Cytokinesis in budding yeast: the relationship between actomyosin ring function and septum formation

Unraveling the mechanisms and evolution of a two-domain module in IQGAP proteins for controlling eukaryotic cytokinesis

The IQGAP family of proteins plays a crucial role in cytokinesis across diverse organisms, but the underlying mechanisms are not fully understood. In this study, we demonstrate that IQGAPs in budding yeast, fission yeast, and human cells use a two-domain module to regulate their localization as well as the assembly and disassembly of the actomyosin ring during cytokinesis. Strikingly, the calponin homology domains (CHDs) in these IQGAPs bind to distinct cellular F-actin structures with varying specificity, whereas the non-conserved domains immediately downstream of the CHDs in these IQGAPs all target the division site, but differ in timing, localization strength, and binding partners. We also demonstrate that human IQGAP3 acts in parallel to septins and myosin-IIs to mediate the role of anillin in cytokinesis. Collectively, our findings highlight the two-domain mechanism by which IQGAPs regulate cytokinesis in distantly related organisms as well as their evolutionary conservation and divergence.

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.

•

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

•

Actomyosin ring constricts in a biphasic manner that is elaborately regulated

•

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

•

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

Comparative Analysis of the Roles of Non-muscle Myosin-IIs in Cytokinesis in Budding Yeast, Fission Yeast, and Mammalian Cells

The contractile ring, which plays critical roles in cytokinesis in fungal and animal cells, has fascinated biologists for decades. However, the basic question of how the non-muscle myosin-II and actin filaments are assembled into a ring structure to drive cytokinesis remains poorly understood. It is even more mysterious why and how the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe, and humans construct the ring structure with one, two, and three myosin-II isoforms, respectively. Here, we provide a comparative analysis of the roles of the non-muscle myosin-IIs in cytokinesis in these three model systems, with the goal of defining the common and unique features and highlighting the major questions regarding this family of proteins.

Cytokinesis in Eukaryotic Cells: The Furrow Complexity at a Glance

The duplication cycle is the fascinating process that, starting from a cell, results in the formation of two daughter cells and it is essential for life. Cytokinesis is the final step of the cell cycle, it is a very complex phase, and is a concert of forces, remodeling, trafficking, and cell signaling. All of the steps of cell division must be properly coordinated with each other to faithfully segregate the genetic material and this task is fundamental for generating viable cells. Given the importance of this process, molecular pathways and proteins that are involved in cytokinesis are conserved from yeast to humans. In this review, we describe symmetric and asymmetric cell division in animal cell and in a model organism, budding yeast. In addition, we illustrate the surveillance mechanisms that ensure a proper cell division and discuss the connections with normal cell proliferation and organs development and with the occurrence of human diseases.

Distinct Roles of Myosin-II Isoforms in Cytokinesis under Normal and Stressed Conditions

To address the question of why more than one myosin-II isoform is expressed in a single cell to drive cytokinesis, we analyzed the roles of the myosin-II isoforms, Myo2 and Myp2, of the fission yeast Schizosaccharomyces pombe, in cytokinesis under normal and stressed conditions. We found that Myp2 controls the disassembly, stability, and constriction initiation of the Myo2 ring in response to high-salt stress. A C-terminal coiled-coil domain of Myp2 is required for its immobility and contractility during cytokinesis, and when fused to the tail of the dynamic Myo2, renders the chimera the low-turnover property. We also found, by following distinct processes in real time at the single-cell level, that Myo2 and Myp2 are differentially required but collectively essential for guiding extracellular matrix remodeling during cytokinesis. These results suggest that the dynamic and immobile myosin-II isoforms are evolved to carry out cytokinesis with robustness under different growth conditions.

•

The myosin-II isoforms Myo2 and Myp2 display distinct responses to cellular stress

•

Myp2 controls the constriction initiation of Myo2 during stress response

•

A C-terminal region of Myp2 is required for its immobility during cytokinesis

•

Myo2 and Myp2 are differentially required for guiding ECM remodeling during cytokinesis

Mechanics and regulation of cytokinesis in budding yeast

Cytokinesis is essential for the survival of all organisms. It requires concerted functions of cell signaling, force production, exocytosis, and extracellular matrix remodeling. Due to the conservation in core components and mechanisms between fungal and animal cells, the budding yeast Saccharomyces cerevisiae has served as an attractive model for studying this fundamental process. In this review, we discuss the mechanics and regulation of distinct events of cytokinesis in budding yeast, including the assembly, constriction, and disassembly of the actomyosin ring, septum formation, abscission, and their spatiotemporal coordination. We also highlight the key concepts and questions that are common to animal and fungal cytokinesis.

Dephosphorylation of Iqg1 by Cdc14 regulates cytokinesis in budding yeast

Cytokinesis separates cells by contraction of a ring composed of filamentous actin (F-actin) and type II myosin. Iqg1, an IQGAP family member, is an essential protein in Saccharomyces cerevisiae required for assembly and contraction of the actomyosin ring. Localization of F-actin to the ring occurs only after anaphase and is mediated by the calponin homology domain (CHD) of Iqg1, but the regulatory mechanisms that temporally restrict actin ring assembly are not well defined. We tested the hypothesis that dephosphorylation of four perfect cyclin-dependent kinase (Cdk) sites flanking the CHD promotes actin ring formation, using site-specific alanine mutants. Cells expressing the nonphosphorylatable iqg1-4A allele formed actin rings before anaphase and exhibited defects in myosin contraction and cytokinesis. The Cdc14 phosphatase is required for normal cytokinesis and acts on specific Cdk phosphorylation sites. Overexpression of Cdc14 resulted in premature actin ring assembly, whereas inhibition of Cdc14 function prevented actin ring formation. Cdc14 associated with Iqg1, dependent on several CHD-flanking Cdk sites, and efficiently dephosphorylated these sites in vitro. Of importance, the iqg1-4A mutant rescued the inability of cdc14-1 cells to form actin rings. Our data support a model in which dephosphorylation of Cdk sites around the Iqg1 CHD by Cdc14 is both necessary and sufficient to promote actin ring formation. Temporal control of actin ring assembly by Cdk and Cdc14 may help to ensure that cytokinesis onset occurs after nuclear division is complete.

Myosin‑II heavy chain and formin mediate the targeting of myosin essential light chain to the division site before and during cytokinesis

MLC1 is a haploinsufficient gene encoding the essential light chain for Myo1, the sole myosin‑II heavy chain in the budding yeast Saccharomyces cerevisiae. Mlc1 defines an essential hub that coordinates actomyosin ring function, membrane trafficking, and septum formation during cytokinesis by binding to IQGAP, myosin‑II, and myosin‑V. However, the mechanism of how Mlc1 is targeted to the division site during the cell cycle remains unsolved. By constructing a GFP‑tagged MLC1 under its own promoter control and using quantitative live‑cell imaging coupled with yeast mutants, we found that septin ring and actin filaments mediate the targeting of Mlc1 to the division site before and during cytokinesis, respectively. Both mechanisms contribute to and are collectively required for the accumulation of Mlc1 at the division site during cytokinesis. We also found that Myo1 plays a major role in the septin‑dependent Mlc1 localization before cytokinesis, whereas the formin Bni1 plays a major role in the actin filament-dependent Mlc1 localization during cytokinesis. Such a two‑tiered mechanism for Mlc1 localization is presumably required for the ordered assembly and robustness of cytokinesis machinery and is likely conserved across species.

F-BAR domain protein Rga7 collaborates with Cdc15 and Imp2 to ensure proper cytokinesis in fission yeast

F-BAR domain proteins act as linkers between the cell cortex and cytoskeleton, and are involved in membrane binding and bending. Rga7 is one of the seven F-BAR proteins present in the fission yeast Schizosaccharomyces pombe. In addition to the F-BAR domain in the N-terminal region, Rga7 possesses a Rho GTPase-activating protein (GAP) domain at its C-terminus. We show here that Rga7 is necessary to prevent fragmentation of the contracting ring and incorrect septum synthesis. Accordingly, cultures of cells lacking Rga7 contain a higher percentage of dividing cells and more frequent asymmetric or aberrant septa, which ultimately might cause cell death. The Rga7 F-BAR domain is necessary for the protein localization to the division site and to the cell tips, and also for the Rga7 roles in cytokinesis. In contrast, Rga7 GAP catalytic activity seems to be dispensable. Moreover, we demonstrate that Rga7 cooperates with the two F-BAR proteins Cdc15 and Imp2 to ensure proper cytokinesis. We have also detected association of Rga7 with Imp2, and its binding partners Fic1 and Pxl1. Taken together, our findings suggest that Rga7 forms part of a protein complex that coordinates the late stages of cytokinesis.

The budding yeast amphiphysin complex is required for contractile actin ring (CAR) assembly and post-contraction GEF-independent accumulation of Rho1-GTP

The late events of the budding yeast cell division cycle, cytokinesis and cell separation, require the assembly of a contractile actomyosin ring (CAR), primary and secondary septum formation followed by enzymatic degradation of the primary septum. Here we present evidence that demonstrates a role for the budding yeast amphiphysin complex, a heterodimer comprising Rvs167 and Rvs161, in CAR assembly and cell separation. The iqg1-1 allele is synthetically lethal with both rvs167 and rvs161 null mutations. We show that both Iqg1 and the amphiphysin complex are required for CAR assembly in early anaphase but cells are able to complete assembly in late anaphase when these activities are, respectively, either compromised or absent. Amphiphysin dependent CAR assembly is dependent upon the Rvs167 SH3 domain, but this function is insufficient to explain the observed synthetic lethality. Dosage suppression of the iqg1-1 allele demonstrates that endocytosis is required for the default cell separation pathway in the absence of CAR contraction but is unlikely to be required to maintain viability. The amphiphysin complex is required for normal, post-mitotic, localization of Chs3 and the Rho1 GEF, Rom2, which are responsible for secondary septum deposition and the accumulation of GTP bound Rho1 at the bud neck. It is concluded that a failure of polarity establishment in the absence of CAR contraction and amphiphysin function leads to loss of viability as a result of the consequent cell separation defect.

Rho-GTPase effector ROCK phosphorylates cofilin in actin-meditated cytokinesis during mouse oocyte meiosis

During oocyte meiosis, a spindle forms in the central cytoplasm and migrates to the cortex. Subsequently, the oocyte extrudes a small body and forms a highly polarized egg; this process is regulated primarily by actin. ROCK is a Rho-GTPase effector that is involved in various cellular functions, such as stress fiber formation, cell migration, tumor cell invasion, and cell motility. In this study, we investigated possible roles for ROCK in mouse oocyte meiosis. ROCK was localized around spindles after germinal vesicle breakdown and was colocalized with cytoplasmic actin and mitochondria. Disrupting ROCK activity by RNAi or an inhibitor resulted in cell cycle progression and polar body extrusion failure. Time-lapse microscopy showed that this may have been due to spindle migration and cytokinesis defects, as chromosomes segregated but failed to extrude a polar body and then realigned. Actin expression at oocyte membranes and in cytoplasm was significantly decreased after these treatments. Actin caps were also disrupted, which was confirmed by a failure to form cortical granule-free domains. The mitochondrial distribution was also disrupted, which indicated that mitochondria were involved in the ROCK-mediated actin assembly. In addition, the phosphorylation levels of Cofilin, a downstream molecule of ROCK, decreased after disrupting ROCK activity. Thus, our results indicated that a ROCK-Cofilin-actin pathway regulated meiotic spindle migration and cytokinesis during mouse oocyte maturation.

Cdk1-dependent phosphorylation of Iqg1 governs actomyosin ring assembly prior to cytokinesis

Contraction of the actomyosin ring (AMR) provides the centripetal force that drives cytokinesis. In budding yeast (Saccharomyces cerevisiae), assembly and contraction of the AMR is coordinated with membrane deposition and septum formation at the bud neck. A central player in this process is Iqg1, which promotes recruitment of actin to the myosin ring and links AMR assembly with that of septum-forming components. We observed early actin recruitment in response to inhibition of cyclin-dependent kinase 1 (Cdk1) activity, and we find that the Cdk1-dependent phosphorylation state of Iqg1 is a determining factor in the timing of bud neck localization of both Iqg1 and actin, with both proteins accumulating prematurely in cells expressing nonphosphorylatable Iqg1 mutants. We also identified the primary septum regulator Hof1 as a binding partner of Iqg1, providing a regulatory link between the septation and contractile pathways that cooperate to complete cytokinesis.

Saccharomyces cerevisiae Dma proteins participate in cytokinesis by controlling two different pathways

Cytokinesis completion in the budding yeast S. cerevisiae is driven by tightly regulated pathways, leading to actomyosin ring contraction coupled to plasma membrane constriction and to centripetal growth of the primary septum, respectively. These pathways can partially substitute for each other, but their concomitant inactivation leads to cytokinesis block and cell death. Here we show that both the lack of the functionally redundant FHA-RING ubiquitin ligases Dma1 and Dma2 and moderate Dma2 overproduction affect actomyosin ring contraction as well as primary septum deposition, although they do not apparently alter cell cycle progression of otherwise wild-type cells. In addition, overproduction of Dma2 impairs the interaction between Tem1 and Iqg1, which is thought to be required for AMR contraction, and causes asymmetric primary septum deposition as well as mislocalization of the Cyk3-positive regulator of this process. In agreement with these multiple inhibitory effects, a Dma2 excess that does not cause any apparent defect in wild-type cells leads to lethal cytokinesis block in cells lacking the Hof1 protein, which is essential for primary septum formation in the absence of Cyk3. Altogether, these findings suggest that the Dma proteins act as negative regulators of cytokinesis.

Mitotic exit and separation of mother and daughter cells

Productive cell proliferation involves efficient and accurate splitting of the dividing cell into two separate entities. This orderly process reflects coordination of diverse cytological events by regulatory systems that drive the cell from mitosis into G1. In the budding yeast Saccharomyces cerevisiae, separation of mother and daughter cells involves coordinated actomyosin ring contraction and septum synthesis, followed by septum destruction. These events occur in precise and rapid sequence once chromosomes are segregated and are linked with spindle organization and mitotic progress by intricate cell cycle control machinery. Additionally, critical paarts of the mother/daughter separation process are asymmetric, reflecting a form of fate specification that occurs in every cell division. This chapter describes central events of budding yeast cell separation, as well as the control pathways that integrate them and link them with the cell cycle.

Targeting and functional mechanisms of the cytokinesis-related F-BAR protein Hof1 during the cell cycle

Hof1 targets to the division site by interacting with septins and myosin II sequentially during the cell cycle. It plays a role in cytokinesis by coupling actomyosin ring constriction to primary septum formation through interactions with Myo1 and Chs2.

F-BAR proteins are membrane‑associated proteins believed to link the plasma membrane to the actin cytoskeleton in cellular processes such as cytokinesis and endocytosis. In the budding yeast Saccharomyces cerevisiae, the F‑BAR protein Hof1 localizes to the division site in a complex pattern during the cell cycle and plays an important role in cytokinesis. However, the mechanisms underlying its localization and function are poorly understood. Here we show that Hof1 contains three distinct targeting domains that contribute to cytokinesis differentially. The N‑terminal half of Hof1 localizes to the bud neck and the sites of polarized growth during the cell cycle. The neck localization is mediated mainly by an interaction between the second coiled‑coil region in the N‑terminus and the septin Cdc10, whereas the localization to the sites of polarized growth is mediated entirely by the F‑BAR domain. In contrast, the C‑terminal half of Hof1 interacts with Myo1, the sole myosin‑II heavy chain in budding yeast, and localizes to the bud neck in a Myo1‑dependent manner from the onset to the completion of cytokinesis. We also show that the SH3 domain in the C‑terminus plays an important role in maintaining the symmetry of Myo1 ring constriction during cytokinesis and that Hof1 interacts with Chs2, a chitin synthase that is required for primary septum formation. Together these data define a mechanism that accounts for the localization of Hof1 during the cell cycle and suggest that Hof1 may function in cytokinesis by coupling actomyosin ring constriction to primary septum formation through interactions with Myo1 and Chs2.

Cytokinesis in plant male meiosis

In somatic cell division, cytokinesis is the final step of the cell cycle and physically divides the mother cytoplasm into two daughter cells. In the meiotic cell division, however, pollen mother cells (PMCs) undergo two successive nuclear divisions without an intervening S-phase and consequently generate four haploid daughter nuclei out of one parental cell. In line with this, the physical separation of meiotic nuclei does not follow the conventional cytokinesis pathway, but instead is mediated by alternative processes, including polar-based phragmoplast outgrowth and RMA-mediated cell wall positioning. In this review, we outline the different cytological mechanisms of cell plate formation operating in different types of PMCs and additionally focus on some important features associated with male meiotic cytokinesis, including cytoskeletal dynamics and callose deposition. We also provide an up-to-date overview of the main molecular actors involved in PMC wall formation and additionally highlight some recent advances on the effect of cold stress on meiotic cytokinesis in plants.

Immobile myosin-II plays a scaffolding role during cytokinesis in budding yeast

Core components of cytokinesis are conserved from yeast to human, but how these components are assembled into a robust machine that drives cytokinesis remains poorly understood. In this paper, we show by fluorescence recovery after photobleaching analysis that Myo1, the sole myosin-II in budding yeast, was mobile at the division site before anaphase and became immobilized shortly before cytokinesis. This immobility was independent of actin filaments or the motor domain of Myo1 but required a small region in the Myo1 tail that is thought to be involved in higher-order assembly. As expected, proteins involved in actin ring assembly (tropomyosin and formin) and membrane trafficking (myosin-V and exocyst) were dynamic during cytokinesis. Strikingly, proteins involved in septum formation (the chitin synthase Chs2) and/or its coordination with the actomyosin ring (essential light chain, IQGAP, F-BAR, etc.) displayed Myo1-dependent immobility during cytokinesis, suggesting that Myo1 plays a scaffolding role in the assembly of a cytokinesis machine.

Mutation of Hof1 PEST motif phosphorylation sites leads to retention of Hof1 at the bud neck and a decrease in the rate of myosin contraction

Regulation of actomyosin ring contraction is important for the coordination of cytokinesis with mitosis. Hof1, a member of the Pombe Cdc15 homology (PCH) family of proteins, is required for efficient cytokinesis in budding yeast. Phosphorylation of Hof1 depends on the mitotic exit network (MEN), and its degradation at the end of mitosis depends on its PEST motif and interaction with the E3 ligase Grr1. To test the hypothesis that targeted destruction of Hof1 temporally couples mitotic exit with contraction of the actomyosin ring, we mutated the Hof1 PEST motif to prevent phosphorylation and subsequent degradation. These mutations increased the amount of Hof1 at the bud neck during cytokinesis, resulted in smaller bud neck diameter, and slowed the rate of myosin contraction. However, Hof1 PEST motif phosphorylation site mutants did not have cytokinesis defects, indicating that regulation of Hof1 levels does not control the onset of actomyosin ring contraction as predicted.

Comparing contractile apparatus-driven cytokinesis mechanisms across kingdoms

Cytokinesis is the final stage of the cell cycle during which a cell physically divides into two daughters through the assembly of new membranes (and cell wall in some cases) between the forming daughters. New membrane assembly can either proceed centripetally behind a contractile apparatus, as in the case of prokaryotes, archaea, fungi, and animals or expand centrifugally, as in the case of higher plants. In this article, we compare the mechanisms of cytokinesis in diverse organisms dividing through the use of a contractile apparatus. While an actomyosin ring participates in cytokinesis in almost all centripetally dividing eukaryotes, the majority of bacteria and archaea (except Crenarchaea) divide using a ring composed of the tubulin-related protein FtsZ. Curiously, despite molecular conservation of the division machinery components, division site placement and its cell cycle regulation occur by a variety of unrelated mechanisms even among organisms from the same kingdom. While molecular motors and cytoskeletal polymer dynamics contribute to force generation during eukaryotic cytokinesis, cytoskeletal polymer dynamics alone appears to be sufficient for force generation during prokaryotic cytokinesis. Intriguingly, there are life forms on this planet that appear to lack molecules currently known to participate in cytokinesis and how these cells perform cytokinesis remains a mystery waiting to be unravelled.

Cell polarization and cytokinesis in budding yeast

Asymmetric cell division, which includes cell polarization and cytokinesis, is essential for generating cell diversity during development. The budding yeast Saccharomyces cerevisiae reproduces by asymmetric cell division, and has thus served as an attractive model for unraveling the general principles of eukaryotic cell polarization and cytokinesis. Polarity development requires G-protein signaling, cytoskeletal polarization, and exocytosis, whereas cytokinesis requires concerted actions of a contractile actomyosin ring and targeted membrane deposition. In this chapter, we discuss the mechanics and spatial control of polarity development and cytokinesis, emphasizing the key concepts, mechanisms, and emerging questions in the field.

Sho1p connects the plasma membrane with proteins of the cytokinesis network through multiple isomeric interaction states

An understanding of cytokinesis at the molecular level requires a detailed description of the protein complexes that perform central activities during this process. The proteins Hof1p, Cyk3p, Inn1p and Myo1p each represent one of the four genetically defined and partially complementary pathways of cytokinesis in the yeast Saccharomyces cerevisiae. Here we show that the osmosensor Sho1p is required for correct cell-cell separation. Shortly before cytokinesis Sho1p sequentially assembles with Hof1p, Inn1p and Cyk3p, into a complex (the HICS complex) that might help to connect the membrane with the actin-myosin ring. The HICS complex is formed exclusively through interactions between three SH3 domains located in Cyk3p, Hof1p and Sho1p, and five acceptor sites found in Cyk3p, Hof1p and Inn1p. Owing to the overlapping binding specificities of its members the HICS complex is best described as ensembles of isomeric interaction states that precisely coordinate the different functions of the interactors during cytokinesis.

Mitotic exit kinase Dbf2 directly phosphorylates chitin synthase Chs2 to regulate cytokinesis in budding yeast

How cell cycle machinery regulates extracellular matrix (ECM) remodeling during cytokinesis remains poorly understood. In the budding yeast Saccharomyces cerevisiae, the primary septum (PS), a functional equivalent of animal ECM, is synthesized during cytokinesis by the chitin synthase Chs2. Here, we report that Dbf2, a conserved mitotic exit kinase, localizes to the division site after Chs2 and directly phosphorylates Chs2 on several residues, including Ser-217. Both phosphodeficient (chs2-S217A) and phosphomimic (chs2-S217D) mutations cause defects in cytokinesis, suggesting that dynamic phosphorylation-dephosphorylation of Ser-217 is critical for Chs2 function. It is striking that Chs2-S217A constricts asymmetrically with the actomyosin ring (AMR), whereas Chs2-S217D displays little or no constriction and remains highly mobile at the division site. These data suggest that Chs2 phosphorylation by Dbf2 triggers its dissociation from the AMR during the late stage of cytokinesis. Of interest, both chs2-S217A and chs2-S217D mutants are robustly suppressed by increased dosage of Cyk3, a cytokinesis protein that displays Dbf2-dependent localization and also stimulates Chs2-mediated chitin synthesis. Thus Dbf2 regulates PS formation through at least two independent pathways: direct phosphorylation and Cyk3-mediated activation of Chs2. Our study establishes a mechanism for direct cell cycle control of ECM remodeling during cytokinesis.

Genetic interactions with mutations affecting septin assembly reveal ESCRT functions in budding yeast cytokinesis

Membrane trafficking via targeted exocytosis to the Saccharomyces cerevisiae bud neck provides new membrane and membrane-associated factors that are critical for cytokinesis. It remains unknown whether yeast plasma membrane abscission, the final step of cytokinesis, occurs spontaneously following extensive vesicle fusion, as in plant cells, or requires dedicated membrane fission machinery, as in cultured mammalian cells. Components of the endosomal sorting complexes required for transport (ESCRT) pathway, or close relatives thereof, appear to participate in cytokinetic abscission in various cell types, but roles in cell division had not been documented in budding yeast, where ESCRTs were first characterized. By contrast, the septin family of filament-forming cytoskeletal proteins were first identified by their requirement for yeast cell division. We show here that mutations in ESCRT-encoding genes exacerbate the cytokinesis defects of cla4Δ or elm1Δ mutants, in which septin assembly is perturbed at an early stage in cell division, and alleviate phenotypes of cells carrying temperature-sensitive alleles of a septin-encoding gene, CDC10. Elevated chitin synthase II (Chs2) levels coupled with aberrant morphogenesis and chitin deposition in elm1Δ cells carrying ESCRT mutations suggest that ESCRTs normally enhance the efficiency of cell division by promoting timely endocytic turnover of key cytokinetic enzymes.

Membrane shaping by the Bin/amphiphysin/Rvs (BAR) domain protein superfamily

BAR domain superfamily proteins have emerged as central regulators of dynamic membrane remodeling, thereby playing important roles in a wide variety of cellular processes, such as organelle biogenesis, cell division, cell migration, secretion, and endocytosis. Here, we review the mechanistic and structural basis for the membrane curvature-sensing and deforming properties of BAR domain superfamily proteins. Moreover, we summarize the present state of knowledge with respect to their regulation by autoinhibitory mechanisms or posttranslational modifications, and their interactions with other proteins, in particular with GTPases, and with membrane lipids. We postulate that BAR superfamily proteins act as membrane-deforming scaffolds that spatiotemporally orchestrate membrane remodeling.

Evidence that a septin diffusion barrier is dispensable for cytokinesis in budding yeast

Septins are essential for cytokinesis in Saccharomyces cerevisiae, but their precise roles remain elusive. Currently, it is thought that before cytokinesis, the hourglass-shaped septin structure at the mother-bud neck acts as a scaffold for assembly of the actomyosin ring (AMR) and other cytokinesis factors. At the onset of cytokinesis, the septin hourglass splits to form a double ring that sandwiches the AMR and may function as diffusion barriers to restrict diffusible cytokinesis factors to the division site. Here, we show that in cells lacking the septin Cdc10 or the septin-associated protein Bud4, the septins form a ring-like structure at the mother-bud neck that fails to re-arrange into a double ring early in cytokinesis. Strikingly, AMR assembly and constriction, the localization of membrane-trafficking and extracellular-matrix-remodeling factors, cytokinesis, and cell-wall-septum formation all occur efficiently in cdc10Δ and bud4Δ mutants. Thus, diffusion barriers formed by the septin double ring do not appear to be critical for S. cerevisiae cytokinesis. However, an AMR mutation and a septin mutation have synergistic effects on cytokinesis and the localization of cytokinesis proteins, suggesting that tethering to the AMR and a septin diffusion barrier may function redundantly to localize proteins to the division site.

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.

Phosphorylation-dependent regulation of the F-BAR protein Hof1 during cytokinesis

Spatial and timely coordination of cytokinesis is crucial for the maintenance of organelle inheritance and genome integrity. The mitotic exit network (MEN) pathway controls both the timely initiation of mitotic exit and cytokinesis in budding yeast. Here we identified the conserved F-BAR protein Hof1 as a substrate of the MEN kinase complex Dbf2-Mob1 during cytokinesis. We show that polo-like kinase Cdc5 first phosphorylates Hof1 to allow subsequent phosphorylation by Dbf2-Mob1. This releases Hof1 from the septin ring and facilitates Hof1 binding to the medial actomyosin ring (AMR), where Hof1 promotes AMR contraction and membrane ingression. Domain structure analysis established that the central, unstructured, region of Hof1, named the ring localization sequence (RLS), is sufficient to mediate Hof1's binding to the medial ring in a cell cycle-dependent manner. Genetic and functional data support a model in which Dbf2-Mob1 regulates Hof1 by inducing domain rearrangements, leading to the exposure of the Hof1 RLS domain during telophase.

Biphasic targeting and cleavage furrow ingression directed by the tail of a myosin II

Cytokinesis in animal and fungal cells utilizes a contractile actomyosin ring (AMR). However, how myosin II is targeted to the division site and promotes AMR assembly, and how the AMR coordinates with membrane trafficking during cytokinesis, remains poorly understood. Here we show that Myo1 is a two-headed myosin II in Saccharomyces cerevisiae, and that Myo1 localizes to the division site via two distinct targeting signals in its tail that act sequentially during the cell cycle. Before cytokinesis, Myo1 localization depends on the septin-binding protein Bni5. During cytokinesis, Myo1 localization depends on the IQGAP Iqg1. We also show that the Myo1 tail is sufficient for promoting the assembly of a "headless" AMR, which guides membrane deposition and extracellular matrix remodeling at the division site. Our study establishes a biphasic targeting mechanism for myosin II and highlights an underappreciated role of the AMR in cytokinesis beyond force generation.

Heterogeneous distribution of Candida albicans cell-surface antigens demonstrated with an Als1-specific monoclonal antibody

Despite an abundance of data describing expression of genes in the Candida albicans ALS (agglutinin-like sequence) gene family, little is known about the production of Als proteins on individual cells, their spatial localization or stability. Als proteins are most commonly discussed with respect to function in adhesion of C. albicans to host and abiotic surfaces. Development of a mAb specific for Als1, one of the eight large glycoproteins encoded by the ALS family, provided the opportunity to detect Als1 during growth of yeast and hyphae, both in vitro and in vivo, and to demonstrate the utility of the mAb in blocking C. albicans adhesion to host cells. Although most C. albicans yeast cells in a saturated culture are Als1-negative by indirect immunofluorescence, Als1 is detected on the surface of nearly all cells shortly after transfer into fresh growth medium. Als1 covers the yeast cell surface, with the exception of bud scars. Daughters of the inoculum cells, and sometimes granddaughters, also have detectable Als1, but Als1 is not detectable on cells from subsequent generations. On germ tubes and hyphae, most Als1 is localized proximal to the mother yeast. Once deposited on yeasts or hyphae, Als1 persists long after the culture has reached saturation. Growth stage-dependent production of Als1, coupled with its persistence on the cell surface, results in a heterogeneous population of cells within a C. albicans culture. Anti-Als1 immunolabelling patterns vary depending on the source of the C. albicans cells, with obvious differences between cells recovered from culture and those from a murine model of disseminated candidiasis. Results from this work highlight the temporal parallels for ALS1 expression and Als1 production in yeasts and germ tubes, the specialized spatial localization and persistence of Als1 on the C. albicans cell surface, and the differences in Als1 localization that occur in vitro and in vivo.

Spatial uncoupling of mitosis and cytokinesis during appressorium-mediated plant infection by the rice blast fungus Magnaporthe oryzae

To infect plants, many pathogenic fungi develop specialized infection structures called appressoria. Here, we report that appressorium development in the rice blast fungus Magnaporthe oryzae involves an unusual cell division, in which nuclear division is spatially uncoupled from the site of cytokinesis and septum formation. The position of the appressorium septum is defined prior to mitosis by formation of a heteromeric septin ring complex, which was visualized by spatial localization of Septin4:green fluorescent protein (GFP) and Septin5:GFP fusion proteins. Mitosis in the fungal germ tube is followed by long-distance nuclear migration and rapid formation of an actomyosin contractile ring in the neck of the developing appressorium, at a position previously marked by the septin complex. By contrast, mutants impaired in appressorium development, such as Deltapmk1 and DeltacpkA regulatory mutants, undergo coupled mitosis and cytokinesis within the germ tube. Perturbation of the spatial control of septation, by conditional mutation of the SEPTATION-ASSOCIATED1 gene of M. oryzae, prevented the fungus from causing rice blast disease. Overexpression of SEP1 did not affect septation during appressorium formation, but instead led to decoupling of nuclear division and cytokinesis in nongerminated conidial cells. When considered together, these results indicate that SEP1 is essential for determining the position and frequency of cell division sites in M. oryzae and demonstrate that differentiation of appressoria requires a cytokinetic event that is distinct from cell divisions within hyphae.

An emerging role for IQGAP1 in regulating protein traffic

IQGAP1, an effector of CDC42p GTPase, is a widely conserved, multifunctional protein that bundles F-actin through its N-terminus and binds microtubules through its C-terminus to modulate the cell architecture. It has emerged as a potential oncogene associated with diverse human cancers. Therefore, IQGAP1 has been heavily investigated; regardless, its precise cellular function remains unclear. Work from yeast suggests that IQGAP1 plays an important role in directed cell growth, which is a conserved feature crucial to morphogenesis, division axis, and body plan determination. New evidence suggests a conserved role for IQGAP1 in protein synthesis and membrane traffic, which may help to explain the diversity of its cellular functions. Membrane traffic mediates infections by intracellular pathogens and a range of degenerative human diseases arise from dysfunctions in intracellular traffic; thus, elucidating the mechanisms of cellular traffic will be important in order to understand the basis of a wide range of inherited and acquired human diseases. Recent evidence suggests that IQGAP1 plays its role in cell growth through regulating the conserved mTOR pathway. The mTOR signaling cascade has been implicated in membrane traffic and is activated in nearly all human cancers, but clinical response to the mTOR-specific inhibitor rapamycin has been disappointing. Thus, understanding the regulators of this pathway will be crucial in order to identify predictors of rapamycin sensitivity. In this review, I discuss emerging evidence that supports a potential role of IQGAP1 in regulating membrane traffic via regulating the mTOR pathway.

Targeted localization of Inn1, Cyk3 and Chs2 by the mitotic-exit network regulates cytokinesis in budding yeast

The mitotic-exit network (MEN) is a signaling pathway that is essential for the coordination of mitotic exit and cytokinesis. Whereas the role of the MEN in mitotic exit is well established, the molecular mechanisms by which MEN components regulate cytokinesis remain poorly understood. Here, we show that the MEN controls components involved in septum formation, including Inn1, Cyk3 and Chs2. MEN-deficient mutants, forced to exit mitosis as a result of Cdk1 inactivation, show defects in targeting Cyk3 and Inn1 to the bud-neck region. In addition, we found that the chitin synthase Chs2 did not efficiently localize at the bud neck in the absence of MEN activity. Ultrastructural analysis of the bud neck revealed that low MEN activity led to unilateral, uncoordinated extension of the primary and secondary septa. This defect was partially suppressed by increased levels of Cyk3. We therefore propose that the MEN directly controls cytokinesis via targeting of Inn1, Cyk3 and Chs2 to the bud neck.

Role of Inn1 and its interactions with Hof1 and Cyk3 in promoting cleavage furrow and septum formation in S. cerevisiae

Cytokinesis requires coordination of actomyosin ring (AMR) contraction with rearrangements of the plasma membrane and extracellular matrix. In Saccharomyces cerevisiae, new membrane, the chitin synthase Chs2 (which forms the primary septum [PS]), and the protein Inn1 are all delivered to the division site upon mitotic exit even when the AMR is absent. Inn1 is essential for PS formation but not for Chs2 localization. The Inn1 C-terminal region is necessary for localization, and distinct PXXP motifs in this region mediate functionally important interactions with SH3 domains in the cytokinesis proteins Hof1 (an F-BAR protein) and Cyk3 (whose overexpression can restore PS formation in inn1Δ cells). The Inn1 N terminus resembles C2 domains but does not appear to bind phospholipids; nonetheless, when overexpressed or fused to Hof1, it can provide Inn1 function even in the absence of the AMR. Thus, Inn1 and Cyk3 appear to cooperate in activating Chs2 for PS formation, which allows coordination of AMR contraction with ingression of the cleavage furrow.

Vesicular transport across the fungal cell wall

Recent findings indicate that fungi use vesicular transport to deliver substances across their cell walls. Fungal vesicles are similar to mammalian exosomes and could originate from cytoplasmic multivesicular bodies. Vesicular transport enables the export of large molecules across the cell wall, and vesicles contain lipids, proteins and polysaccharides, many of which are associated with virulence. Concentration of fungal products in vesicles could increase their efficiency in food acquisition and/or delivering potentially noxious substances to other cells, such as amoebae or phagocytes. The discovery of vesicular transport in fungi opens many new avenues for investigation in basic cell biology and pathogenesis.

Of bars and rings: Hof1-dependent cytokinesis in multiseptated hyphae of Ashbya gossypii

We analyzed the development of multiple septa in elongated multinucleated cells (hyphae) of the filamentous ascomycete Ashbya gossypii in which septation is apparently uncoupled from nuclear cycles. A key player for this compartmentalization is the PCH protein Hof1. Hyphae that are lacking this protein form neither actin rings nor septa but still elongate at wild-type speed. Using in vivo fluorescence microscopy, we present for the first time the coordination of cytokinesis and septation in multiseptated and multinucleated cells. Hof1, the type II myosin Myo1, the landmark protein Bud3, and the IQGAP Cyk1 form collars of cortical bars already adjacent to hyphal tips, thereby marking the sites of septation. While hyphae continue to elongate, these proteins gradually form cortical rings. This bar-to-ring transition depends on Hof1 and Cyk1 but not Myo1 and is required for actin ring assembly. The Fes/CIP4 homology (FCH) domain of Hof1 ensures efficient localization of Hof1, whereas ring integrity is conferred by the Src homology 3 (SH3) domain. Up to several hours after site selection, actin ring contraction leads to membrane invagination and subsequent cytokinesis. Simultaneously, a septum forms between the adjacent hyphal compartments, which do not separate. During evolution, A. gossypii lost the homologs of two enzymes essential for cell separation in Saccharomyces cerevisiae.

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 syp1Delta 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.

Shs1 plays separable roles in septin organization and cytokinesis in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, five septins (Cdc3, Cdc10, Cdc11, Cdc12, and Shs1/Sep7) form the septin ring at the bud neck during vegetative growth. We show here that disruption of SHS1 caused cold-sensitive growth in the W303 background, with cells arrested in chains, indicative of a cytokinesis defect. Surprisingly, the other four septins appeared to form an apparently normal septin ring in shs1Delta cells grown under the restrictive condition. We found that Myo1 and Iqg1, two components of the actomyosin contractile ring, and Cyk3, a component of the septum formation, were either delocalized or mislocalized in shs1Delta cells, suggesting that Shs1 plays supportive roles in cytokinesis. We also found that deletion of SHS1 enhanced or suppressed the septin defect in cdc10Delta and cdc11Delta cells, respectively, suggesting that Shs1 is involved in septin organization, exerting different effects on septin-ring assembly, depending on the composition of the septin subunits. Furthermore, we constructed an shs1-100c allele that lacks the coding sequence for the C-terminal 32 amino acids. This allele still displayed the genetic interactions with the septin mutants, but did not show cytokinesis defects as described above, suggesting that the roles of Shs1 in septin organization and cytokinesis are separable.

Localization of RHO-4 indicates differential regulation of conidial versus vegetative septation in the filamentous fungus Neurospora crassa

Rho-4 mutants of the filamentous fungus Neurospora crassa lack septa and asexual spores (conidia) and grow slowly. In this report, localization of green fluorescent protein-tagged RHO-4 is used to elucidate the differences in factors controlling RHO-4 localization during vegetative growth versus asexual development. RHO-4 forms a ring at incipient vegetative septation sites that constricts with the formation of the septum toward the septal pore; RHO-4 persists around the septal pore after septum completion. During the formation of conidia, RHO-4 localizes to the primary septum but subsequently is relocalized to the cytoplasm after the placement of the secondary septum. Cytoplasmic localization and inactivation of RHO-4 are mediated by a direct physical interaction with RDI-1, a RHO guanosine nucleotide dissociation inhibitor. Inappropriate activation of the cyclic AMP-dependent protein kinase A pathway during vegetative growth causes mislocalization of RHO-4 away from septa to the cytoplasm, a process which was dependent upon RDI-1. An adenylate cyclase cr-1 mutant partially suppresses the aconidial defect of rho-4 mutants but only rarely suppresses the vegetative septation defect, indicating that conidial septation is negatively regulated by CR-1. These data highlight the differences in the regulation of septation during conidiation versus vegetative septation in filamentous fungi.

Central roles of small GTPases in the development of cell polarity in yeast and beyond

SUMMARY

The establishment of cell polarity is critical for the development of many organisms and for the function of many cell types. A large number of studies of diverse organisms from yeast to humans indicate that the conserved, small-molecular-weight GTPases function as key signaling proteins involved in cell polarization. The budding yeast Saccharomyces cerevisiae is a particularly attractive model because it displays pronounced cell polarity in response to intracellular and extracellular cues. Cells of S. cerevisiae undergo polarized growth during various phases of their life cycle, such as during vegetative growth, mating between haploid cells of opposite mating types, and filamentous growth upon deprivation of nutrition such as nitrogen. Substantial progress has been made in deciphering the molecular basis of cell polarity in budding yeast. In particular, it becomes increasingly clear how small GTPases regulate polarized cytoskeletal organization, cell wall assembly, and exocytosis at the molecular level and how these GTPases are regulated. In this review, we discuss the key signaling pathways that regulate cell polarization during the mitotic cell cycle and during mating.

The yeast actin cytoskeleton: from cellular function to biochemical mechanism

All cells undergo rapid remodeling of their actin networks to regulate such critical processes as endocytosis, cytokinesis, cell polarity, and cell morphogenesis. These events are driven by the coordinated activities of a set of 20 to 30 highly conserved actin-associated proteins, in addition to many cell-specific actin-associated proteins and numerous upstream signaling molecules. The combined activities of these factors control with exquisite precision the spatial and temporal assembly of actin structures and ensure dynamic turnover of actin structures such that cells can rapidly alter their cytoskeletons in response to internal and external cues. One of the most exciting principles to emerge from the last decade of research on actin is that the assembly of architecturally diverse actin structures is governed by highly conserved machinery and mechanisms. With this realization, it has become apparent that pioneering efforts in budding yeast have contributed substantially to defining the universal mechanisms regulating actin dynamics in eukaryotes. In this review, we first describe the filamentous actin structures found in Saccharomyces cerevisiae (patches, cables, and rings) and their physiological functions, and then we discuss in detail the specific roles of actin-associated proteins and their biochemical mechanisms of action.

Four novel suppressors of gic1 gic2 and their roles in cytokinesis and polarized cell growth in Saccharomyces cerevisiae

Gic1 and Gic2 are two Cdc42/Rac interactive binding (CRIB) domain-containing effectors of Cdc42-GTPase that promote polarized cell growth in S. cerevisiae. To identify novel genes that functionally interact with Gic1 and Gic2, we screened for high-copy suppressors of a gic1 gic2 temperature-sensitive strain. We identified two pairs of structurally related genes, SKG6-TOS2 and VHS2-MLF3. These genes have been implicated in polarized cell growth, but their functions have not previously been characterized. We found that overproduction of Skg6 and Tos2 in wild-type cells causes aberrant localization of Cdc3 septin and actin structures as well as defective recruitment of Hof1 and impaired formation of the septum at the mother-bud neck. These data suggest a negative regulatory function for Skg6 and Tos2 in cytokinesis. Consistent with this model, deletion of SKG6 suppresses the growth defects associated with loss of HOF1, a positive regulator of cytokinesis. Our analysis of the second pair of gic1 gic2 suppressors, VHS2 and MLF3, suggests that they regulate polarization of the actin cytoskeleton and cell growth and function in a pathway distinct from and parallel to GIC1 and GIC2.

Exit from mitosis triggers Chs2p transport from the endoplasmic reticulum to mother-daughter neck via the secretory pathway in budding yeast

Budding yeast chitin synthase 2 (Chs2p), which lays down the primary septum, localizes to the mother–daughter neck in telophase. However, the mechanism underlying the timely neck localization of Chs2p is not known. Recently, it was found that a component of the exocyst complex, Sec3p–green fluorescent protein, arrives at the neck upon mitotic exit. It is not clear whether the neck localization of Chs2p, which is a cargo of the exocyst complex, was similarly regulated by mitotic exit. We report that Chs2p was restrained in the endoplasmic reticulum (ER) during metaphase. Furthermore, mitotic exit was sufficient to cause Chs2p neck localization specifically by triggering the Sec12p-dependent transport of Chs2p out of the ER. Chs2p was “forced” prematurely to the neck by mitotic kinase inactivation at metaphase, with chitin deposition occurring between mother and daughter cells. The dependence of Chs2p exit from the ER followed by its transport to the neck upon mitotic exit ensures that septum formation occurs only after the completion of mitotic events.

The fission yeast Chs2 protein interacts with the type-II myosin Myo3p and is required for the integrity of the actomyosin ring

In Schizosaccharomyces pombe cytokinesis requires the function of a contractile actomyosin ring. Fission yeast Chs2p is a transmembrane protein structurally similar to chitin synthases that lacks such enzymatic activity. Chs2p localisation and assembly into a ring that contracts during division requires the general system for polarised secretion, some components of the actomyosin ring, and an active septation initiation network. Chs2p interacts physically with the type-II myosin Myo3p revealing a physical link between the plasma membrane and the ring. In chs2Delta mutants, actomyosin ring integrity is compromised during the last stages of contraction and it remains longer in the midzone. In synchronous cultures, chs2Delta cells exhibit a delay in septation with respect to the control strain. All these results show that Chs2p participates in the correct functioning of the medial ring.

Actin and non-muscle myosin II facilitate apical exocytosis of tear proteins in rabbit lacrimal acinar epithelial cells

The acinar epithelial cells of the lacrimal gland exocytose the contents of mature secretory vesicles containing tear proteins at their apical membranes in response to secretagogues. Here we use time-lapse confocal fluorescence microscopy and fluorescence recovery after photobleaching to investigate the changes in actin filaments located beneath the apical membrane during exocytosis evoked by the muscarinic agonist, carbachol (100 microM). Time-lapse confocal fluorescence microscopy of apical actin filaments in reconstituted rabbit lacrimal acini transduced with replication-deficient adenovirus containing GFP-actin revealed a relatively quiescent apical actin array in resting acini. Carbachol markedly increased apical actin filament turnover and also promoted transient actin assembly around apparent fusion intermediates. Fluorescence recovery after photobleaching measurements revealed significant (P< or =0.05) increases and decreases, respectively, in mobile fraction (Mf) and turnover times (t1/2) for apical actin filaments in carbachol-stimulated acini relative to untreated acini. The myosin inhibitors, 2,3-butanedione monoxime (BDM, 10 mM, 15 minutes) and ML-7 (40 microM, 15 minutes), significantly decreased carbachol-stimulated secretion of bulk protein and the exogenous secretory vesicle marker, syncollin-GFP; these agents also promoted accumulation of actin-coated structures which were enriched, in transduced acini, in syncollin-GFP, confirming their identity as fusion intermediates. Actin-coated fusion intermediates were sized consistent with incorporation of multiple rather than single secretory vesicles; moreover, BDM and ML-7 caused a shift towards formation of multiple secretory vesicle aggregates while significantly increasing the diameter of actin-coated fusion intermediates. Our findings suggest that the increased turnover of apical actin filaments and the interaction of actin with non-muscle myosin II assembled around aggregates of secretory vesicles facilitate exocytosis in lacrimal acinar epithelial cells.

Split decisions: coordinating cytokinesis in yeast

Cytokinesis in eukaryotes involves the regulated assembly and contraction of a ring comprising filamentous (F)-actin and myosin II. Assembly of the contractile ring occurs through the accumulation of cortical cues at the specified division plane, followed by recruitment of F-actin, myosin II and accessory proteins involved in generating the mature ring. Ring contraction is temporally regulated to occur only after chromosome segregation and, in yeast, it is controlled by a conserved signaling cascade that becomes active only after Cdk1-Cyclin-B inactivation. In this article (which is part of the Cytokinesis series), we discuss recent studies that have begun to clarify both the spatial and the temporal order of ring assembly and that have illuminated the signals that trigger ring contraction in yeast. These studies add to the growing knowledge of the processes that control eukaryotic cell division.

Severing all ties between mother and daughter: cell separation in budding yeast

At the end of nuclear division in the budding yeast, acto-myosin ring contraction and cytokinesis occur between mother and daughter cells. This is followed by cell separation, after which mother and daughter cells go their separate ways. While cell separation may be the last event that takes place between the two cells, it is nonetheless under tight regulation which ensures that both cells are viable upon separation. It is becoming increasingly clear that the components of the cell separation machinery are controlled at various levels, including the temporal and spatial regulation of the genes encoding for the components and the specific localization of the components to the neck. In addition, these regulatory controls are co-ordinated with exit from mitosis, thereby placing a mechanistic link between the end of mitosis and cell separation. More importantly, the success of the cell separation event is contingent upon the presence of a trilaminar septum, whose assembly is dependent on a host of proteins which localize to the neck over the span of one cell division cycle.