Saccharomyces cerevisiae septins: supramolecular organization of heterooligomers and the mechanism of filament assembly.
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ABSTRACT:
SUBMITTER: Bertin A
PROVIDER: S-EPMC2426963 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:The highly conserved family of septin proteins has important functions in cytokinesis in mitotically proliferating cells. A different form of cytokinesis occurs during gametogenesis in Saccharomyces cerevisiae, in which four haploid meiotic products become encased by prospore membrane (PSMs) and specialized, stress-resistant spore walls. Septins are known to localize in a series of structures near the growing PSM, but previous studies noted only mild sporulation defects upon septin mutation. We report that directed PSM extension fails in many septin-mutant cells, and, for those that do succeed, walls are abnormal, leading to increased susceptibility to heating, freezing, and digestion by the Drosophila gut. Septin mutants mislocalize the leading-edge protein (LEP) complex required for normal PSM and wall biogenesis, and ectopic expression of the LEP protein Ssp1 perturbs mitotic septin localization and function, suggesting a functional interaction. Strikingly, extra copies of septin CDC10 rescue sporulation and LEP localization in cells lacking Sma1, a phospholipase D-associated protein dispensable for initiation of PSM assembly and PSM curvature but required for PSM extension. These findings point to key septin functions in directing efficient membrane and cell wall synthesis during budding yeast gametogenesis.
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Project description:Sumoylation regulates a wide range of cellular processes. However, little is known about the regulation of the SUMO machinery. In this study, we demonstrate that two lysine residues (Lys-153 and Lys-157) in the C-terminal region of the yeast E2-conjugating enzyme Ubc9 are the major and minor autosumoylation sites, respectively. Surprisingly, mutation of Lys-157 (ubc9(K157R)) significantly stimulates the level of Ubc9 autosumoylation at Lys-153. The functional role of Ubc9 autosumoylation is exemplified in our findings that cell cycle-dependent sumoylation of cytoskeletal septin proteins is inversely correlated with the Ubc9 autosumoylation level and that mutation of the Ubc9 autosumoylation sites results in aberrant cell morphology. Our study elucidates a regulatory mechanism that utilizes automodification of the E2 enzyme of the sumoylation machinery to control substrate sumoylation.
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Project description: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.
| S-EPMC4772501 | biostudies-literature
Project description:Septins are conserved GTP-binding proteins involved in membrane compartmentalization and remodeling. In budding yeast, five mitotic septins localize at the bud neck, where the plasma membrane is enriched in phosphatidylinositol-4,5-bisphosphate (PtdIns4,5P(2)). We previously established the subunit organization within purified yeast septin complexes and how these hetero-octamers polymerize into filaments in solution and on PtdIns4,5P(2)-containing lipid monolayers. How septin ultrastructure in vitro relates to the septin-containing filaments observed at the neck in fixed cells by thin-section electron microscopy was unclear. A morphological description of these filaments in the crowded space of the cell is challenging, given their small cross section. To examine septin organization in situ, sections of dividing yeast cells were analyzed by electron tomography of freeze-substituted cells, as well as by cryo-electron tomography. We found networks of filaments both perpendicular and parallel to the mother-bud axis that resemble septin arrays on lipid monolayers, displaying a repeat pattern that mirrors the molecular dimensions of the corresponding septin preparations in vitro. Thus these in situ structures most likely represent septin filaments. In viable mutants lacking a single septin, in situ filaments are still present, although more disordered, consistent with other evidence that the in vivo function of septins requires filament formation.
| S-EPMC3268722 | biostudies-literature
Project description:The GTPase dynamin is required for endocytic vesicle formation. Dynamin has also been implicated in regulating the actin cytoskeleton, but the mechanism by which it does so is unclear. Through interactions via its proline-rich domain (PRD), dynamin binds several proteins, including cortactin, profilin, syndapin, and murine Abp1, that regulate the actin cytoskeleton. We investigated the interaction of dynamin2 and cortactin in regulating actin assembly in vivo and in vitro. When expressed in cultured cells, a dynamin2 mutant with decreased affinity for GTP decreased actin dynamics within the cortical actin network. Expressed mutants of cortactin that have decreased binding of Arp2/3 complex or dynamin2 also decreased actin dynamics. Dynamin2 influenced actin nucleation by purified Arp2/3 complex and cortactin in vitro in a biphasic manner. Low concentrations of dynamin2 enhanced actin nucleation by Arp2/3 complex and cortactin, and high concentrations were inhibitory. Dynamin2 promoted the association of actin filaments nucleated by Arp2/3 complex and cortactin with phosphatidylinositol 4,5-bisphosphate (PIP2)-containing lipid vesicles. GTP hydrolysis altered the organization of the filaments and the lipid vesicles. We conclude that dynamin2, through an interaction with cortactin, regulates actin assembly and actin filament organization at membranes.
| S-EPMC9590655 | biostudies-literature
Project description:The ligninolytic enzymatic consortium produced by white-rot fungi is one of the most efficient oxidative systems found in nature, with many potential applications that range from the production of 2nd generation biofuels to chemicals synthesis. In the current study, two high redox potential oxidoreductase fusion genes (laccase -Lac- and versatile peroxidase -Vp-) that had been evolved in the laboratory were re-assembled in Saccharomyces cerevisiae. First, cell viability and secretion were assessed after co-transforming the Lac and Vp genes into yeast. Several expression cassettes were inserted in vivo into episomal bi-directional vectors in order to evaluate inducible promoter and/or terminator pairs of different strengths in an individual and combined manner. The synthetic white-rot yeast model harboring Vp(GAL1/CYC1)-Lac(GAL10/ADH1) displayed up to 1000 and 100 Units per L of peroxidase and laccase activity, respectively, representing a suitable point of departure for future synthetic biology studies.
| S-EPMC4143396 | biostudies-literature
Project description:Septin family proteins are quite similar to each other both within and between eukaryotic species. Typically, multiple discrete septins co-assemble into linear heterooligomers (usually hexameric or octameric rods) with a variety of cellular functions. We know little about how incorporation of different septins confers different properties to such complexes. This issue is especially acute in human cells where 13 separate septin gene products (often produced in multiple forms arising from alternative start codons and differential splicing) are expressed in a tissue-specific manner. Based on sequence alignments and phylogenetic criteria, human septins fall into four distinct groups predictive of their interactions, that is, members of the same group appear to occupy the same position within oligomeric septin protomers, which are "palindromic" (have twofold rotational symmetry about a central homodimeric pair). Many such protomers are capable of end-to-end polymerization, generating filaments. Over a decade ago, a study using X-ray crystallography and single-particle electron microscopy deduced the arrangement within recombinant heterohexamers comprising representatives of three human septin groups-SEPT2, SEPT6, and SEPT7. This model greatly influenced subsequent studies of human and other septin complexes, including how incorporating a septin from a fourth group forms heterooctamers, as first observed in budding yeast. Two recent studies, including one in this issue of Cytoskeleton, provide clear evidence that, in fact, the organization of subunits within human septin heterohexamers and heterooctamers is inverted relative to the original model. These findings are discussed here in a broader context, including possible causes for the initial confusion.
| S-EPMC6872917 | biostudies-literature
Project description:BackgroundProteins in organisms, rather than act alone, usually form protein complexes to perform cellular functions. We analyze the topological network structure of protein complexes and their component proteins in the budding yeast in terms of the bipartite network and its projections, where the complexes and proteins are its two distinct components. Compared to conventional protein-protein interaction networks, the networks from the protein complexes show more homogeneous structures than those of the binary protein interactions, implying the formation of complexes that cause a relatively more uniform number of interaction partners. In addition, we suggest a new optimization method to determine the abundance and function of protein complexes, based on the information of their global organization. Estimating abundance and biological functions is of great importance for many researches, by providing a quantitative description of cell behaviors, instead of just a "catalogues" of the lists of protein interactions.ResultsWith our new optimization method, we present genome-wide assignments of abundance and biological functions for complexes, as well as previously unknown abundance and functions of proteins, which can provide significant information for further investigations in proteomics. It is strongly supported by a number of biologically relevant examples, such as the relationship between the cytoskeleton proteins and signal transduction and the metabolic enzyme Eno2's involvement in the cell division process.ConclusionsWe believe that our methods and findings are applicable not only to the specific area of proteomics, but also to much broader areas of systems biology with the concept of optimization principle.
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Project description:Previous studies showed that the K342E substitution in the Saccharomyces cerevisiae Rad51 protein increases the interaction with Rad54 protein in the two-hybrid system, leads to increased sensitivity to the alkylating agent MMS and hyper-recombination in an oligonucleotide-mediated gene targeting assay. K342 localizes in loop 2, a region of Rad51 whose function is not well understood. Here, we show that Rad51-K342E displays DNA-independent and DNA-dependent ATPase activities, owing to its ability to form filaments in the absence of a DNA lattice. These filaments exhibit a compressed pitch of 81 A, whereas filaments of wild-type Rad51 and Rad51-K342E on DNA form extended filaments with a 97 A pitch. Rad51-K342E shows near normal binding to ssDNA, but displays a defect in dsDNA binding, resulting in less stable protein-dsDNA complexes. The mutant protein is capable of catalyzing the DNA strand exchange reaction and is insensitive to inhibition by the early addition of dsDNA. Wild-type Rad51 protein is inhibited under such conditions, because of its ability to bind dsDNA. No significant changes in the interaction between Rad51-K342E and Rad54 could be identified. These findings suggest that loop 2 contributes to the primary DNA-binding site in Rad51, controlling filament formation and ATPase activity.
| S-EPMC2615628 | biostudies-literature
Project description:Septins are a conserved family of GTP-binding proteins that assemble into symmetric linear heterooligomeric complexes, which in turn are able to polymerize into apolar filaments and higher-order structures. In budding yeast (Saccharomyces cerevisiae) and other eukaryotes, proper septin organization is essential for processes that involve membrane remodeling, such as the execution of cytokinesis. In yeast, four septin subunits form a Cdc11-Cdc12-Cdc3-Cdc10-Cdc10-Cdc3-Cdc12-Cdc11 heterooctameric rod that polymerizes into filaments thought to form a collar around the bud neck in close contact with the inner surface of the plasma membrane. To explore septin-membrane interactions, we examined the effect of lipid monolayers on septin organization at the ultrastructural level using electron microscopy. Using this methodology, we have acquired new insights into the potential effect of septin-membrane interactions on filament assembly and, more specifically, on the role of phosphoinositides. Our studies demonstrate that budding yeast septins interact specifically with phosphatidylinositol-4,5-bisphosphate (PIP2) and indicate that the N terminus of Cdc10 makes a major contribution to the interaction of septin filaments with PIP2. Furthermore, we found that the presence of PIP2 promotes filament polymerization and organization on monolayers, even under conditions that prevent filament formation in solution or for mutants that prevent filament formation in solution. In the extreme case of septin complexes lacking the normally terminal subunit Cdc11 or the normally central Cdc10 doublet, the combination of the PIP2-containing monolayer and nucleotide permitted filament formation in vitro via atypical Cdc12-Cdc12 and Cdc3-Cdc3 interactions, respectively.
| S-EPMC3005623 | biostudies-literature