The anillin-related region of Bud4 is the major functional determinant for Bud4's function in septin organization during bud growth and axial bud site selection in budding yeast

Characterization of Bud3 domains sufficient for bud neck targeting in S. cerevisiae

The cytoskeleton serves a diverse set of functions in both multi- and unicellular organisms, including movement, transport, morphology, cell division and cell signalling. The septin family of cytoskeletal proteins are found within all fungi and metazoans and can generate three-dimensional scaffolds in vivo that promote membrane curvature, serve as physical barriers and coordinate cell cycle checkpoints. In budding yeast, the septins organize into polymerized filaments that decorate the division site between mother and daughter cells during mitosis; assembly of this structure at the ‘bud neck’ is critical for completion of cytokinesis and execution of numerous other cellular events. One such pathway includes bud site selection and the recruitment of proteins such as Bud4 and Bud3 that are responsible for promoting an axial budding pattern in haploid yeast. While Bud4 appears to be recruited to the septins independently of the presence of Bud3, it is likely that Bud3 can localize to the bud neck using both Bud4-dependent and Bud4-independent mechanisms. Furthermore, it remains unclear which precise domain or domains within Bud3 is/are both necessary and sufficient for optimal association at the septin structure. In this study, we examined the localization of GFP-Bud3 constructs in otherwise wild-type (WT) haploid yeast cells expressing Cdc10-mCherry using fluorescence microscopy; we tested a collection of N- and C-terminal truncations and fusions of separate Bud3 protein elements to identify the smallest domain(s) responsible for bud neck localization. We found that the coordinate action of the central amphipathic helix (residues 847–865) and a partially conserved C-terminal motif (residues 1172–1273) was sufficient to promote bud neck recruitment in the presence of endogenous Bud3. This domain is considerably smaller than the previously characterized C-terminal portion required to physically interact with Bud4 (1221–1636) and utilizes a similar mechanism of pairing membrane association, with a separate localization domain, similar to other non-septin proteins targeted to the division site during cell division.

Septin Assembly and Remodeling at the Cell Division Site During the Cell Cycle

The septin family of proteins can assemble into filaments that further organize into different higher order structures to perform a variety of different functions in different cell types and organisms. In the budding yeast Saccharomyces cerevisiae, the septins localize to the presumptive bud site as a cortical ring prior to bud emergence, expand into an hourglass at the bud neck (cell division site) during bud growth, and finally “split” into a double ring sandwiching the cell division machinery during cytokinesis. While much work has been done to understand the functions and molecular makeups of these structures, the mechanisms underlying the transitions from one structure to another have largely remained elusive. Recent studies involving advanced imaging and in vitro reconstitution have begun to reveal the vast complexity involved in the regulation of these structural transitions, which defines the focus of discussion in this mini-review.

Targeted Quantification of Phosphorylation Sites Identifies STRIPAK-Dependent Phosphorylation of the Hippo Pathway-Related Kinase SmKIN3

We showed recently that the germinal center kinase III (GCKIII) SmKIN3 from the fungus Sordaria macrospora is involved in sexual development and hyphal septation. Our recent extensive global proteome and phosphoproteome analysis revealed that SmKIN3 is a target of the striatin-interacting phosphatase and kinase (STRIPAK) multisubunit complex. Here, using protein samples from the wild type and three STRIPAK mutants, we applied absolute quantification by parallel-reaction monitoring (PRM) to analyze phosphorylation site occupancy in SmKIN3 and other septation initiation network (SIN) components, such as CDC7 and DBF2, as well as BUD4, acting downstream of SIN. For SmKIN3, we show that phosphorylation of S668 and S686 is decreased in mutants lacking distinct subunits of STRIPAK, while a third phosphorylation site, S589, was not affected. We constructed SmKIN3 mutants carrying phospho-mimetic and phospho-deficient codons for phosphorylation sites S589, S668, and S686. Investigation of hyphae in a ΔSmkin3 strain complemented by the S668 and S686 mutants showed a hyper-septation phenotype, which was absent in the wild type, the ΔSmkin3 strain complemented with the wild-type gene, and the S589 mutant. Furthermore, localization studies with SmKIN3 phosphorylation variants and STRIPAK mutants showed that SmKIN3 preferentially localizes at the terminal septa, which is distinctly different from the localization of the wild-type strains. We conclude that STRIPAK-dependent phosphorylation of SmKIN3 has an impact on controlled septum formation and on the time-dependent localization of SmKIN3 on septa at the hyphal tip. Thus, STRIPAK seems to regulate SmKIN3, as well as DBF2 and BUD4 phosphorylation, affecting septum formation.

Roles of the PH, coiled-coil and SAM domains of the yeast polarity protein Boi2 in polarity-site localization and function in polarized growth

Boi1 and Boi2 are paralogous proteins essential for bud formation in budding yeast. So far, the domains that target Boi1/Boi2 to the polarity sites and function in bud formation are not well understood. Here, we report that a coiled-coil domain of Boi2 cooperates with the adjacent PH domain to confer Boi2's bud-cortex localization and major function in cell growth. The PH domain portion of the PH-CC bi-domain interacts with the Rho GTPases Cdc42 and Rho3 and both interactions are independent of the GTP/GDP-bound state of each GTPase. Interestingly, high-copy RHO3 and BOI2 but not CDC42 suppressed the growth defect of RGA1-C538 overexpression and the sec15-1 mutant and this BOI2 function depends on RHO3, suggesting that Boi2 may function in the Rho3 pathway. The SAM domain of Boi2 plays an essential role in high-copy suppression of the two mutants as well as in the early bud-neck localization of Boi2. The SAM domain and the CC domain also interact homotypically. They are likely involved in the formation of Boi2-containing protein complex. Our results provide new insights in the localization and function of Boi2 and highlight the importance of the PH-CC bi-domain and the SAM domain in Boi2's localization and function.

Role of Anillin in Tumour: From a Prognostic Biomarker to a Novel Target

Anillin (ANLN), an actin-binding protein, reportedly plays a vital role in cell proliferation and migration, particularly in cytokinesis. Although there have been findings pointing to a contribution of ANLN to the development of cancer, the association of ANLN to cancer remains not fully understood. Here, we gather evidence to determine the applicability of ANLN as a prognostic tool for some types of cancer, and the impact that ANLN has on the hallmarks of cancer. We searched academic repositories including PubMed and Google Scholar to find and review studies related to cancer and ANLN. The conclusion is that ANLN could be a potent target for cancer treatment, but the roles ANLN, other than in cytokinesis and its influence on tumour microenvironment remodeling in cancer development, must be further elucidated, and specific ANLN inhibitors should be found.

Critical Roles of a RhoGEF-Anillin Module in Septin Architectural Remodeling during Cytokinesis

How septin architecture is remodeled from an hourglass to a double ring during cytokinesis in fungal and animal cells remains unknown. Here, we show that during the hourglass-to-double-ring transition in budding yeast, septins acquire a "zonal architecture" in which paired septin filaments that are organized along the mother-bud axis associate with circumferential single septin filaments, the Rho guanine-nucleotide-exchange factor (RhoGEF) Bud3, and the anillin-like protein Bud4 exclusively at the outer zones and with myosin-II filaments in the middle zone. Deletion of Bud3 or its Bud4-interacting domain, but not its RhoGEF domain, leads to a complete loss of the single filaments, whereas deletion of Bud4 or its Bud3-interacting domain destabilizes the transitional hourglass, especially at the mother side, with partial loss of both filament types. Deletion of Bud3 and Bud4 together further weakens the transitional structure and abolishes the double ring formation while causing no obvious defect in actomyosin ring constriction. This and further analyses suggest that Bud3 stabilizes the single filaments, whereas Bud4 strengthens the interaction between the paired and single filaments at the outer zones of the transitional hourglass, as well as in the double ring. This study reveals a striking zonal architecture for the transitional hourglass that pre-patterns two cytokinetic structures-a septin double ring and an actomyosin ring-and also defines the essential roles of a RhoGEF-anillin module in septin architectural remodeling during cytokinesis at the filament level.

Molecular mechanisms of contractile-ring constriction and membrane trafficking in cytokinesis

In this review, we discuss the molecular mechanisms of cytokinesis from plants to humans, with a focus on contribution of membrane trafficking to cytokinesis. Selection of the division site in fungi, metazoans, and plants is reviewed, as well as the assembly and constriction of a contractile ring in fungi and metazoans. We also provide an introduction to exocytosis and endocytosis, and discuss how they contribute to successful cytokinesis in eukaryotic cells. The conservation in the coordination of membrane deposition and cytoskeleton during cytokinesis in fungi, metazoans, and plants is highlighted.

The anillin-related Int1 protein and the Sep7 septin collaborate to maintain cellular ploidy in Candida albicans

Variation in cell ploidy is a common feature of Candida albicans clinical isolates that are resistant to the antifungal drug fluconazole. Here, we report that the anillin-related protein Int1 interacts with septins for coupling cytokinesis with nuclear segregation. Loss of Int1 results in a rapid disassembly of duplicated septin rings from the bud neck at the onset of actomyosin ring contraction. Strikingly, this has no major impact on cytokinesis and septum formation. However, Int1 genetically interacts with the Sep7 septin, maintaining the diffusion barrier at the bud neck and guarantying a faithful nuclear segregation. Indeed, int1ΔΔ sep7ΔΔ mutant cells, in contrast to int1ΔΔ cdc10ΔΔ, undergo a premature activation of mitotic exit prior to the alignment of the mitotic spindle with the division axis, producing large multinucleated cells. Some of these multinucleated cells arise from trimeras similar to those observed upon fluconazole exposure. Finally, the defects in nuclear segregation could be in part due to the inability to maintain the Lte1 mitotic exit activator at the cortex of the daughter cell. These results suggest that Int1 and Sep7 play a role in maintaining genome stability by acting as a diffusion barrier for Lte1.

Dynamic localization of a yeast development-specific PP1 complex during prospore membrane formation is dependent on multiple localization signals and complex formation

Prospore membrane formation of Saccharomyces cerevisiae provides a powerful model for understanding the mechanisms of de novo membrane formation. Protein phosphatase type1, Glc7, and a sporulation-specific targeting subunit, Gip1, show dynamic localization using multiple localization signals and regulate membrane growth during sporulation.

During the developmental process of sporulation in Saccharomyces cerevisiae, membrane structures called prospore membranes are formed de novo, expand, extend, acquire a round shape, and finally become plasma membranes of the spores. GIP1 encodes a regulatory/targeting subunit of protein phosphatase type 1 that is required for sporulation. Gip1 recruits the catalytic subunit Glc7 to septin structures that form along the prospore membrane; however, the molecular basis of its localization and function is not fully understood. Here we show that Gip1 changes its localization dynamically and is required for prospore membrane extension. Gip1 first associates with the spindle pole body as the prospore membrane forms, moves onto the prospore membrane and then to the septins as the membrane extends, distributes around the prospore membrane after closure, and finally translocates into the nucleus in the maturing spore. Deletion and mutation analyses reveal distinct sequences in Gip1 that are required for different localizations and for association with Glc7. Binding to Glc7 is also required for proper localization. Strikingly, localization to the prospore membrane, but not association with septins, is important for Gip1 function. Further, our genetic analysis suggests that a Gip1–Glc7 phosphatase complex regulates prospore membrane extension in parallel to the previously reported Vps13, Spo71, Spo73 pathway.

Roles of an N-terminal coiled-coil-containing domain in the localization and function of Bem3, a Rho GTPase-activating protein in budding yeast

GTPase-activating proteins (GAPs) play critical roles in the spatial and temporal control of small GTPases. The budding yeast Bem3 is a GAP for Cdc42, a Rho GTPase crucial for actin and septin organization. Bem3 localizes to the sites of polarized growth. However, the amino acid sequence determinants mediating recruitment of Bem3 to its physiological sites of action and those important for Bem3 function are not clear. Here, we show that Bem3's localization is guided by two distinct targeting regions-the PX-PH-domain-containing TD1 and the coiled-coil-containing TD2. TD2 localization is largely mediated by its interaction with the polarisome component Epo1 via heterotypic coiled-coil interaction. This finding reveals a novel role for the polarisome in linking Bem3 to its functional target, Cdc42. We also show that the coiled-coil domain of Bem3 interacts homotypically and this interaction is important for the regulation of Cdc42 by Bem3. Moreover, we show that overexpression of a longer version of the TD2 domain disrupts septin-ring assembly in a RhoGAP-independent manner, suggesting that TD2 may be capable of interacting with proteins implicated in septin-ring assembly. Furthermore, we show that the longer version of TD2 interacts with Kss1, a MAPK involved in filamentous growth. Kss1 is reported to localize mainly in the nucleus. We find that Kss1 also localizes to the sites of polarized growth and Bem3 interacts with Kss1 at the septin-ring assembly site. Our study provides new insights in Bem3's localization and function.

The final cut: cell polarity meets cytokinesis at the bud neck in S. cerevisiae

Cell division is a fundamental but complex process that gives rise to two daughter cells. It includes an ordered set of events, altogether called "the cell cycle", that culminate with cytokinesis, the final stage of mitosis leading to the physical separation of the two daughter cells. Symmetric cell division equally partitions cellular components between the two daughter cells, which are therefore identical to one another and often share the same fate. In many cases, however, cell division is asymmetrical and generates two daughter cells that differ in specific protein inheritance, cell size, or developmental potential. The budding yeast Saccharomyces cerevisiae has proven to be an excellent system to investigate the molecular mechanisms governing asymmetric cell division and cytokinesis. Budding yeast is highly polarized during the cell cycle and divides asymmetrically, producing two cells with distinct sizes and fates. Many components of the machinery establishing cell polarization during budding are relocalized to the division site (i.e., the bud neck) for cytokinesis. In this review we recapitulate how budding yeast cells undergo polarized processes at the bud neck for cell division.

Detection of protein-protein interactions at the septin collar in Saccharomyces cerevisiae using a tripartite split-GFP system

A tripartite split-GFP system faithfully reports the order of the subunits in septin hetero-octamers (and thus can serve as a “molecular ruler”), conversely yields little or no false signal even with very highly expressed cytosolic proteins, and detects authentic interactions of other cellular proteins that are bona fide septin-binding proteins.

Various methods can provide a readout of the physical interaction between two biomolecules. A recently described tripartite split-GFP system has the potential to report by direct visualization via a fluorescence signal the intimate association of minimally tagged proteins expressed at their endogenous level in their native cellular milieu and can capture transient or weak interactions. Here we document the utility of this tripartite split-GFP system to assess in living cells protein–protein interactions in a dynamic cytoskeletal structure—the septin collar at the yeast bud neck. We show, first, that for septin–septin interactions, this method yields a robust signal whose strength reflects the known spacing between the subunits in septin filaments and thus serves as a “molecular ruler.” Second, the method yields little or no spurious signal even with highly abundant cytosolic proteins readily accessible to the bud neck (including molecular chaperone Hsp82 and glycolytic enzyme Pgk1). Third, using two proteins (Bni5 and Hsl1) that have been shown by other means to bind directly to septins at the bud neck in vivo, we validate that the tripartite split-GFP method yields the same conclusions and further insights about specificity. Finally, we demonstrate the capacity of this approach to uncover additional new information by examining whether three other proteins reported to localize to the bud neck (Nis1, Bud4, and Hof1) are able to interact physically with any of the subunits in the septin collar and, if so, with which ones.

Septins and Generation of Asymmetries in Fungal Cells

Polarized growth is critical for the development and maintenance of diverse organisms and tissues but particularly so in fungi, where nutrient uptake, communication, and reproduction all rely on cell asymmetries. To achieve polarized growth, fungi spatially organize both their cytosol and cortical membranes. Septins, a family of GTP-binding proteins, are key regulators of spatial compartmentalization in fungi and other eukaryotes. Septins form higher-order structures on fungal plasma membranes and are thought to contribute to the generation of cell asymmetries by acting as molecular scaffolds and forming diffusional barriers. Here we discuss the links between septins and polarized growth and consider molecular models for how septins contribute to cellular asymmetry in fungi.

Coordinate action of distinct sequence elements localizes checkpoint kinase Hsl1 to the septin collar at the bud neck in Saccharomyces cerevisiae

A long-standing conundrum is resolved about the underlying sequence determinants and molecular mechanism responsible for the recruitment of the protein kinase Hsl1 (an indispensable component of the so-called “morphogenesis checkpoint”) exclusively to the septin collar at the bud neck.

Passage through the eukaryotic cell cycle requires processes that are tightly regulated both spatially and temporally. Surveillance mechanisms (checkpoints) exert quality control and impose order on the timing and organization of downstream events by impeding cell cycle progression until the necessary components are available and undamaged and have acted in the proper sequence. In budding yeast, a checkpoint exists that does not allow timely execution of the G2/M transition unless and until a collar of septin filaments has properly assembled at the bud neck, which is the site where subsequent cytokinesis will occur. An essential component of this checkpoint is the large (1518-residue) protein kinase Hsl1, which localizes to the bud neck only if the septin collar has been correctly formed. Hsl1 reportedly interacts with particular septins; however, the precise molecular determinants in Hsl1 responsible for its recruitment to this cellular location during G2 have not been elucidated. We performed a comprehensive mutational dissection and accompanying image analysis to identify the sequence elements within Hsl1 responsible for its localization to the septins at the bud neck. Unexpectedly, we found that this targeting is multipartite. A segment of the central region of Hsl1 (residues 611–950), composed of two tandem, semiredundant but distinct septin-associating elements, is necessary and sufficient for binding to septin filaments both in vitro and in vivo. However, in addition to 611–950, efficient localization of Hsl1 to the septin collar in the cell obligatorily requires generalized targeting to the cytosolic face of the plasma membrane, a function normally provided by the C-terminal phosphatidylserine-binding KA1 domain (residues 1379–1518) in Hsl1 but that can be replaced by other, heterologous phosphatidylserine-binding sequences.

The Carboxy-Terminal Tails of Septins Cdc11 and Shs1 Recruit Myosin-II Binding Factor Bni5 to the Bud Neck in Saccharomyces cerevisiae

Septins are a conserved family of GTP-binding proteins that form heterooctameric complexes that assemble into higher-order structures. In yeast, septin superstructure at the bud neck serves as a barrier to separate a daughter cell from its mother and as a scaffold to recruit the proteins that execute cytokinesis. However, how septins recruit specific factors has not been well characterized. In the accompanying article in this issue, (Finnigan et al. 2015), we demonstrated that the C-terminal extensions (CTEs) of the alternative terminal subunits of septin heterooctamers, Cdc11 and Shs1, share a role required for optimal septin function in vivo. Here we describe our use of unbiased genetic approaches (both selection of dosage suppressors and analysis of synthetic interactions) that pinpointed Bni5 as a protein that interacts with the CTEs of Cdc11 and Shs1. Furthermore, we used three independent methods-construction of chimeric proteins, noncovalent tethering mediated by a GFP-targeted nanobody, and imaging by fluorescence microscopy-to confirm that a physiologically important function of the CTEs of Cdc11 and Shs1 is optimizing recruitment of Bni5 and thereby ensuring efficient localization at the bud neck of Myo1, the type II myosin of the actomyosin contractile ring.Related article in

GENETICS

Finnigan, G. C. et al., 2015 Comprehensive Genetic Analysis of Paralogous Terminal Septin Subunits Shs1 and Cdc11 in Saccharomyces cerevisiae. Genetics 200: 841-861.