Fission yeast septation

A novel cell biological tool to explain mechanics and dynamics in fission yeast

The Rho guanosine triphosphatase hydrolase enzyme (GTPase) is required for the control of the actin cytoskeleton, but its activation in vivo condition is unknown. The study's goal was to find a new synthetic nanobody VHH (P-36 tagged with mNeonGreen) that interacts strongly with the Rho GTPase. We present the first novel synthetic nanobody, VHH (P-36 tagged with mNeonGreen), tested in fission yeast cells and found to have a particular interaction with Rho1GTPase. Plasmids were constructed by using of certain enzymes to digest the pDUAL-pef1a vector plasmid to produce a protein that was encoded by cloned genes. A varied VHH library was created synthetically, then transformed into yeast cells, and positive clones were chosen using chemical agents. To investigate protein interactions and cellular reactions, several studies were carried out, such as live cell imaging, growth curve analysis, coimmunoprecipitation, structural analysis, and cell therapies. Prism and RStudio were used for the statistical analysis. The presence of VHH (P-36) has no effect on the growth pattern making it an appropriate model for studying cytokinesis in vivo. According to a computational biological study, its affinity to interact with Rho1GTPase with all the complementarity-determining region (CDR) regions found on VHH (P-36) is extremely strong. We were able to track its subcellular target by localization using a fluorescent confocal microscope, ensuring the maintenance of cell polarity and morphology. Spheroplast analysis revealed a circular-shaped cell with an even distribution of Rho1 tagged VHH (P-36), indicating that the interaction occurs near the plasma membrane. The introduction of latrunculin-A (Lat-A) disrupted Rho GTPase localization, demonstrating the control over actin production, and the cell did not show evidence of mitotic phase commencement while Lat-A was present. Finally, this important biological tool can aid in our understanding of the mechanics and dynamics of cytokinesis in relation to Rho1GTPase.

The Roles of Septins in Regulating Fission Yeast Cytokinesis

Cytokinesis is required to separate two daughter cells at the end of mitosis, and septins play crucial roles in many aspects of cytokinesis. While septins have been intensively studied in many model organisms, including the budding yeast Saccharomyces cerevisiae, septins have been relatively less characterized in the fission yeast Schizosaccharomyces pombe, which has proven to be an excellent model organism for studying fundamental cell biology. In this review, we summarize the findings of septins made in fission yeasts mainly from four aspects: the domain structure of septins, the localization of septins during the cell cycle, the roles of septins in regulating cytokinesis, and the regulatory proteins of septins.

Cdc42 prevents precocious Rho1 activation during cytokinesis in a Pak1-dependent manner

During cytokinesis, a series of coordinated events partition a dividing cell. Accurate regulation of cytokinesis is essential for proliferation and genome integrity. In fission yeast, these coordinated events ensure that the actomyosin ring and septum start ingressing only after chromosome segregation. How cytokinetic events are coordinated remains unclear. The GTPase Cdc42 promotes recruitment of certain cell wall-building enzymes whereas the GTPase Rho1 activates these enzymes. We show that Cdc42 prevents early Rho1 activation during fission yeast cytokinesis. Using an active Rho probe, we find that although the Rho1 activators Rgf1 and Rgf3 localize to the division site in early anaphase, Rho1 is not activated until late anaphase, just before the onset of ring constriction. We find that loss of Cdc42 activation enables precocious Rho1 activation in early anaphase. Furthermore, we provide functional and genetic evidence that Cdc42-dependent Rho1 inhibition is mediated by the Cdc42 target Pak1 kinase. Our work proposes a mechanism of Rho1 regulation by active Cdc42 to coordinate timely septum formation and cytokinesis fidelity.

Cdc42 GTPase activating proteins Rga4 and Rga6 coordinate septum synthesis and membrane trafficking at the division plane during cytokinesis

Fission yeast cytokinesis is driven by simultaneous septum synthesis, membrane furrowing and actomyosin ring constriction. The septum consists of a primary septum flanked by secondary septa. First, delivery of the glucan synthase Bgs1 and membrane vesicles initiate primary septum synthesis and furrowing. Next, Bgs4 is delivered for secondary septum formation. It is unclear how septum synthesis is coordinated with membrane furrowing. Cdc42 promotes delivery of Bgs1 but not Bgs4. We find that after primary septum initiation, Cdc42 inactivators Rga4 and Rga6 localize to the division site. In rga4Δrga6Δ mutants, Cdc42 activity is enhanced during late cytokinesis and cells take longer to separate. Electron micrographs of the division site in these mutants exhibit malformed septum with irregular membrane structures. These mutants have a larger division plane with enhanced Bgs1 delivery but fail to enhance accumulation of Bgs4 and several exocytic proteins. Additionally, these mutants show endocytic defects at the division site. This suggests that Cdc42 regulates primary septum formation and only certain membrane trafficking events. As cytokinesis progresses Rga4 and Rga6 localize to the division site to decrease Cdc42 activity to allow coupling of Cdc42-independent membrane trafficking events with septum formation for proper septum morphology.

Involvement of Smi1 in cell wall integrity and glucan synthase Bgs4 localization during fission yeast cytokinesis

Cytokinesis is the final step of the cell-division cycle. In fungi, it relies on the coordination of constriction of an actomyosin contractile ring and construction of the septum at the division site. Glucan synthases synthesize glucans, which are the major components in fungal cell walls and division septa. It is known that Rho1 and Rho2 GTPases regulate glucan synthases Bgs1, Bgs4, and Ags1, and that Sbg1 and the F-BAR protein Cdc15 play roles in Bgs1 stability and delivery to the plasma membrane. Here we characterize Smi1, an intrinsically disordered protein that interacts with Bgs4 and regulates its trafficking and localization in fission yeast. Smi1 is important for septum integrity, and its absence causes severe lysis during cytokinesis. Smi1 localizes to secretory vesicles and moves together with Bgs4 toward the division site. The concentrations of the glucan synthases Bgs1 and Bgs4 and the glucanases Agn1 and Bgl2 decrease at the division site in the smi1 mutant, but Smi1 seems to be more specific to Bgs4. Mistargeting of Smi1 to mitochondria mislocalizes Bgs4 but not Bgs1. Together, our data reveal a novel regulator of glucan synthases and glucanases, Smi1, which is more important for Bgs4 trafficking, stability, and localization during cytokinesis.

Cdc42 reactivation at growth sites is regulated by local cell-cycle-dependent loss of its GTPase-activating protein Rga4 in fission yeast

In fission yeast, polarized cell growth stops during division and resumes after cytokinesis completes and cells separate. It is unclear how growth reactivation is timed to occur immediately after cell separation. We uncoupled these sequential events by delaying cytokinesis with a temporary Latrunculin A treatment. Mitotic cells recovering from treatment initiate end growth during septation, displaying a polar elongation simultaneous with septation (PrESS) phenotype. PrESS cell ends reactivate Cdc42, a major regulator of polarized growth, during septation, but at a fixed time after anaphase B. A candidate screen implicates Rga4, a negative regulator of Cdc42, in this process. We show that Rga4 appears punctate at the cell sides during G2, but is diffuse during mitosis, extending to the ends. Although the Morphogenesis Orb6 (MOR) pathway is known to promote cell separation and growth by activating protein synthesis, we find that, for polarized growth, removal of Rga4 from the ends is also necessary. Therefore, we propose that growth resumes after division once the MOR pathway is activated and the ends lose Rga4 in a cell-cycle-dependent manner.

The Role of the Cell Integrity Pathway in Septum Assembly in Yeast

Cytokinesis divides a mother cell into two daughter cells at the end of each cell cycle and proceeds via the assembly and constriction of a contractile actomyosin ring (CAR). Ring constriction promotes division furrow ingression, after sister chromatids are segregated to opposing sides of the cleavage plane. Cytokinesis contributes to genome integrity because the cells that fail to complete cytokinesis often reduplicate their chromosomes. While in animal cells, the last steps of cytokinesis involve extracellular matrix remodelling and mid-body abscission, in yeast, CAR constriction is coupled to the synthesis of a polysaccharide septum. To preserve cell integrity during cytokinesis, fungal cells remodel their cell wall through signalling pathways that connect receptors to downstream effectors, initiating a cascade of biological signals. One of the best-studied signalling pathways is the cell wall integrity pathway (CWI) of the budding yeast Saccharomyces cerevisiae and its counterpart in the fission yeast Schizosaccharomyces pombe, the cell integrity pathway (CIP). Both are signal transduction pathways relying upon a cascade of MAP kinases. However, despite strong similarities in the assembly of the septa in both yeasts, there are significant mechanistic differences, including the relationship of this process with the cell integrity signalling pathways.

Callose deposition is essential for the completion of cytokinesis in the unicellular alga Penium margaritaceum

Cytokinesis in land plants involves the formation of a cell plate that develops into the new cell wall. Callose, a β-1,3 glucan, accumulates at later stages of cell plate development, presumably to stabilize this delicate membrane network during expansion. Cytokinetic callose is considered specific to multicellular plant species, because it has not been detected in unicellular algae. Here we present callose at the cytokinesis junction of the unicellular charophyte, Penium margaritaceum Callose deposition at the division plane of P. margaritaceum showed distinct, spatiotemporal patterns likely representing distinct roles of this polymer in cytokinesis. Pharmacological inhibition of callose deposition by endosidin 7 resulted in cytokinesis defects, consistent with the essential role for this polymer in P. margaritaceum cell division. Cell wall deposition at the isthmus zone was also affected by the absence of callose, demonstrating the dynamic nature of new wall assembly in P. margaritaceum The identification of candidate callose synthase genes provides molecular evidence for callose biosynthesis in P. margaritaceum The evolutionary implications of cytokinetic callose in this unicellular zygnematopycean alga is discussed in the context of the conquest of land by plants.This article has an associated First Person interview with the first author of the paper.

Cdc42 promotes Bgs1 recruitment for septum synthesis and glucanase localization for cell separation during cytokinesis in fission yeast

Cytokinesis in fission yeast involves actomyosin ring constriction concurrent to septum synthesis followed by septum digestion resulting in cell separation. A recent report indicates that endocytosis is required for septum synthesis and cell separation. The conserved GTPase Cdc42 is required for membrane trafficking and promotes endocytosis. Cdc42 is activated by Guanine nucleotide exchange factors (GEFs). Cdc42 GEFs have been shown to promote timely initiation of septum synthesis and proper septum morphology. Here we show that Cdc42 promotes the recruitment of the major primary septum synthesizing enzyme Bgs1 and consequent ring constriction. Cdc42 is also required for proper localization of the septum digesting glucanases at the division site. Thus, Cdc42 is required to promote multiple steps during cytokinesis.

Coordinating septum formation and the actomyosin ring during cytokinesis in Schizosaccharomyces pombe

During cytokinesis, animal and fungal cells form a membrane furrow via actomyosin ring constriction. Our understanding of actomyosin ring-driven cytokinesis stems extensively from the fission yeast model system. However, unlike animal cells, actomyosin ring constriction occurs simultaneously with septum formation in fungi. While the formation of an actomyosin ring is essential for cytokinesis in fission yeast, proper furrow formation also requires septum deposition. The molecular mechanisms of spatiotemporal coordination of septum deposition with actomyosin ring constriction are poorly understood. Although the role of the actomyosin ring as a mechanical structure driving furrow formation is better understood, its role as a spatiotemporal landmark for septum deposition is not widely discussed. Here we review and discuss the recent advances describing how the actomyosin ring spatiotemporally regulates membrane traffic to promote septum-driven cytokinesis in fission yeast. Finally, we explore emerging questions in cytokinesis, and discuss the role of extracellular matrix during cytokinesis in other organisms.

gas1 mutation extends chronological lifespan via Pmk1 and Sty1 MAPKs in Schizosaccharomyces pombe

In the longevity research by using yeasts, chronological lifespan is defined as the survival time after entry into stationary phase. Previously, screening for long lived mutants of Schizosaccharomyces pombe was performed to identify the novel factors involved in longevity. From this screening, one long lived mutant called as No.36 was obtained. In this study, we identified the mutation caused in gas1⁺, which encodes glucanosyltransferase (gas1-287 mutation) is responsible for the longevity of No.36 mutant. Through the analysis of this mutant, we found that cell wall perturbing agent micafungin also extends chronological lifespan in fission yeast. This lifespan extension depended on both Pmk1 and Sty1 MAP kinases, and longevity caused by the gas1-287 mutation also depended on these kinases. In summary, we propose that the gas1-287 mutation causes longevity as the similar mechanism as cell wall stress depending on Pmk1 and Sty1 MAPK pathways.

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.

Roles of the fission yeast UNC-13/Munc13 protein Ync13 in late stages of cytokinesis

Cytokinesis is a complicated yet conserved step of the cell-division cycle that requires the coordination of multiple proteins and cellular processes. Here we describe a previously uncharacterized protein, Ync13, and its roles during fission yeast cytokinesis. Ync13 is a member of the UNC-13/Munc13 protein family, whose animal homologues are essential priming factors for soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex assembly during exocytosis in various cell types, but no roles in cytokinesis have been reported. We find that Ync13 binds to lipids in vitro and dynamically localizes to the plasma membrane at cell tips during interphase and at the division site during cytokinesis. Deletion of Ync13 leads to defective septation and exocytosis, uneven distribution of cell-wall enzymes and components of cell-wall integrity pathway along the division site and massive cell lysis during cell separation. Interestingly, loss of Ync13 compromises endocytic site selection at the division plane. Collectively, we find that Ync13 has a novel function as an UNC-13/Munc13 protein in coordinating exocytosis, endocytosis, and cell-wall integrity during fission yeast cytokinesis.

Septins regulate the equatorial dynamics of the separation initiation network kinase Sid2p and glucan synthases to ensure proper cytokinesis

Septins generally function as scaffolds and as cortical barriers to restrict the diffusion of membrane proteins. In the fission yeast Schizosaccharomyces pombe, septins form a ring structure at the septum after spindle breakdown during the constriction of the contractile actomyosin ring (CAR) and serve as a scaffold to recruit glucanases to mediate ultimate daughter cell separation. Despite this, it remains unclear if septins play any significant roles before the cell separation during cytokinesis. Employing live cell microscopy, we carefully examined SIN (Septation Initiation Network) signaling and glucan synthases, two key factors ensuring proper function of the CAR. In the absence of the core septin component Spn1p, the formation of a compact CAR is advanced and the CAR constriction rate is slightly but significantly decreased. Moreover, the SIN kinase Sid2p and the glucan synthases Bgs1p and Ags1p form an equatorial ring quite prematurely, but their maintenance at the equatorial region is diminished spn1Δ cells. These findings suggest that septins act as key players in an accurate establishment and the maintenance of CAR by orchestrating the equatorial dynamics of Sid2p and glucan synthases. Hence, this work demonstrates that, in addition to their function during ultimate cell septation, septins have important roles in regulating earlier cytokinetic events, including CAR assembly and constriction, SIN signaling, and the cortical dynamics of the glucan synthases.