The Multiple Roles of Cyk1p in the Assembly and Function of the Actomyosin Ring in Budding Yeast

The budding yeast IQGAP-like protein Cyk1p/Iqg1p localizes to the mother-bud junction during anaphase and has been shown to be required for the completion of cytokinesis. In this study, video microscopy analysis of cells expressing green fluorescent protein-tagged Cyk1p/Iqg1p demonstrates that Cyk1p/Iqg1p is a dynamic component of the contractile ring during cytokinesis. Furthermore, in the absence of Cyk1p/Iqg1p, myosin II fails to undergo the contraction-like size change at the end of mitosis. To understand the mechanistic role of Cyk1p/Iqg1p in actomyosin ring assembly and dynamics, we have investigated the role of the structural domains that Cyk1p/Iqg1p shares with IQGAPs. An amino terminal portion containing the calponin homology domain binds to actin filaments and is required for the assembly of actin filaments to the ring. This result supports the hypothesis that Cyk1p/Iqg1p plays a direct role in F-actin recruitment. Deletion of the domain harboring the eight IQ motifs abolishes the localization of Cyk1p/Iqg1p to the bud neck, suggesting that Cyk1p/Iqg1p may be localized through interactions with a calmodulin-like protein. Interestingly, deletion of the COOH-terminal GTPase-activating protein-related domain does not affect Cyk1p/Iqg1p localization or actin recruitment to the ring but prevents actomyosin ring contraction. In vitro binding experiments show that Cyk1p/Iqg1p binds to calmodulin, Cmd1p, in a calcium-dependent manner, and to Tem1p, a small GTP-binding protein previously found to be required for the completion of anaphase. These results demonstrate the critical function of Cyk1p/Iqg1p in regulating various steps of actomyosin ring assembly and cytokinesis.     

Rho1 directs formin-mediated actin ring assembly during budding yeast cytokinesis

In eukaryotic cells, dynamic rearrangement of the actin cytoskeleton is critical for cell division. In the yeast Saccharomyces cerevisiae, three main structures constitute the actin cytoskeleton: cortical actin patches, cytoplasmic actin cables, and the actin-based cytokinetic ring. The conserved Arp2/3 complex and a WASP-family protein mediate actin patch formation, whereas the yeast formins (Bni1 and Bnr1) promote assembly of actin cables. However, the mechanism of actin ring formation is currently unclear. Here, we show that actin filaments are required for cytokinesis in S. cerevisiae, and that the actin ring is a highly dynamic structure that undergoes constant turnover. Assembly of the actin ring requires the formin-like proteins and profilin, but is not Arp2/3-mediated. Furthermore, the formin-dependent actin ring assembly pathway is regulated by the Rho-type GTPase Rho1 but not Cdc42. Finally, we show that the formins are not required for localization of Cyk1/Iqg1, an IQGAP-like protein previously shown to be required for actin ring formation, suggesting that formin-like proteins and Cyk1 act synergistically but independently in assembly of the actin ring.     

Identification of yeast IQGAP (Iqg1p) as an anaphase-promoting-complex substrate and its role in actomyosin-ring-independent cytokinesis

In the yeast Saccharomyces cerevisiae, a ring of myosin II forms in a septin-dependent manner at the budding site in late G1. This ring remains at the bud neck until the onset of cytokinesis, when actin is recruited to it. The actomyosin ring then contracts, septum formation occurs concurrently, and cytokinesis is soon completed. Deletion of MYO1 (the only myosin II gene) is lethal on rich medium in the W303 strain background and causes slow-growth and delayed-cell-separation phenotypes in the S288C strain background. These phenotypes can be suppressed by deletions of genes encoding nonessential components of the anaphase-promoting complex (APC/C). This suppression does not seem to result simply from a delay in mitotic exit, because overexpression of a nondegradable mitotic cyclin does not suppress the same phenotypes. Overexpression of either IQG1 or CYK3 also suppresses the myo1Delta phenotypes, and Iqg1p (an IQGAP protein) is increased in abundance and abnormally persistent after cytokinesis in APC/C mutants. In vitro assays showed that Iqg1p is ubiquitinated directly by APC/C(Cdh1) via a novel recognition sequence. A nondegradable Iqg1p (lacking this recognition sequence) can suppress the myo1Delta phenotypes even when expressed at relatively low levels. Together, the data suggest that compromise of APC/C function allows the accumulation of Iqg1p, which then promotes actomyosin-ring-independent cytokinesis at least in part by activation of Cyk3p.     

A myosin light chain mediates the localization of the budding yeast IQGAP-like protein during contractile ring formation

Cytokinesis in animal cells is accomplished through constriction of an actomyosin ring [1] [2] [3], which must assemble at the correct time and place in order to ensure proper division of genetic material and organelles. Budding yeast is a useful model system for determining the biochemical pathway of contractile ring assembly. The budding yeast IQGAP-like protein, Cyk1/Iqg1p, has multiple roles in the assembly and contraction of the actomyosin ring [4] [5] [6]. Previously, the IQ motifs of Cyk1/Iqg1p were shown to be required for the localization of this protein at the bud neck [6]. We have investigated the binding partner of the IQ motifs, which are predicted to interact with calmodulin-like proteins. Mlc1p was originally identified as a light chain for a type V myosin, Myo2p; however, a cytokinesis defect associated with disruption of the MLC1 gene suggested that the essential function of Mlc1p may involve interactions with other proteins [7]. We show that Mlc1p binds the IQ motifs of Cyk1/Iqg1p and present evidence that this interaction recruits Cyk1/Iqg1p to the bud neck. Immunofluorescence staining shows that Mlc1p is localized to sites of polarized cell growth as well as the bud neck before and independently of Cyk1p. These results demonstrate that Mlc1p is important for the assembly of the actomyosin ring in budding yeast and that this function is mediated through interaction with Cyk1/Iqg1p.     

Cyk3 acts in actomyosin ring independent cytokinesis by recruiting Inn1 to the yeast bud neck

Cytokinesis in yeast can be achieved by plasma membrane ingression, which is dependent on actomyosin ring constriction. Inn1 presumably couples these processes by interaction with both the plasma membrane and the temporary actomyosin ring component Hof1. In addition, an actomyosin ring independent cytokinesis pathway exists in yeast. We here identified Cyk3, a key component of the alternative pathway, as a novel interaction partner of Inn1. The carboxy-terminal proline rich part of Inn1 binds the SH3 domains of either Cyk3 or Hof1. Strains with truncated proteins lacking either of these SH3 domains do not display any severe phenotypes, but are synthetically lethal, demonstrating their crucial role in cytokinesis. Overexpression of CYK3 leads to an actomyosin ring independent recruitment of Inn1 to the bud neck, further supporting the significance of this interaction in vivo. Moreover, overexpression of CYK3 in a myo1 or an iqg1 deletion not only restores viability, but also the recruitment of Inn1 to the bud neck. We propose that Cyk3 is part of an actomyosin ring independent cytokinesis pathway, which acts as a rescue mechanism to recruit Inn1 to the bud neck.     

Sequential Assembly of Myosin II, an IQGAP-like Protein, and Filamentous Actin to a Ring Structure Involved in Budding Yeast Cytokinesis

We have identified a Saccharomyces cerevisiae protein, Cyk1p, that exhibits sequence similarity to the mammalian IQGAPs. Gene disruption of Cyk1p results in a failure in cytokinesis without affecting other events in the cell cycle. Cyk1p is diffused throughout most of the cell cycle but localizes to a ring structure at the mother–bud junction after the initiation of anaphase. This ring contains filamentous actin and Myo1p, a myosin II homologue. In vivo observation with green fluorescent protein–tagged Myo1p showed that the ring decreases drastically in size during cell division and therefore may be contractile. These results indicate that cytokinesis in budding yeast is likely to involve an actomyosin-based contractile ring. The assembly of this ring occurs in temporally distinct steps: Myo1p localizes to a ring that overlaps the septins at the G1-S transition slightly before bud emergence; Cyk1p and actin then accumulate in this ring after the activation of the Cdc15 pathway late in mitosis. The localization of myosin is abolished by a mutation in Cdc12p, implicating a role for the septin filaments in the assembly of the actomyosin ring. The accumulation of actin in the cytokinetic ring was not observed in cells depleted of Cyk1p, suggesting that Cyk1p plays a role in the recruitment of actin filaments, perhaps through a filament-binding activity similar to that demonstrated for mammalian IQGAPs.     

Dual Function of Cyk2, a cdc15/PSTPIP Family Protein,in Regulating Actomyosin Ring Dynamics and Septin Distribution

We previously showed that the budding yeast Saccharomyces cerevisiae assembles an actomyosin-based ring that undergoes a contraction-like size change during cytokinesis. To learn more about the biochemical composition and activity of this ring, we have characterized the in vivo distribution and function of Cyk2p, a budding yeast protein that exhibits significant sequence similarity to the cdc15/PSTPIP family of cleavage furrow proteins. Video microscopy of cells expressing green fluorescent protein (GFP)-tagged Cyk2p revealed that Cyk2p forms a double ring that coincides with the septins through most of the cell cycle. During cytokinesis, however, the Cyk2 double ring merges with the actomyosin ring and exhibits a contraction-like size change that is dependent on Myo1p. The septin double ring, in contrast, does not undergo the contraction-like size change but the separation between the two rings increases during cytokinesis. These observations suggest that the septin-containing ring is dynamically distinct from the actomyosin ring and that Cyk2p transits between the two types of structures. Gene disruption of CYK2 does not affect the assembly of the actomyosin ring but results in rapid disassembly of the ring during the contraction phase, leading to incomplete cytokinesis, suggesting that Cyk2p has an important function in modulating the stability of the actomyosin ring during contraction. Overexpression of Cyk2p also blocks cytokinesis, most likely due to a loss of the septins from the bud neck, indicating that Cyk2p may also play a role in regulating the localization of the septins.     

Fission yeast Cyk3p is a transglutaminase-like protein that participates in cytokinesis and cell morphogenesis

Cell morphogenesis is a complex process that relies on a diverse array of proteins and pathways. We have identified a transglutaminase-like protein (Cyk3p) that functions in fission yeast morphogenesis. The phenotype of a cyk3 knockout strain indicates a primary role for Cyk3p in cytokinesis. Correspondingly, Cyk3p localizes both to the actomyosin contractile ring and the division septum, promoting ring constriction, septation, and subsequent cell separation following ring disassembly. In addition, Cyk3p localizes to polarized growth sites and plays a role in cell shape determination, and it also appears to contribute to cell integrity during stationary phase, given its accumulation as dynamic puncta at the cortex of such cells. Our results and the conservation of Cyk3p across fungi point to a role in cell wall synthesis and remodeling. Cyk3p possesses a transglutaminase domain that is essential for function, even though it lacks the catalytic active site. In a wider sense, our work illustrates the physiological importance of inactive members of the transglutaminase family, which are found throughout eukaryotes. We suggest that the proposed evolution of animal transglutaminase cross-linking activity from ancestral bacterial thiol proteases was accompanied by the emergence of a subclass whose function does not depend on enzymatic activity.     

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.     

Iqg1p links spatial and secretion landmarks to polarity and cytokinesis

Cytokinesis requires the polarization of the actin cytoskeleton, the secretion machinery, and the correct positioning of the division axis. Budding yeast cells commit to their cytokinesis plane by choosing a bud site and polarizing their growth. Iqg1p (Cyk1p) was previously implicated in cytokinesis (Epp and Chant, 1997; Lippincott and Li, 1998; Osman and Cerione, 1998), as well as in the establishment of polarity and protein trafficking (Osman and Cerione, 1998). To better understand how Iqg1p influences these processes, we performed a two-hybrid screen and identified the spatial landmark Bud4p as a binding partner. Iqg1p can be coimmunoprecipitated with Bud4p, and Bud4p requires Iqg1p for its proper localization. Iqg1p also appears to specify axial bud-site selection and mediates the proper localization of the septin, Cdc12p, as well as binds and helps localize the secretion landmark, Sec3p. The double mutants iqg1Deltasec3Delta and bud4Deltasec3Delta display defects in polarity, budding pattern and cytokinesis, and electron microscopic studies reveal that these cells have aberrant septal deposition. Taken together, these findings suggest that Iqg1p recruits landmark proteins to form a targeting patch that coordinates axial budding with cytokinesis.     

An IQGAP-related protein controls actin-ring formation and cytokinesis in yeast

Background: Proteins of the IQGAP family have been identified as candidate effectors for the Rho family of GTPases; however, little is known about their cellular functions. The domain structures of IQGAP family members make them excellent candidates as regulators of the cytoskeleton: their sequences include an actin-binding domain homologous to that found in calponin, IQ motifs for interaction with calmodulin, and a GTPase-binding domain.Results: The genomic sequence of Saccharomyces cerevisiae revealed a single gene encoding an IQGAP family member (denoted IQGAP-related protein: Iqg1). Iqg1 and IQGAPs share similarity along their entire length, with an amino-terminal calponin-homology (CH) domain, IQ repeats, and a conserved carboxyl terminus. In contrast to IQGAPs, Iqg1 lacks an identifiable GAP motif, a WW domain, and IR repeats, although the functions of these domains in IQGAPs are not well defined. Deletion of the IQG1 gene resulted in lethality. Cellular defects included a deficiency in cytokinesis, altered actin organization, aberrant nuclear segregation, and cell lysis. The primary defect appeared to be a cytokinesis defect, and the other problems possibly arose as a consequence of this initial defect. Consistent with a role in cytokinesis, Iqg1 co-localizes with an actin ring encircling the mother–bud neck late in the cell cycle –a putative cytokinetic ring. IQG1 overexpression resulted in premature actin-ring formation, suggesting that Iqg1 activity temporally controls formation of this structure during the cell cycle.Conclusions: Yeast IQGAP-related protein, Iqg1, is an important regulator of cellular morphogenesis, inducing actin-ring formation in association with cytokinesis.     

A novel function of Saccharomyces cerevisiae CDC5 in cytokinesis

Coordination of mitotic exit with timely initiation of cytokinesis is critical to ensure completion of mitotic events before cell division. The Saccharomyces cerevisiae polo kinase Cdc5 functions in a pathway leading to the degradation of mitotic cyclin Clb2, thereby permitting mitotic exit. Here we provide evidence that Cdc5 also plays a role in regulating cytokinesis and that an intact polo-box, a conserved motif in the noncatalytic COOH-terminal domain of Cdc5, is required for this event. Depletion of Cdc5 function leads to an arrest in cytokinesis. Overexpression of the COOH-terminal domain of Cdc5 (cdc5DeltaN), but not the corresponding polo-box mutant, resulted in connected cells. These cells shared cytoplasms with incomplete septa, and possessed aberrant septin ring structures. Provision of additional copies of endogenous CDC5 remedied this phenotype, suggesting a dominant-negative inhibition of cytokinesis. The polo-box-dependent interactions between Cdc5 and septins (Cdc11 and Cdc12) and genetic interactions between the dominant-negative cdc5DeltaN and Cyk2/Hof1 or Myo1 suggest that direct interactions between cdc5DeltaN and septins resulted in inhibition of Cyk2/Hof1- and Myo1-mediated cytokinetic pathways. Thus, we propose that Cdc5 may coordinate mitotic exit with cytokinesis by participating in both anaphase promoting complex activation and a polo-box-dependent cytokinetic pathway.     

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.     

Saccharomyces cerevisiae Mob1p is required for cytokinesis and mitotic exit

The Saccharomyces cerevisiae mitotic exit network (MEN) is a conserved set of genes that mediate the transition from mitosis to G(1) by regulating mitotic cyclin degradation and the inactivation of cyclin-dependent kinase (CDK). Here, we demonstrate that, in addition to mitotic exit, S. cerevisiae MEN gene MOB1 is required for cytokinesis and cell separation. The cytokinesis defect was evident in mob1 mutants under conditions in which there was no mitotic-exit defect. Observation of live cells showed that yeast myosin II, Myo1p, was present in the contractile ring at the bud neck but that the ring failed to contract and disassemble. The cytokinesis defect persisted for several mitotic cycles, resulting in chains of cells with correctly segregated nuclei but with uncontracted actomyosin rings. The cytokinesis proteins Cdc3p (a septin), actin, and Iqg1p/ Cyk1p (an IQGAP-like protein) appeared to correctly localize in mob1 mutants, suggesting that MOB1 functions subsequent to actomyosin ring assembly. We also examined the subcellular distribution of Mob1p during the cell cycle and found that Mob1p first localized to the spindle pole bodies during mid-anaphase and then localized to a ring at the bud neck just before and during cytokinesis. Localization of Mob1p to the bud neck required CDC3, MEN genes CDC5, CDC14, CDC15, and DBF2, and spindle pole body gene NUD1 but was independent of MYO1. The localization of Mob1p to both spindle poles was abolished in cdc15 and nud1 mutants and was perturbed in cdc5 and cdc14 mutants. These results suggest that the MEN functions during the mitosis-to-G(1) transition to control cyclin-CDK inactivation and cytokinesis.     

The Saccharomyces cerevisiae actin cytoskeletal component Bsp1p has an auxiliary role in actomyosin ring function and in the maintenance of bud-neck structure

Iqg1p is a component of the actomyosin contractile ring that is required for actin recruitment and septum deposition. Cells lacking Iqg1p function have an altered bud-neck structure and fail to form a functional actomyosin contractile ring resulting in a block to cytokinesis and septation. Here it is demonstrated that increased expression of the actin cytoskeleton associated protein Bsp1p bypasses the requirement for contractile ring function. This also correlates with reduced bud-neck width and remedial septum formation. Increased expression of this protein in a temperature-sensitive iqg1-1 background causes remedial septum formation at the bud neck that is reliant upon chitin synthase III activity and restores cell separation. The observed suppression correlates with a restoration of normal bud-neck structure. While Bsp1p is a component of the contractile ring, its recruitment to the bud neck is not required for the observed suppression. Loss of Bsp1p causes a brief delay in the redistribution of the actin cytoskeleton normally observed at the end of actin ring contraction. Compromise of Iqg1p function, in the absence of Bsp1p function, leads to a profound change in the distribution of actin and the pattern of cell growth accompanied by a failure to complete cytokinesis and cell separation.     

An IQGAP-related protein,encoded by AgCYK1, is required for septation in the filamentous fungus Ashbya gossypii

In filamentous ascomycetes hyphae are compartmentalized by septation in which the cytoplasm of the compartments are interconnected via septal pores. Thus, septation in filamentous fungi is different from cytokinesis in yeast like fungi. We have identified an Ashbya gossypii orthologue of the Saccharomyces cerevisiae CYK1 gene which belongs to the IQGAP-protein family. In contrast to S. cerevisiae disruption of AgCYK1 yields viable mutant strains that exhibit wildtype-like polarized hyphal growth rates. In the Agcyk1 mutant cortical actin patches localize to growing hyphal tips like wildtype, however, mutant hyphae are totally devoid of actin rings at presumptive septal sites. Septation in wildtype results in the formation of chitin rings. Agcyk1 mutant hyphae are aseptate and do not accumulate chitin in their cell walls. Agcyk1 mutant strains are completely asporogenous indicating that septation is essential for the formation of sporangia in A. gossypii. AgCyk1p-GFP localizes to sites of future septation as a ring prior to chitin depositioning. Furthermore, decrease in Cyk1p-ring diameter was found to be a prerequisite for the accumulation of chitin and septum formation.     

Vrp1p functions in both actomyosin ring-dependent and Hof1p-dependent pathways of cytokinesis

Vrp1p/verprolin/End5p is a Saccharomyces cerevisiae proline-rich protein, structurally and functionally related to human Wiskott–Aldrich syndrome protein-interacting protein. Vrp1p is required for viability at 37°C, but not 24°C. Here, we show that loss of Vrp1p ( vrp1Δ ) leads to a 3–4-fold delay in cytokinesis, wide bud necks, abnormal actomyosin rings, and aberrant septa even at 24°C. Like other mutations affecting the actomyosin ring, vrp1Δ is synthetic lethal with deletion of HOF1 (or CYK2 ), which encodes a protein related to mammalian proline serine threonine phosphatase-interacting protein and Schizosaccharomyces pombe Cdc15p required for an actomyosin ring-independent pathway of cytokinesis in S. cerevisiae . At 37°C, vrp1Δ cells rapidly cease dividing and exhibit a novel terminal phenotype: a single large bud, two well-separated nuclei, and an interphase microtubule array. The arrested cells have a persistent ring containing both actin and myosin at the bud neck. Many also exhibit some polarisation of cortical actin patches to the bud neck. Vrp1p binds an SH3-domain-containing fragment of Hof1p in vitro . Vrp1p is required in vivo for Hof1p relocalisation to a single ring at the bud neck prior to cytokinesis at 37°C, but not at 24°C. Vrp1p thus acts in both actomyosin ring formation and function, as well as in Hof1p localisation during cytokinesis.     

Ingression Progression Complexes Control Extracellular Matrix Remodelling during Cytokinesis in Budding Yeast

Eukaryotic cells must coordinate contraction of the actomyosin ring at the division site together with ingression of the plasma membrane and remodelling of the extracellular matrix (ECM) to support cytokinesis, but the underlying mechanisms are still poorly understood. In eukaryotes, glycosyltransferases that synthesise ECM polysaccharides are emerging as key factors during cytokinesis. The budding yeast chitin synthase Chs2 makes the primary septum, a special layer of the ECM, which is an essential process during cell division. Here we isolated a group of actomyosin ring components that form complexes together with Chs2 at the cleavage site at the end of the cell cycle, which we named ‘ingression progression complexes’ (IPCs). In addition to type II myosin, the IQGAP protein Iqg1 and Chs2, IPCs contain the F-BAR protein Hof1, and the cytokinesis regulators Inn1 and Cyk3. We describe the molecular mechanism by which chitin synthase is activated by direct association of the C2 domain of Inn1, and the transglutaminase-like domain of Cyk3, with the catalytic domain of Chs2. We used an experimental system to find a previously unanticipated role for the C-terminus of Inn1 in preventing the untimely activation of Chs2 at the cleavage site until Cyk3 releases the block on Chs2 activity during late mitosis. These findings support a model for the co-ordinated regulation of cell division in budding yeast, in which IPCs play a central role.     

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.     

Contributions of turgor pressure, the contractile ring, and septum assembly to forces in cytokinesis in fission yeast

A paradigm of cytokinesis in animal cells is that the actomyosin contractile ring provides the primary force to divide the cell [1]. In the fission yeast Schizosaccharomyces pombe, cytokinesis also involves a conserved cytokinetic ring, which has been generally assumed to provide the force for cleavage [2-4] (see also [5]). However, in contrast to animal cells, cytokinesis in yeast cells also requires the assembly of a cell wall septum [6], which grows centripetally inward as the ring closes. Fission yeast, like other walled cells, also possess high (MPa) turgor pressure [7-9]. Here, we show that turgor pressure is an important factor in the mechanics of cytokinesis. Decreasing effective turgor pressure leads to an increase in cleavage rate, suggesting that the inward force generated by the division apparatus opposes turgor pressure. The contractile ring, which is predicted to provide only a tiny fraction of the mechanical stress required to overcome turgor, is largely dispensable for ingression; once septation has started, cleavage can continue in the absence of the contractile ring. Scaling arguments and modeling suggest that the large forces for cytokinesis are not produced by the contractile ring but are driven by the assembly of cell wall polymers in the growing septum.