formation of a gauzelike meshwork. These results show that a single alternative terminal subunit is sufficient to confer a distinctive higher-order septin ultrastructure that can be further regulated by phosphorylation.     

Role of nucleotide binding in septin-septin interactions and septin localization in Saccharomyces cerevisiae

Septins are a conserved family of eukaryotic GTP-binding, filament-forming proteins. In Saccharomyces cerevisiae, five septins (Cdc3p, Cdc10p, Cdc11p, Cdc12p, and Shs1p) form a complex and colocalize to the incipient bud site and as a collar of filaments at the neck of budded cells. Septins serve as a scaffold to localize septin-associated proteins involved in diverse processes and as a barrier to diffusion of membrane-associated proteins. Little is known about the role of nucleotide binding in septin function. Here, we show that Cdc3p, Cdc10p, Cdc11p, and Cdc12p all bind GTP and that P-loop and G4 motif mutations affect nucleotide binding and result in temperature-sensitive defects in septin localization and function. Two-hybrid, in vitro, and in vivo analyses show that for all four septins nucleotide binding is important in septin-septin interactions and complex formation. In the absence of complete complexes, septins do not localize to the cortex, suggesting septin localization factors interact only with complete complexes. When both complete and partial complexes are present, septins localize to the cortex but do not form a collar, perhaps because of an inability to form filaments. We find no evidence that nucleotide binding is specifically involved in the interaction of septins with septin-associated proteins.     

A Septin from the Filamentous Fungus A. nidulans Induces Atypical Pseudohyphae in the Budding Yeast S. cerevisiae

Background

Septins, novel cytoskeletal proteins, form rings at the bases of emerging round buds in yeasts and at the bases of emerging elongated hyphal initials in filamentous fungi.

Methodology/Principal Findings

When introduced into the yeast Saccharomyces cerevisiae, the septin AspC from the filamentous fungus Aspergillus nidulans induced highly elongated atypical pseudohyphae and spore-producing structures similar to those of hyphal fungi. AspC induced atypical pseudohyphae when S. cerevisiae pseudohyphal or haploid invasive genes were deleted, but not when the CDC10 septin gene was deleted. AspC also induced atypical pseudohyphae when S. cerevisiae genes encoding Cdc12-interacting proteins Bem4, Cla4, Gic1 and Gic2 were deleted, but not when BNI1, a Cdc12-interacting formin gene, was deleted. AspC localized to bud and pseudohypha necks, while its S. cerevisiae ortholog, Cdc12, localized only to bud necks.

Conclusions/Significance

Our results suggest that AspC competes with Cdc12 for incorporation into the yeast septin scaffold and once there alters cell shape by altering interactions with the formin Bni1. That introduction of the A. nidulans septin AspC into S. cerevisiae induces a shift from formation of buds to formation of atypical pseudohyphae suggests that septins play an important role in the morphological plasticity of fungi.     

Interplay of septin amphipathic helices in sensing membrane-curvature and filament bundling

The curvature of the membrane defines cell shape. Septins are GTP-binding proteins that assemble into heteromeric complexes and polymerize into filaments at areas of micron-scale membrane curvature. An amphipathic helix (AH) domain within the septin complex is necessary and sufficient for septins to preferentially assemble onto micron-scale curvature. Here we report that the nonessential fungal septin, Shs1, also has an AH domain capable of recognizing membrane curvature. In a septin mutant strain lacking a fully functional Cdc12 AH domain (cdc12-6), the C-terminal extension of Shs1, containing an AH domain, becomes essential. Additionally, we find that the Cdc12 AH domain is important for regulating septin filament bundling, suggesting septin AH domains have multiple, distinct functions and that bundling and membrane binding may be coordinately controlled.     

Bni5p,a septin-interacting protein,is required for normal septin function and cytokinesis in Saccharomyces cerevisiae

In the budding yeast Saccharomyces cerevisiae, the Cdc3p, Cdc10p, Cdc11p, Cdc12p, and Sep7p/Shs1p septins assemble early in the cell cycle in a ring that marks the future cytokinetic site. The septins appear to be major structural components of a set of filaments at the mother-bud neck and function as a scaffold for recruiting proteins involved in cytokinesis and other processes. We isolated a novel gene, BNI5, as a dosage suppressor of the cdc12-6 growth defect. Overexpression of BNI5 also suppressed the growth defects of cdc10-1, cdc11-6, and sep7Delta strains. Loss of BNI5 resulted in a cytokinesis defect, as evidenced by the formation of connected cells with shared cytoplasms, and deletion of BNI5 in a cdc3-6, cdc10-1, cdc11-6, cdc12-6, or sep7Delta mutant strain resulted in enhanced defects in septin localization and cytokinesis. Bni5p localizes to the mother-bud neck in a septin-dependent manner shortly after bud emergence and disappears from the neck approximately 2 to 3 min before spindle disassembly. Two-hybrid, in vitro binding, and protein-localization studies suggest that Bni5p interacts with the N-terminal domain of Cdc11p, which also appears to be sufficient for the localization of Cdc11p, its interaction with other septins, and other critical aspects of its function. Our data suggest that the Bni5p-septin interaction is important for septin ring stability and function, which is in turn critical for normal cytokinesis.     

GTP binding induces filament assembly of a recombinant septin

The septins are a family of GTPases involved in cytokinesis in budding yeast, Drosophila, and vertebrates (see for review). 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, we have reconstituted filament assembly 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.