Hof1 and Chs4 Interact via F-BAR Domain and Sel1-like Repeats to Control Extracellular Matrix Deposition during Cytokinesis

Central Role of the Actomyosin Ring in Coordinating Cytokinesis Steps in Budding Yeast

Eukaryotic cells must accurately transfer their genetic material and cellular components to their daughter cells. Initially, cells duplicate their chromosomes and subsequently segregate them toward the poles. The actomyosin ring, a crucial molecular machinery normally located in the middle of the cells and underneath the plasma membrane, then physically divides the cytoplasm and all components into two daughter cells, each ready to start a new cell cycle. This process, known as cytokinesis, is conserved throughout evolution. Defects in cytokinesis can lead to the generation of genetically unstable tetraploid cells, potentially initiating uncontrolled proliferation and cancer. This review focuses on the molecular mechanisms by which budding yeast cells build the actomyosin ring and the preceding steps involved in forming a scaffolding structure that supports the challenging structural changes throughout cytokinesis. Additionally, we describe how cells coordinate actomyosin ring contraction, plasma membrane ingression, and extracellular matrix deposition to successfully complete cytokinesis. Furthermore, the review discusses the regulatory roles of Cyclin-Dependent Kinase (Cdk1) and the Mitotic Exit Network (MEN) in ensuring the precise timing and execution of cytokinesis. Understanding these processes in yeast provides insights into the fundamental aspects of cell division and its implications for human health.

Septin Organization and Dynamics for Budding Yeast Cytokinesis

Cytokinesis, the process by which the cytoplasm divides to generate two daughter cells after mitosis, is a crucial stage of the cell cycle. Successful cytokinesis must be coordinated with chromosome segregation and requires the fine orchestration of several processes, such as constriction of the actomyosin ring, membrane reorganization, and, in fungi, cell wall deposition. In Saccharomyces cerevisiae, commonly known as budding yeast, septins play a pivotal role in the control of cytokinesis by assisting the assembly of the cytokinetic machinery at the division site and controlling its activity. Yeast septins form a collar at the division site that undergoes major dynamic transitions during the cell cycle. This review discusses the functions of septins in yeast cytokinesis, their regulation and the implications of their dynamic remodelling for cell division.

The matrisome landscape controlling in vivo germ cell fates

The developmental fate of cells is regulated by intrinsic factors and the extracellular environment. The extracellular matrix (matrisome) delivers chemical and mechanical cues that can modify cellular development. However, comprehensive understanding of how matrisome factors control cells in vivo is lacking. Here we show that specific matrisome factors act individually and collectively to control germ cell development. Surveying development of undifferentiated germline stem cells through to mature oocytes in the Caenorhabditis elegans germ line enabled holistic functional analysis of 443 conserved matrisome-coding genes. Using high-content imaging, 3D reconstruction, and cell behavior analysis, we identify 321 matrisome genes that impact germ cell development, the majority of which (>80%) are undescribed. Our analysis identifies key matrisome networks acting autonomously and non-autonomously to coordinate germ cell behavior. Further, our results demonstrate that germ cell development requires continual remodeling of the matrisome landscape. Together, this study provides a comprehensive platform for deciphering how extracellular signaling controls cellular development and anticipate this will establish new opportunities for manipulating cell fates.

Transient septin sumoylation steers a Fir1-Skt5 protein complex between the split septin ring

Ubiquitylation and phosphorylation control composition and architecture of the cell separation machinery in yeast and other eukaryotes. The significance of septin sumoylation on cell separation remained an enigma. Septins form an hourglass structure at the bud neck of yeast cells that transforms into a split septin double ring during mitosis. We discovered that sumoylated septins recruit the cytokinesis checkpoint protein Fir1 to the peripheral side of the septin hourglass just before its transformation into the double-ring configuration. As this transition occurs, Fir1 is released from the septins and seamlessly relocates between the split septin rings through synchronized binding to the scaffold Spa2. Fir1 binds and carries the membrane-bound Skt5 on its route to the division plane where the Fir1-Skt5 complex serves as receptor for chitin synthase III.

Live-cell imaging of septins and cell polarity proteins in the growing dikaryotic vegetative hypha of the model mushroom Coprinopsis cinerea

The developmental biology underlying the morphogenesis of mushrooms remains poorly understood despite the essential role of fungi in the terrestrial environment and global carbon cycle. The mushroom Coprinopsis cinerea is a leading model system for the molecular and cellular basis of fungal morphogenesis. The dikaryotic vegetative hyphae of this fungus grow by tip growth with clamp cell formation, conjugate nuclear division, septation, subapical peg formation, and fusion of the clamp cell to the peg. Studying these processes provides many opportunities to gain insights into fungal cell morphogenesis. Here, we report the dynamics of five septins, as well as the regulators CcCla4, CcSpa2, and F-actin, visualized by tagging with fluorescent proteins, EGFP, PA-GFP or mCherry, in the growing dikaryotic vegetative hyphae. We also observed the nuclei using tagged Sumo proteins and histone H1. The five septins colocalized at the hyphal tip in the shape of a dome with a hole (DwH). CcSpa2-EGFP signals were observed in the hole, while CcCla4 signals were observed as the fluctuating dome at the hyphal tip. Before septation, CcCla4-EGFP was also occasionally recruited transiently around the future septum site. Fluorescent protein-tagged septins and F-actin together formed a contractile ring at the septum site. These distinct specialized growth machineries at different sites of dikaryotic vegetative hyphae provide a foundation to explore the differentiation program of various types of cells required for fruiting body formation.

Chitin Synthesis in Yeast: A Matter of Trafficking

Chitin synthesis has attracted scientific interest for decades as an essential part of fungal biology and for its potential as a target for antifungal therapies. While this interest remains, three decades ago, pioneering molecular studies on chitin synthesis regulation identified the major chitin synthase in yeast, Chs3, as an authentic paradigm in the field of the intracellular trafficking of integral membrane proteins. Over the years, researchers have shown how the intracellular trafficking of Chs3 recapitulates all the steps in the intracellular trafficking of integral membrane proteins, from their synthesis in the endoplasmic reticulum to their degradation in the vacuole. This trafficking includes specific mechanisms for sorting in the trans-Golgi network, regulated endocytosis, and endosomal recycling at different levels. This review summarizes the work carried out on chitin synthesis regulation, mostly focusing on Chs3 as a molecular model to study the mechanisms involved in the control of the intracellular trafficking of proteins.

Identification of chitin synthase activator in Aspergillus niger and its application in citric acid fermentation

The biosynthesis of citric acid (CA) using Aspergillus niger as a carrier is influenced by mycelium morphology, which is determined by the expression level of morphology-related genes. As a key component of the fungal cell wall, chitin content has an important effect on morphogenesis, and to investigate the effects of this on fermentation performance, we used RNA interference to knockdown chitin synthase C (CHSC) and chitin synthase activator (CHS3) to obtain the single-gene mutant strains A. niger chs3 and chsC and the double mutant A. niger chs3C. We found that the CA fermentation performance of the two single mutants was significantly better than that of the double mutant. The mutant A. niger chs3-4 exhibited CA production potential compared to that of the parent strain in scale-up fermentation; we determined certain characteristics of CA high-yielding strain fermentation pellets. In addition, when chsC alone was silenced, there was very little change in chs3 mRNA levels, whereas those of chsC were significantly reduced when only chs3 was silenced. As this may be because of a synergistic effect between chsC and chs3, and we speculated that the latent activation target of CHS3 is CHSC, our results confirmed this hypothesis. This study is the first application of a separation and combination silence strategy of chitin synthase and chitin synthase activator in the morphology of A. niger CA fermentation. Furthermore, it provides new insights into the method for the morphological study of A. niger fermentation and the interaction of homologous genes. KEY POINTS: • The function of chitin synthase C (chsC) and chitin synthase activator (chs3) is tightly interrelated. • Mycelial morphology was optimized by knockdown of CHS3, resulting in the overproduction of citric acid. • The separation and combination silence strategies are promising tools for the interaction of homologous housekeeping genes.

Fission yeast paxillin contains two Cdc15 binding motifs for robust recruitment to the cytokinetic ring

The F-BAR protein Cdc15 mediates attachment of the cytokinetic ring (CR) to the plasma membrane and is essential for cytokinesis in Schizosaccharomyces pombe. While its N-terminal F-BAR domain is responsible for oligomerization and membrane binding, its C-terminal SH3 domain binds other partners at a distance from the membrane. We previously demonstrated that the essential cytokinetic formin Cdc12, through an N-terminal motif, directly binds the cytosolic face of the F-BAR domain. Here, we show that paxillin-like Pxl1, which is important for CR stability, contains a motif highly related to that in formin Cdc12, and also binds the Cdc15 F-BAR domain directly. Interestingly, Pxl1 has a second site for binding the Cdc15 SH3 domain. To understand the importance of these two Pxl1-Cdc15 interactions, we mapped and disrupted both. Disrupting the Pxl1-Cdc15 F-BAR domain interaction reduced Pxl1 levels in the CR, whereas disrupting Pxl1’s interaction with the Cdc15 SH3 domain, did not. Unexpectedly, abolishing Pxl1-Cdc15 interaction greatly reduced but did not eliminate CR Pxl1 and did not significantly affect cytokinesis. These data point to another mechanism of Pxl1 CR recruitment and show that very little CR Pxl1 is sufficient for its cytokinetic function.

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.

Essential role of the endocytic site-associated protein Ecm25 in stress-induced cell elongation

How cells adopt a different morphology to cope with stress is not well understood. Here, we show that budding yeast Ecm25 associates with polarized endocytic sites and interacts with the polarity regulator Cdc42 and several late-stage endocytic proteins via distinct regions, including an actin filament-binding motif. Deletion of ECM25 does not affect Cdc42 activity or cause any strong defects in fluid-phase and clathrin-mediated endocytosis but completely abolishes hydroxyurea-induced cell elongation. This phenotype is accompanied by depolarization of the spatiotemporally coupled exo-endocytosis in the bud cortex while maintaining the overall mother-bud polarity. These data suggest that Ecm25 provides an essential link between the polarization signal and the endocytic machinery to enable adaptive morphogenesis under stress conditions.

How cells adopt a different morphology to cope with stress is not well understood. Duan et al. report that the budding yeast protein Ecm25 plays an essential role in stress-induced cell elongation by linking the polarity regulator Cdc42 to the late-stage endocytic machinery.

The state of F-BAR domains as membrane-bound oligomeric platforms

Fes/Cip4 homology Bin/amphiphysin/Rvs (F-BAR) domains, like all BAR domains, are dimeric units that oligomerize and bind membranes. F-BAR domains are generally coupled to additional domains that function in protein binding or have enzymatic activity. Because of their crescent shape and ability to oligomerize, F-BAR domains have been traditionally viewed as membrane-deformation modules. However, multiple independent studies have provided no evidence that certain F-BAR domains are able to tubulate membrane. Instead, a growing body of literature featuring structural, biochemical, biophysical, and microscopy-based studies supports the idea that the F-BAR domain family can be unified only by their ability to form oligomeric assemblies on membranes to provide platforms for molecular assembly.

The kinetic landscape and interplay of protein networks in cytokinesis

Cytokinesis is executed by protein networks organized into functional modules. Individual proteins within each module have been characterized to various degrees. However, the collective behavior and interplay of the modules remain poorly understood. In this study, we conducted quantitative time-lapse imaging to analyze the accumulation kinetics of more than 20 proteins from different modules of cytokinesis in budding yeast. This analysis has led to a comprehensive picture of the kinetic landscape of cytokinesis, from actomyosin ring (AMR) assembly to cell separation. It revealed that the AMR undergoes biphasic constriction and that the switch between the constriction phases is likely triggered by AMR maturation and primary septum formation. This analysis also provided further insights into the functions of actin filaments and the transglutaminase-like protein Cyk3 in cytokinesis and, in addition, defined Kre6 as the likely enzyme that catalyzes β-1,6-glucan synthesis to drive cell wall maturation during cell growth and division.

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Cytokinesis is executed by protein modules each with a unique kinetic signature

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Actomyosin ring constricts in a biphasic manner that is elaborately regulated

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The transglutaminase-like domain in Cyk3 plays a dual role in cytokinesis

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Kre6 catalyzes β-1,6-glucan synthesis at the cell surface during growth and division

Opposite Surfaces of the Cdc15 F-BAR Domain Create a Membrane Platform That Coordinates Cytoskeletal and Signaling Components for Cytokinesis

Many eukaryotes assemble an actin- and myosin-based cytokinetic ring (CR) on the plasma membrane (PM) for cell division, but how it is anchored there remains unclear. In Schizosaccharomyces pombe, the F-BAR protein Cdc15 links the PM via its F-BAR domain to proteins in the CR’s interior via its SH3 domain. However, Cdc15’s F-BAR domain also directly binds formin Cdc12, suggesting that Cdc15 may polymerize a protein network directly adjacent to the membrane. Here, we determine that the F-BAR domain binds Cdc12 using residues on the face opposite its membrane-binding surface. These residues also bind paxillin-like Pxl1, promoting its recruitment with calcineurin to the CR. Mutation of these F-BAR domain residues results in a shallower CR, with components localizing ~35% closer to the PM than in wild type, and aberrant CR constriction. Thus, F-BAR domains serve as oligomeric membrane-bound platforms that can modulate the architecture of an entire actin structure.

Multiple F-BAR domains link actin structures to membrane. Snider et al. show that the flat Cdc15 F-BAR domain utilizes opposite surfaces to bind the plasma membrane and cytokinetic ring proteins simultaneously. Disrupting Cdc15 F-BAR domain’s interaction with proteins results in an overall compression of the entire cytokinetic ring architecture.

A septin-Hof1 scaffold at the yeast bud neck binds and organizes actin cables

Cellular actin arrays are often highly organized, with characteristic patterns critical to their in vivo functions, yet the mechanisms for establishing these higher order geometries remain poorly understood. In Saccharomyces cerevisiae, formin-polymerized actin cables are spatially organized and aligned along the mother–bud axis to facilitate polarized vesicle traffic. Here, we show that the bud neck–associated F-BAR protein Hof1, independent of its functions in regulating the formin Bnr1, binds to actin filaments and organizes actin cables in vivo. Hof1 bundles actin filaments and links them to septins in vitro. F-actin binding is mediated by the “linker” domain of Hof1, and its deletion leads to cable organization defects in vivo. Using superresolution imaging, we show that Hof1 and septins are patterned at the bud neck into evenly spaced axial pillars (∼200 nm apart), from which actin cables emerge and grow into the mother cell. These results suggest that Hof1, while bound to septins at the bud neck, not only regulates Bnr1 activity, but also binds to actin cables and aligns them along the mother–bud axis. More broadly, these findings provide a strong example of how an actin regulatory protein can be spatially patterned at the cell cortex to govern actin network geometry.

Implications of maintenance of mother-bud neck size in diverse vital processes of Saccharomyces cerevisiae

The mother-bud neck is defined as the boundary between the mother cell and bud in budding microorganisms, wherein sequential morphological events occur throughout the cell cycle. This study was designed to quantitatively investigate the morphology of the mother-bud neck in budding yeast Saccharomyces cerevisiae. Observation of yeast cells with time-lapse microscopy revealed an increase of mother-bud neck size through the cell cycle. After screening of yeast non-essential gene-deletion mutants with the image processing software CalMorph, we comprehensively identified 274 mutants with broader necks during S/G2 phase. Among these yeasts, we extensively analyzed 19 representative deletion mutants with defects in genes annotated to six gene ontology terms (polarisome, actin reorganization, endosomal tethering complex, carboxy-terminal domain protein kinase complex, DNA replication, and maintenance of DNA trinucleotide repeats). The representative broad-necked mutants exhibited calcofluor white sensitivity, suggesting defects in their cell walls. Correlation analysis indicated that maintenance of mother-bud neck size is important for cellular processes such as cell growth, system robustness, and replicative lifespan. We conclude that neck-size maintenance in budding yeast is regulated by numerous genes and has several aspects that are physiologically significant.

A cell separation checkpoint that enforces the proper order of late cytokinetic events

Eukaryotic cell division requires dependency relationships in which late processes commence only after early ones are appropriately completed. We have discovered a system that blocks late events of cytokinesis until early ones are successfully accomplished. In budding yeast, cytokinetic actomyosin ring contraction and membrane ingression are coupled with deposition of an extracellular septum that is selectively degraded in its primary septum immediately after its completion by secreted enzymes. We find this secretion event is linked to septum completion and forestalled when the process is slowed. Delay of septum degradation requires Fir1, an intrinsically disordered protein localized to the cytokinesis site that is degraded upon septum completion but stabilized when septation is aberrant. Fir1 protects cytokinesis in part by inhibiting a separation-specific exocytosis function of the NDR/LATS kinase Cbk1, a key component of "hippo" signaling that induces mother-daughter separation. We term this system enforcement of cytokinesis order, a checkpoint ensuring proper temporal sequence of mechanistically incompatible processes of cytokinesis.

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.

Transcriptional responses of the marine green alga Micromonas pusilla and an infecting prasinovirus under different phosphate conditions

Prasinophytes are widespread marine algae for which responses to nutrient limitation and viral infection are not well understood. We studied the picoprasinophyte, Micromonas pusilla, grown under phosphate-replete (0.65 ± 0.07 d^-1 ) and 10-fold lower (low)-phosphate (0.11 ± 0.04 d^-1 ) conditions, and infected by the phycodnavirus MpV-SP1. Expression of 17% of Micromonas genes in uninfected cells differed by >1.5-fold (q < 0.01) between nutrient conditions, with genes for P-metabolism and the uniquely-enriched Sel1-like repeat (SLR) family having higher relative transcript abundances, while phospholipid-synthesis genes were lower in low-P than P-replete. Approximately 70% (P-replete) and 30% (low-P) of cells were lysed 24 h post-infection, and expression of ≤5.8% of host genes changed relative to uninfected treatments. Host genes for CAZymes and glycolysis were activated by infection, supporting importance in viral production, which was significantly lower in slower growing (low-P) hosts. All MpV-SP1 genes were expressed, and our analyses suggest responses to differing host-phosphate backgrounds involve few viral genes, while the temporal program of infection involves many more, and is largely independent of host-phosphate background. Our study (i) identifies genes previously unassociated with nutrient acclimation or viral infection, (ii) provides insights into the temporal program of prasinovirus gene expression by hosts and (iii) establishes cell biological aspects of an ecologically important host-prasinovirus system that differ from other marine algal-virus systems.