Roles of Hof1p, Bni1p, Bnr1p, and myo1p in cytokinesis in Saccharomyces cerevisiae

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.

Saccharomyces cerevisiae Kelch proteins and Bud14 protein form a stable 520-kDa formin regulatory complex that controls actin cable assembly and cell morphogenesis

Formins perform essential roles in actin assembly and organization in vivo, but they also require tight regulation of their activities to produce properly functioning actin structures. Saccharomyces cerevisiae Bud14 is one member of an emerging class of formin regulators that target the FH2 domain to inhibit actin polymerization, but little is known about how these regulators are themselves controlled in vivo. Kelch proteins are critical for cell polarity and morphogenesis in a wide range of organisms, but their mechanistic roles in these processes are still largely undefined. Here, we report that S. cerevisiae Kelch proteins, Kel1 and Kel2, associate with Bud14 in cell extracts to form a stable 520-kDa complex with an apparent stoichiometry of 2:2:1 Bud14/Kel1/Kel2. Using pairwise combinations of GFP- and red fluorescent protein-tagged proteins, we show that Kel1, Kel2, and Bud14 interdependently co-localize at polarity sites. By analyzing single, double, and triple mutants, we show that Kel1 and Kel2 function in the same pathway as Bud14 in regulating Bnr1-mediated actin cable formation. Loss of any component of the complex results in long, bent, and hyper-stable actin cables, accompanied by defects in secretory vesicle traffic during polarized growth and septum formation during cytokinesis. These observations directly link S. cerevisiae Kelch proteins to the control of formin activity, and together with previous observations made for S. pombe homologues tea1p and tea3p, they have broad implications for understanding Kelch function in other systems.

The yeast actin cytoskeleton

The actin cytoskeleton is a complex network of dynamic polymers, which plays an important role in various fundamental cellular processes, including maintenance of cell shape, polarity, cell division, cell migration, endocytosis, vesicular trafficking, and mechanosensation. Precise spatiotemporal assembly and disassembly of actin structures is regulated by the coordinated activity of about 100 highly conserved accessory proteins, which nucleate, elongate, cross-link, and sever actin filaments. Both in vivo studies in a wide range of organisms from yeast to metazoans and in vitro studies of purified proteins have helped shape the current understanding of actin dynamics and function. Molecular genetics, genome-wide functional analysis, sophisticated real-time imaging, and ultrastructural studies in concert with biochemical analysis have made yeast an attractive model to understand the actin cytoskeleton, its molecular dynamics, and physiological function. Studies of the yeast actin cytoskeleton have contributed substantially in defining the universal mechanism regulating actin assembly and disassembly in eukaryotes. Here, we review some of the important insights generated by the study of actin cytoskeleton in two important yeast models the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe.

Aim44p regulates phosphorylation of Hof1p to promote contractile ring closure during cytokinesis in budding yeast

Aim44p undergoes septin-dependent localization to the actomyosin ring and regulates contractile ring closure and the abundance, phosphorylation, and dynamics of Hof1p, a regulator of actomyosin ring closure. It also interacts directly with Hof1p. Thus Aim44p is a novel regulator of contractile ring closure in budding yeast.

Whereas actomyosin and septin ring organization and function in cytokinesis are thoroughly described, little is known regarding the mechanisms by which the actomyosin ring interacts with septins and associated proteins to coordinate cell division. Here we show that the protein product of YPL158C, Aim44p, undergoes septin-dependent recruitment to the site of cell division. Aim44p colocalizes with Myo1p, the type II myosin of the contractile ring, throughout most of the cell cycle. The Aim44p ring does not contract when the actomyosin ring closes. Instead, it forms a double ring that associates with septin rings on mother and daughter cells after cell separation. Deletion of AIM44 results in defects in contractile ring closure. Aim44p coimmunoprecipitates with Hof1p, a conserved F-BAR protein that binds both septins and type II myosins and promotes contractile ring closure. Deletion of AIM44 results in a delay in Hof1p phosphorylation and altered Hof1p localization. Finally, overexpression of Dbf2p, a kinase that phosphorylates Hof1p and is required for relocalization of Hof1p from septin rings to the contractile ring and for Hof1p-triggered contractile ring closure, rescues the cytokinesis defect observed in aim44∆ cells. Our studies reveal a novel role for Aim44p in regulating contractile ring closure through effects on Hof1p.

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.

The formins Cdc12 and For3 cooperate during contractile ring assembly in cytokinesis

Both de novo-assembled actin filaments at the division site and existing filaments recruited by directional cortical transport contribute to contractile ring formation during cytokinesis. However, it is unknown which source is more important. Here, we show that fission yeast formin For3 is responsible for node condensation into clumps in the absence of formin Cdc12. For3 localization at the division site depended on the F-BAR protein Cdc15, and for3 deletion was synthetic lethal with mutations that cause defects in contractile ring formation. For3 became essential in cells expressing N-terminal truncations of Cdc12, which were more active in actin assembly but depended on actin filaments for localization to the division site. In tetrad fluorescence microscopy, double mutants of for3 deletion and cdc12 truncations were severely defective in contractile ring assembly and constriction, although cortical transport of actin filaments was normal. Together, these data indicate that different formins cooperate in cytokinesis and that de novo actin assembly at the division site is predominant for contractile ring formation.

Cell-cycle regulation of formin-mediated actin cable assembly

Assembly of appropriately oriented actin cables nucleated by formin proteins is necessary for many biological processes in diverse eukaryotes. However, compared with knowledge of how nucleation of dendritic actin filament arrays by the actin-related protein-2/3 complex is regulated, the in vivo regulatory mechanisms for actin cable formation are less clear. To gain insights into mechanisms for regulating actin cable assembly, we reconstituted the assembly process in vitro by introducing microspheres functionalized with the C terminus of the budding yeast formin Bni1 into extracts prepared from yeast cells at different cell-cycle stages. EM studies showed that unbranched actin filament bundles were reconstituted successfully in the yeast extracts. Only extracts enriched in the mitotic cyclin Clb2 were competent for actin cable assembly, and cyclin-dependent kinase 1 activity was indispensible. Cyclin-dependent kinase 1 activity also was found to regulate cable assembly in vivo. Here we present evidence that formin cell-cycle regulation is conserved in vertebrates. The use of the cable-reconstitution system to test roles for the key actin-binding proteins tropomyosin, capping protein, and cofilin provided important insights into assembly regulation. Furthermore, using mass spectrometry, we identified components of the actin cables formed in yeast extracts, providing the basis for comprehensive understanding of cable assembly and regulation.

Inhibition of Cdc42 during mitotic exit is required for cytokinesis

A decrease in Cdc42 activation during mitotic exit is necessary to allow localization of key cytokinesis regulators and proper septum formation.

The role of Cdc42 and its regulation during cytokinesis is not well understood. Using biochemical and imaging approaches in budding yeast, we demonstrate that Cdc42 activation peaks during the G₁/S transition and during anaphase but drops during mitotic exit and cytokinesis. Cdc5/Polo kinase is an important upstream cell cycle regulator that suppresses Cdc42 activity. Failure to down-regulate Cdc42 during mitotic exit impairs the normal localization of key cytokinesis regulators—Iqg1 and Inn1—at the division site, and results in an abnormal septum. The effects of Cdc42 hyperactivation are largely mediated by the Cdc42 effector p21-activated kinase Ste20. Inhibition of Cdc42 and related Rho guanosine triphosphatases may be a general feature of cytokinesis in eukaryotes.

Drosophila F-BAR protein Syndapin contributes to coupling the plasma membrane and contractile ring in cytokinesis

Cytokinesis is a highly ordered cellular process driven by interactions between central spindle microtubules and the actomyosin contractile ring linked to the dynamic remodelling of the plasma membrane. The mechanisms responsible for reorganizing the plasma membrane at the cell equator and its coupling to the contractile ring in cytokinesis are poorly understood. We report here that Syndapin, a protein containing an F-BAR domain required for membrane curvature, contributes to the remodelling of the plasma membrane around the contractile ring for cytokinesis. Syndapin colocalizes with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) at the cleavage furrow, where it directly interacts with a contractile ring component, Anillin. Accordingly, Anillin is mislocalized during cytokinesis in Syndapin mutants. Elevated or diminished expression of Syndapin leads to cytokinesis defects with abnormal cortical dynamics. The minimal segment of Syndapin, which is able to localize to the cleavage furrow and induce cytokinesis defects, is the F-BAR domain and its immediate C-terminal sequences. Phosphorylation of this region prevents this functional interaction, resulting in reduced ability of Syndapin to bind to and deform membranes. Thus, the dephosphorylated form of Syndapin mediates both remodelling of the plasma membrane and its proper coupling to the cytokinetic machinery.

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.

Ligand-induced activation of a formin-NPF pair leads to collaborative actin nucleation

Formins associate with other nucleators and nucleation-promoting factors (NPFs) to stimulate collaborative actin assembly, but the mechanisms regulating these interactions have been unclear. Yeast Bud6 has an established role as an NPF for the formin Bni1, but whether it also directly regulates the formin Bnr1 has remained enigmatic. In this paper, we analyzed NPF-impaired alleles of bud6 in a bni1Δ background and found that Bud6 stimulated Bnr1 activity in vivo. Furthermore, Bud6 bound directly to Bnr1, but its NPF effects were masked by a short regulatory sequence, suggesting that additional factors may be required for activation. We isolated a novel in vivo binding partner of Bud6, Yor304c-a/Bil1, which colocalized with Bud6 and functioned in the Bnr1 pathway for actin assembly. Purified Bil1 bound to the regulatory sequence in Bud6 and triggered NPF effects on Bnr1. These observations define a new mode of formin regulation, which has important implications for understanding NPF-nucleator pairs in diverse systems.

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.

Type II myosin gene in Fusarium graminearum is required for septation, development, mycotoxin biosynthesis and pathogenicity

Type II myosin is required for cytokinesis/septation in yeast and filamentous fungi, including Fusarium graminearum, a prevalent cause of Fusarium head blight in China. A type II myosin gene from the Chinese F. graminearum strain 5035, isolated from infected wheat spikes, was identified by screening a mutant library generated by restriction enzyme-mediated integration. Disruption of the Myo2 gene reduced mycelial growth by 50% and conidiation by 76-fold, and abolished sexual reproduction on wheat kernels. The Δmyo2 mutants also had a 97% decrease in their pathogenicity on wheat, and mycotoxin production fell to just 3.4% of the normal level. The distribution of nuclei and septa was abnormal in the mutants, and the septal ultrastructure appeared disorganized. Time-lapse imaging of septation provided direct evidence that Myo2 is required for septum initiation and formation, and revealed the dynamic behavior of GFP-tagged Myo2 during hyphal and macroconidia development, particularly in the delimiting septum of phialides and macroconidial spores. Microarray analysis identified many genes with altered expression profiles in the Δmyo2 mutant, indicating that Myo2 is required for several F. graminearum developmental processes and biological activities.

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.

Hof1 and Rvs167 have redundant roles in actomyosin ring function during cytokinesis in budding yeast

The Hof1 protein (Homologue of Fifteen) regulates formation of the primary septum during cytokinesis in the budding yeast Saccharomyces cerevisiae, whereas the orthologous Cdc15 protein in fission yeast regulates the actomyosin ring by using its F-BAR domain to recruit actin nucleators to the cleavage site. Here we show that budding yeast Hof1 also contributes to actin ring assembly in parallel with the Rvs167 protein. Simultaneous deletion of the HOF1 and RVS167 genes is lethal, and cells fail to assemble the actomyosin ring as they progress through mitosis. Although Hof1 and Rvs167 are not orthologues, they both share an analogous structure, with an F-BAR or BAR domain at the amino terminus, capable of inducing membrane curvature, and SH3 domains at the carboxyl terminus that bind to specific proline-rich targets. The SH3 domain of Rvs167 becomes essential for assembly of the actomyosin ring in cells lacking Hof1, suggesting that it helps to recruit a regulator of the actin cytoskeleton. This new function of Rvs167 appears to be independent of its known role as a regulator of the Arp2/3 actin nucleator, as actin ring assembly is not abolished by the simultaneous inactivation of Hof1 and Arp2/3. Instead we find that recruitment to the bud-neck of the Iqg1 actin regulator is defective in cells lacking Hof1 and Rvs167, though future studies will be needed to determine if this reflects a direct interaction between these factors. The redundant role of Hof1 in actin ring assembly suggests that the mechanism of actin ring assembly has been conserved to a greater extent across evolution than anticipated previously.

Dual function of the NDR-kinase Dbf2 in the regulation of the F-BAR protein Hof1 during cytokinesis

The conserved NDR-kinase Dbf2 plays a critical role in cytokinesis in budding yeast. Among its cytokinesis-related substrates is the F-BAR protein Hof1. Hof1 colocalizes at the cell division site with the septin complex and, as mitotic exit progresses, moves to the actomyosin ring (AMR). Neither the function of Hof1 at the septin complex nor the mechanism by which Hof1 supports AMR constriction is understood. Here we establish that Dbf2 has a dual function in Hof1 regulation. First, we show that the coiled-coil region, which is adjacent to the conserved F-BAR domain, is required for the binding of Hof1 to septins. The Dbf2-dependent phosphorylation of Hof1 at a single serine residue (serine 313) in this region diminishes the recruitment of Hof1 to septins both in vitro and in vivo. Genetic and functional analysis indicates that the binding of Hof1 to septins is important for septin rearrangement and integrity during cytokinesis. Furthermore, Dbf2 phosphorylation of Hof1 at serines 533 and 563 promotes AMR constriction most likely by inhibiting the SH3-domain-dependent interactions of Hof1. Thus our data show that Dbf2 coordinates septin and AMR functions during cytokinesis through the regulation/control of Hof1.

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.

Septin rings act as template for myosin higher-order structures and inhibit redundant polarity establishment

The mechanisms of the coordinated assembly and disassembly of the septin/myosin ring is central for the understanding of polar growth and cytokinesis in yeast and other organisms. The septin- and myosin-binding protein Bni5p provides a dual function during the formation and disassembly of septin/myosin rings. Early in the cell cycle Bni5p captures Myo1p at the incipient bud site and actively transforms it into higher-order structures. Additionally, Bni5p stabilizes the septin/myosin ring and is released from the septins shortly before the onset of cytokinesis. Once this Bni5p-dissociation from the septins is artificially prevented, ring disassembly is impaired and the untimely appearance of septin/myosin ring is induced. The prematurely formed septin/myosin rings delay the establishment of a new polarity axis and the progression into a new cell cycle. This observation suggests a negative feedback between septin/myosin ring formation and polarity establishment that might help to guarantee the singular assembly of this structure and the synchronization of its formation with the cell cycle.

Cdc42p regulation of the yeast formin Bni1p mediated by the effector Gic2p

Regulation of the formin Bni1p by Cdc42p in yeast does not require direct interaction between Bni1p and Cdc42p. The Cdc42p effector Gic2p can bind both Bni1p and GTP-Cdc42p, providing a novel regulatory input.

Actin filaments are dynamically reorganized to accommodate ever-changing cellular needs for intracellular transport, morphogenesis, and migration. Formins, a major family of actin nucleators, are believed to function as direct effectors of Rho GTPases, such as the polarity regulator Cdc42p. However, the presence of extensive redundancy has made it difficult to assess the in vivo significance of the low-affinity Rho GTPase–formin interaction and specifically whether Cdc42p polarizes the actin cytoskeleton via direct formin binding. Here we exploit a synthetically rewired budding yeast strain to eliminate the redundancy, making regulation of the formin Bni1p by Cdc42p essential for viability. Surprisingly, we find that direct Cdc42p–Bni1p interaction is dispensable for Bni1p regulation. Alternative paths linking Cdc42p and Bni1p via “polarisome” components Spa2p and Bud6p are also collectively dispensable. We identify a novel regulatory input to Bni1p acting through the Cdc42p effector, Gic2p. This pathway is sufficient to localize Bni1p to the sites of Cdc42p action and promotes a polarized actin organization in both rewired and wild-type contexts. We suggest that an indirect mechanism linking Rho GTPases and formins via Rho effectors may provide finer spatiotemporal control for the formin-nucleated actin cytoskeleton.

The Mitotic Exit Network and Cdc14 phosphatase initiate cytokinesis by counteracting CDK phosphorylations and blocking polarised growth

Polarisation of the actin cytoskeleton must cease during cytokinesis, to support efficient assembly and contraction of the actomyosin ring at the site of cell division, but the underlying mechanisms are still understood poorly in most species. In budding yeast, the Mitotic Exit Network (MEN) releases Cdc14 phosphatase from the nucleolus during anaphase, leading to the inactivation of mitotic forms of cyclin-dependent kinase (CDK) and the onset of septation, before G1-CDK can be reactivated and drive re-polarisation of the actin cytoskeleton to a new bud. Here, we show that premature inactivation of mitotic CDK, before release of Cdc14, allows G1-CDK to divert the actin cytoskeleton away from the actomyosin ring to a new site of polarised growth, thereby delaying progression through cytokinesis. Our data indicate that cells normally avoid this problem via the MEN-dependent release of Cdc14, which counteracts all classes of CDK-mediated phosphorylations during cytokinesis and blocks polarised growth. The dephosphorylation of CDK targets is therefore central to the mechanism by which the MEN and Cdc14 initiate cytokinesis and block polarised growth during late mitosis.

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.

Morphogenesis and the cell cycle

Studies of the processes leading to the construction of a bud and its separation from the mother cell in Saccharomyces cerevisiae have provided foundational paradigms for the mechanisms of polarity establishment, cytoskeletal organization, and cytokinesis. Here we review our current understanding of how these morphogenetic events occur and how they are controlled by the cell-cycle-regulatory cyclin-CDK system. In addition, defects in morphogenesis provide signals that feed back on the cyclin-CDK system, and we review what is known regarding regulation of cell-cycle progression in response to such defects, primarily acting through the kinase Swe1p. The bidirectional communication between morphogenesis and the cell cycle is crucial for successful proliferation, and its study has illuminated many elegant and often unexpected regulatory mechanisms. Despite considerable progress, however, many of the most puzzling mysteries in this field remain to be resolved.

Yeast formin Bni1p has multiple localization regions that function in polarized growth and spindle orientation

There are four distinct localization domains in formin Bni1p of budding yeast. Analysis of the functions of the domains in the actin cytoskeleton and in spindle orientation reveals unexpected complexity in the mechanism of formin localization and function.

Formins are conserved proteins that assemble unbranched actin filaments in a regulated, localized manner. Budding yeast's two formins, Bni1p and Bnr1p, assemble actin cables necessary for polarized cell growth and organelle segregation. Here we define four regions in Bni1p that contribute to its localization to the bud and at the bud neck. The first (residues 1–333) requires dimerization for its localization and encompasses the Rho-binding domain. The second (residues 334–821) covers the Diaphanous inhibitory–dimerization–coiled coil domains, and the third is the Spa2p-binding domain. The fourth region encompasses the formin homology 1–formin homology 2–COOH region of the protein. These four regions can each localize to the bud cortex and bud neck at the right stage of the cell cycle independent of both F-actin and endogenous Bni1p. The first three regions contribute cumulatively to the proper localization of Bni1p, as revealed by the effects of progressive loss of these regions on the actin cytoskeleton and fidelity of spindle orientation. The fourth region contributes to the localization of Bni1p in tiny budded cells. Expression of mislocalized Bni1p constructs has a dominant-negative effect on both growth and nuclear segregation due to mislocalized actin assembly. These results define an unexpected complexity in the mechanism of formin localization and function.

The myosin passenger protein Smy1 controls actin cable structure and dynamics by acting as a formin damper

Formins are a conserved family of proteins with robust effects in promoting actin nucleation and elongation. However, the mechanisms restraining formin activities in cells to generate actin networks with particular dynamics and architectures are not well understood. In S. cerevisiae, formins assemble actin cables, which serve as tracks for myosin-dependent intracellular transport. Here, we show that the kinesin-like myosin passenger-protein Smy1 interacts with the FH2 domain of the formin Bnr1 to decrease rates of actin filament elongation, which is distinct from the formin displacement activity of Bud14. In vivo analysis of smy1Δ mutants demonstrates that this "damper" mechanism is critical for maintaining proper actin cable architecture, dynamics, and function. We directly observe Smy1-3GFP being transported by myosin V and transiently pausing at the neck in a manner dependent on Bnr1. These observations suggest that Smy1 is part of a negative feedback mechanism that detects cable length and prevents overgrowth.

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.

Assembly and architecture of precursor nodes during fission yeast cytokinesis

Mapping of fission yeast precursor node interaction modules and assembly reveals important steps in contractile ring assembly.

The contractile ring is essential for cytokinesis in most fungal and animal cells. In fission yeast, cytokinesis nodes are precursors of the contractile ring and mark the future cleavage site. However, their assembly and architecture have not been well described. We found that nodes are assembled stoichiometrically in a hierarchical order with two modules linked by the positional marker anillin Mid1. Mid1 first recruits Cdc4 and IQGAP Rng2 to form module I. Rng2 subsequently recruits the myosin-II subunits Myo2 and Rlc1. Mid1 then independently recruits the F-BAR protein Cdc15 to form module II. Mid1, Rng2, Cdc4, and Cdc15 are stable node components that accumulate close to the plasma membrane. Both modules recruit the formin Cdc12 to nucleate actin filaments. Myo2 heads point into the cell interior, where they efficiently capture actin filaments to condense nodes into the contractile ring. Collectively, our work characterizing the assembly and architecture of precursor nodes defines important steps and molecular players for contractile ring assembly.

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.

Mitotic exit control of the Saccharomyces cerevisiae Ndr/LATS kinase Cbk1 regulates daughter cell separation after cytokinesis

Saccharomyces cerevisiae cell division ends with destruction of a septum deposited during cytokinesis; this must occur only after the structure's construction is complete. Genes involved in septum destruction are induced by the transcription factor Ace2, which is activated by the kinase Cbk1, an Ndr/LATS-related protein that functions in a system related to metazoan hippo pathways. Phosphorylation of a conserved hydrophobic motif (HM) site regulates Cbk1; at peak levels in late mitosis we found that approximately 3% of Cbk1 carries this modification. HM site phosphorylation prior to mitotic exit occurs in response to activation of the FEAR (Cdc fourteen early anaphase release) pathway. However, HM site phosphorylation is not sufficient for Cbk1 to act on Ace2: the kinase is also negatively regulated prior to cytokinesis, likely by cyclin-dependent kinase (CDK) phosphorylation. Cbk1 cannot phosphorylate Ace2 until after mitotic exit network (MEN)-initiated release of the phosphatase Cdc14. Treatment of Cbk1 with Cdc14 in vitro does not increase its intrinsic enzymatic activity, but Cdc14 is required for Cbk1 function in vivo. Thus, we propose that Cdc14 coordinates cell separation with mitotic exit via FEAR-initiated phosphorylation of the Cbk1 HM site and MEN-activated reversal of mitotic CDK phosphorylations that block both Cbk1 and Ace2 function.

The fungal type II myosin in Penicillium marneffei, MyoB, is essential for chitin deposition at nascent septation sites but not actin localization

Cytokinesis is essential for proliferative growth but also plays equally important roles during morphogenesis and development. The human pathogen Penicillium marneffei is capable of dimorphic switching in response to temperature, growing in a multicellular filamentous hyphal form at 25°C and in a unicellular yeast form at 37°C. P. marneffei also undergoes asexual development at 25°C to produce multicellular differentiated conidiophores. Thus, P. marneffei exhibits cell division with and without cytokinesis and division by budding and fission, depending on the cell type. The type II myosin gene, myoB, from P. marneffei plays important roles in the morphogenesis of these cell types. Deletion of myoB leads to chitin deposition defects at sites of cell division without perturbing actin localization. In addition to aberrant hyphal cells, distinct conidiophore cell types are lacking due to malformed septa and nuclear division defects. At 37°C, deletion of myoB prevents uninucleate yeast cell formation, instead producing long filaments resembling hyphae at 25°C. The ΔmyoB cells also often lyse due to defects in cell wall biogenesis. Thus, MyoB is essential for correct morphogenesis of all cell types regardless of division mode (budding or fission) and defines differences between the different types of growth.

Global analysis of Cdc14 phosphatase reveals diverse roles in mitotic processes

Cdc14 phosphatase regulates multiple events during anaphase and is essential for mitotic exit in budding yeast. Cdc14 is regulated in both a spatial and temporal manner. It is sequestered in the nucleolus for most of the cell cycle by the nucleolar protein Net1 and is released into the nucleus and cytoplasm during anaphase. To identify novel binding partners of Cdc14, we used affinity purification of Cdc14 and mass spectrometric analysis of interacting proteins from strains in which Cdc14 localization or catalytic activity was altered. To alter Cdc14 localization, we used a strain deleted for NET1, which causes full release of Cdc14 from the nucleolus. To alter Cdc14 activity, we generated mutations in the active site of Cdc14 (C283S or D253A), which allow binding of substrates, but not dephosphorylation, by Cdc14. Using this strategy, we identified new interactors of Cdc14, including multiple proteins involved in mitotic events. A subset of these proteins displayed increased affinity for catalytically inactive mutants of Cdc14 compared with the wild-type version, suggesting they are likely substrates of Cdc14. We have also shown that several of the novel Cdc14-interacting proteins, including Kar9 (a protein that orients the mitotic spindle) and Bni1 and Bnr1 (formins that nucleate actin cables and may be important for actomyosin ring contraction) are specifically dephosphorylated by Cdc14 in vitro and in vivo. Our findings suggest the dephosphorylation of the formins may be important for their observed localization change during exit from mitosis and indicate that Cdc14 targets proteins involved in wide-ranging mitotic events.

Conserved components, but distinct mechanisms for the placement and assembly of the cell division machinery in unicellular and filamentous ascomycetes

Cytokinesis is essential for cell proliferation, yet its molecular description is challenging, because >100 conserved proteins must be spatially and temporally co-ordinated. Despite the high importance of a tight co-ordination of cytokinesis with chromosome and organelle segregation, the mechanism for determining the cell division plane is one of the least conserved aspects of cytokinesis in eukaryotic cells. Budding and fission yeast have developed fundamentally distinct mechanisms to ensure proper nuclear segregation. The extent to which these pathways are conserved in multicellular fungi remains unknown. Recent progress indicates common components, but different mechanisms that are required for proper selection of the septation site in the different groups of Ascomycota. Cortical cues are used in yeast- and filament-forming species of the Saccharomycotina clade that are established at the incipient bud site or the hyphal tip respectively. In contrast, septum formation in the filament-forming Pezizomycotina species Aspergillus nidulans and Neurospora crassa seems more closely related to the fission yeast programme in that they may combine mitotic signals with a cell end-based marker system and Rho GTPase signalling. Thus, significant differences in the use and connection of conserved signalling modules become apparent that reflect the phylogenetic relationship of the analysed models.

Dephosphorylation of F-BAR protein Cdc15 modulates its conformation and stimulates its scaffolding activity at the cell division site

Cytokinesis in Schizosaccharomyces pombe requires the function of Cdc15, the founding member of the pombe cdc15 homology (PCH) family of proteins. As an early, abundant contractile ring component with multiple binding partners, Cdc15 plays a key role in organizing the ring. We demonstrate that Cdc15 phosphorylation at many sites generates a closed conformation, inhibits Cdc15 assembly at the division site in interphase, and precludes interaction of Cdc15 with its binding partners. Cdc15 dephosphorylation induces an open conformation, oligomerization, and scaffolding activity during mitosis. Cdc15 mutants with reduced phosphorylation precociously appear at the division site in filament-like structures and display increased association with protein partners and the membrane. Our results indicate that Cdc15 phosphoregulation impels both assembly and disassembly of the contractile apparatus and suggest a regulatory strategy that PCH family and BAR superfamily members might broadly employ to achieve temporal specificity in their roles as linkers between membrane and cytoskeleton.

Isolation and partial purification of the Saccharomyces cerevisiae cytokinetic apparatus

Cytokinesis is the process by which a cell physically divides in two at the conclusion of a cell cycle. In animal and fungal cells, this process is mediated by a conserved set of proteins including actin, type II myosin, IQGAP proteins, F-BAR proteins, and the septins. To facilitate biochemical and ultrastructural analysis of cytokinesis, we have isolated and partially purified the Saccharomyces cerevisiae cytokinetic apparatus. The isolated apparatus contains all components of the actomyosin ring for which we tested-actin, myosin heavy and light chain, and IQGAP-as well as septins and the cytokinetic F-BAR protein, Hof1p. We also present evidence indicating that the actomyosin rings associated with isolated cytokinetic apparati may be contractile in vitro, and show preliminary electron microscopic imaging of the cytokinetic apparatus. This first successful isolation of the cytokinetic apparatus from a genetically tractable organism promises to make possible a deeper understanding of cytokinesis.

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.

The germinal centre kinase Don3 triggers the dynamic rearrangement of higher-order septin structures during cytokinesis in Ustilago maydis

The dimorphic phytopathogenic fungus Ustilago maydis grows in its haploid phase by budding. Cytokinesis and separation of daughter cells are accomplished by the consecutive formation of two distinct septa. Here, we show that both septation events involve the dynamic rearrangement of septin assemblies from hourglass-shaped collars into ring-like structures. Using a chemical genetic approach we demonstrate that the germinal centre kinase Don3 triggers this septin reorganization during secondary septum formation. Although chemical inhibition of an analogue-sensitive version of Don3 prevented septation, a stable septin collar was assembled at the presumptive septation site. Interestingly, the essential light chain of type II myosin, Cdc4, was already associated with this septin collar. Release of Don3 kinase inhibition triggered immediate dispersal of septin filaments and concomitant incorporation of Cdc4 into a contractile actomyosin ring, which also contained the F-BAR domain protein Cdc15. Inhibition of actin polymerization or deletion of the cdc15 gene, did not affect assembly of the initial collar consisting of septin and myosin light chain. However, reassembly of septin filaments into a ring-like structure was prevented in the absence of either F-actin or Cdc15, indicating that septin ring formation in U. maydis depends on a functional contractile actomyosin ring.

Bayesian modeling of the yeast SH3 domain interactome predicts spatiotemporal dynamics of endocytosis proteins

A genome-scale specificity and interaction map for yeast SH3 domain-containing proteins reveal how family members show selective binding to target proteins and predicts the dynamic localization of new candidate endocytosis proteins.

SH3 domains are peptide recognition modules that mediate the assembly of diverse biological complexes. We scanned billions of phage-displayed peptides to map the binding specificities of the SH3 domain family in the budding yeast, Saccharomyces cerevisiae. Although most of the SH3 domains fall into the canonical classes I and II, each domain utilizes distinct features of its cognate ligands to achieve binding selectivity. Furthermore, we uncovered several SH3 domains with specificity profiles that clearly deviate from the two canonical classes. In conjunction with phage display, we used yeast two-hybrid and peptide array screening to independently identify SH3 domain binding partners. The results from the three complementary techniques were integrated using a Bayesian algorithm to generate a high-confidence yeast SH3 domain interaction map. The interaction map was enriched for proteins involved in endocytosis, revealing a set of SH3-mediated interactions that underlie formation of protein complexes essential to this biological pathway. We used the SH3 domain interaction network to predict the dynamic localization of several previously uncharacterized endocytic proteins, and our analysis suggests a novel role for the SH3 domains of Lsb3p and Lsb4p as hubs that recruit and assemble several endocytic complexes.

Significant diversity exists in protein structure and function, yet certain structural domains are used repeatedly across species to execute similar functions. The SH3 domain is one such common structural domain. It is found in signaling proteins and mediates protein–protein interactions by binding to short peptide sequences generally composed of proline. To investigate both the generality and selectivity of peptide binding by SH3 domains, we examined peptide specificity for almost all SH3 domains encoded within the proteome of the budding yeast, Saccharomyces cerevisiae, using a range of experimental methods. We found that although most of the intrinsic binding specificity for SH3 domains can be summarized by the two previously described canonical binding modes, each individual SH3 domain that we studied utilizes unique features of its cognate ligand to achieve binding selectivity. Moreover, some domains exhibit binding specificities that are distinct from the two canonical classes. We integrated peptide-SH3 domain binding data from three complementary screening techniques using a Bayesian statistical model to generate a protein–protein interaction network for the budding yeast SH3 domain family. This network was highly enriched in endocytosis proteins and their interactions. By examining these interactions in detail, we show that our SH3 domain network can be used to predict the temporal localization of several previously uncharacterized proteins to dynamic complexes that orchestrate the process of endocytosis.

The evolution, complex structures and function of septin proteins

The septin family is a conserved GTP-binding protein family and was originally discovered through genetic screening for budding yeast mutants. Septins are implicated in many cellular processes in fungi and metazoa. The function of septins usually depends on septin assembling into oligomeric complexes and highly ordered polymers. The expansion of the septin gene number in vertebrates increased the complex diversity of septins. In this review, we first discuss the evolution, structures and assembly of septin proteins in yeast and metazoa. Then, we review the function of septin proteins in cytokinesis, membrane remodeling and compartmentalization.

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.

Bub2 regulation of cytokinesis and septation in budding yeast

Background

The mitotic exit network (MEN) is required for events at the end of mitosis such as degradation of mitotic cyclins and cytokinesis. Bub2 and its binding partner Bfa1 act as a GTPase activating protein (GAP) to negatively regulate the MEN GTPase Tem1. The Bub2/Bfa1 checkpoint pathway is required to delay the cell cycle in response to mispositioned spindles. In addition to its role in mitotic exit, Tem1 is required for actomyosin ring contraction.

Results

To test the hypothesis that the Bub2 pathway prevents premature actin ring assembly, we compared the timing of actin ring formation in wild type, bub2Δ, mad2Δ, and bub2Δmad2Δ cells both with and without microtubules. There was no difference in the timing of actin ring formation between wild type and mutant cells in a synchronized cell cycle. In the presence of nocodazole, both bub2Δ and mad2Δ cells formed rings after a delay of the same duration. Double mutant bub2Δmad2Δ and bfa1Δmad2Δ cells formed rings at the same time with and without nocodazole. To determine if Bub2 has an effect on actomyosin ring contraction through its regulation of Tem1, we used live cell imaging of Myo1-GFP in a bub2Δ strain. We found a significant decrease in the total time of contraction and an increase in rate of contraction compared to wild type cells. We also examined myosin contraction using Myo1-GFP in cells overexpressing an epitope tagged Bub2. Surprisingly, overexpression of Bub2 also led to a significant increase in the rate of contraction, as well as morphological defects. The chained cell phenotype caused by Bub2 overexpression could be rescued by co-overexpression of Tem1, and was not rescued by deletion of BFA1.

Conclusion

Our data indicate that the Bub2 checkpoint pathway does not have a specific role in delaying actin ring formation. The observed increase in the rate of myosin contraction in the bub2Δ strain provides evidence that the MEN regulates actomyosin ring contraction. Our data suggest that the overexpression of the Bub2 fusion protein acts as a dominant negative, leading to septation defects by a mechanism that is Tem1-dependent.

Systems biology of the cell cycle of Saccharomyces cerevisiae: From network mining to system-level properties

Following a brief description of the operational procedures of systems biology (SB), the cell cycle of budding yeast is discussed as a successful example of a top-down SB analysis. After the reconstruction of the steps that have led to the identification of a sizer plus timer network in the G1 to S transition, it is shown that basic functions of the cell cycle (the setting of the critical cell size and the accuracy of DNA replication) are system-level properties, detected only by integrating molecular analysis with modelling and simulation of their underlying networks. A detailed network structure of a second relevant regulatory step of the cell cycle, the exit from mitosis, derived from extensive data mining, is constructed and discussed. To reach a quantitative understanding of how nutrients control, through signalling, metabolism and transcription, cell growth and cycle is a very relevant aim of SB. Since we know that about 900 gene products are required for cell cycle execution and control in budding yeast, it is quite clear that a purely systematic approach would require too much time. Therefore lines for a modular SB approach, which prioritises molecular and computational investigations for faster cell cycle understanding, are proposed. The relevance of the insight coming from the cell cycle SB studies in developing a new framework for tackling very complex biological processes, such as cancer and aging, is discussed.

Septins and the lateral compartmentalization of eukaryotic membranes

Eukaryotic cells from neurons and epithelial cells to unicellular fungi frequently rely on cellular appendages such as axons, dendritic spines, cilia, and buds for their biology. The emergence and differentiation of these appendages depend on the formation of lateral diffusion barriers at their bases to insulate their membranes from the rest of the cell. Here, we review recent progress regarding the molecular mechanisms and functions of such barriers. This overview underlines the importance and conservation of septin-dependent diffusion barriers, which coordinately compartmentalize both plasmatic and internal membranes. We discuss their role in memory establishment and the control of cellular aging.

Polarized growth in budding yeast in the absence of a localized formin

Polarity is achieved partly through the localized assembly of the cytoskeleton. During growth in budding yeast, the bud cortex and neck localized formins Bni1p and Bnr1p nucleate and assemble actin cables that extend along the bud-mother axis, providing tracks for secretory vesicle delivery. Localized formins are believed to determine the location and polarity of cables, hence growth. However, yeast expressing the nonlocalized actin nucleating/assembly formin homology (FH) 1-FH2 domains of Bnr1p or Bni1p as the sole formin grow well. Although cables are significantly disorganized, analysis of directed transport of secretory vesicles is still biased toward the bud, reflecting a bias in correctly oriented cables, thereby permitting polarized growth. Myosin II, localized at the bud neck, contributes to polarized growth as a mutant unable to interact with F-actin further compromises growth in cells with an unlocalized formin but not with a localized formin. Our results show that multiple mechanisms contribute to cable orientation and polarized growth, with localized formins and myosin II being two major contributors.

Single cells (put a ring on it)

Proper spatial and temporal regulation of the small GTPase RhoA at the equatorial cortex represents a critical step in the specification of the division plane in eukaryotes. Despite increased understanding of the mechanisms whereby RhoA becomes active following chromosome segregation, far less is known about how RhoA is spatially regulated so that it concentrates precisely at the division site. In the April 1, 2009, issue of Genes & Development, Yoshida and colleagues (pp. 810-823) uncovered two genetically separable mechanisms whereby Rho1 is recruited to the bud neck in the budding yeast Saccharomyces cerevisiae to facilitate cytokinesis.