Fission yeast Pob1p, which is homologous to budding yeast Boi proteins and exhibits subcellular localization close to actin patches, is essential for cell elongation and separation

Generation and characterization of conditional yeast mutants affecting each of the 2 essential functions of the scaffolding proteins Boi1/2 and Bem1

Boi1 and Boi2 are closely related yeast scaffolding proteins, either of which can perform an essential function. Previous studies have suggested a role in cell polarity, interacting with lipids, components of the late secretory pathway, and actin nucleators. We report detailed studies of their localization, dynamics, and the generation and characterization of conditional mutants. Boi1/2 are present on the plasma membrane in dynamic patches, then at the bud neck during cytokinesis. These distributions are unaffected by perturbation of the actin cytoskeleton or the secretory pathway. We identify 2 critical aromatic residues, present in both Boi1 and Boi2, in the essential C-terminal Pleckstrin-Homology domain, that cause temperature-sensitive growth resulting in defects in polarized growth leading to cell lysis. The scaffolding protein, Bem1, colocalizes with Boi1 in patches at the growing bud, and at the bud neck, the latter requiring the N-terminal SH3 domain of Boi1p. Loss of function of Boi1-SH3 domain renders Bem1 essential, which can be fully replaced by a fusion of the SH3b and PB1 domains of Bem1. Thus, the 2 essential functions of the Boi1/2/Bem1 proteins can be satisfied by Bem1-SH3b-PB1 and Boi1-Pleckstrin-Homology. Generation and characterization of conditional mutations in the essential function of Bem1 reveal a slow onset of defects in polarized growth, which is difficult to define a specific initial defect. This study provides more details into the functions of Boi1/2 and their relationship with Bem1 and presents the generation of conditional mutants that will be useful for future genetic analysis.

Roles of the PH, coiled-coil and SAM domains of the yeast polarity protein Boi2 in polarity-site localization and function in polarized growth

Boi1 and Boi2 are paralogous proteins essential for bud formation in budding yeast. So far, the domains that target Boi1/Boi2 to the polarity sites and function in bud formation are not well understood. Here, we report that a coiled-coil domain of Boi2 cooperates with the adjacent PH domain to confer Boi2's bud-cortex localization and major function in cell growth. The PH domain portion of the PH-CC bi-domain interacts with the Rho GTPases Cdc42 and Rho3 and both interactions are independent of the GTP/GDP-bound state of each GTPase. Interestingly, high-copy RHO3 and BOI2 but not CDC42 suppressed the growth defect of RGA1-C538 overexpression and the sec15-1 mutant and this BOI2 function depends on RHO3, suggesting that Boi2 may function in the Rho3 pathway. The SAM domain of Boi2 plays an essential role in high-copy suppression of the two mutants as well as in the early bud-neck localization of Boi2. The SAM domain and the CC domain also interact homotypically. They are likely involved in the formation of Boi2-containing protein complex. Our results provide new insights in the localization and function of Boi2 and highlight the importance of the PH-CC bi-domain and the SAM domain in Boi2's localization and function.

Rho Family GTPases in Fission Yeast Cytokinesis

During cytokinesis, actomyosin ring constriction drives furrow formation. In animal cells, Rho GTPases drive this process through the positioning and assembly of the actomyosin ring, and through extracellular matrix remodeling within the furrow. In the fission yeast S. pombe, actomyosin ring constriction and septum formation are concurrent processes. While S. pombe is the primary source from which the mechanics of ring assembly and constriction stem, much less is known about the regulation of Rho GTPases that control these processes. Of the six Rho GTPases encoded in S. pombe, only Rho1, the RhoA homologue, has been shown to be essential for cytokinesis. While Rho3, Rho4, and Cdc42 have defined roles in cytokinesis, Rho2 and Rho5 play minor to no roles in this process. Here we review the roles of the Rho GTPases during cytokinesis, with a focus on their regulation, and discuss whether crosstalk between GTPases, as has been reported in other organisms, exists during cytokinesis in S. pombe.

Sterol-Rich Membrane Domains Define Fission Yeast Cell Polarity

Cell polarization is crucial for the functioning of all organisms. The cytoskeleton is central to the process but its role in symmetry breaking is poorly understood. We study cell polarization when fission yeast cells exit starvation. We show that the basis of polarity generation is de novo sterol biosynthesis, cell surface delivery of sterols, and their recruitment to the cell poles. This involves four phases occurring independent of the polarity factor cdc42p. Initially, multiple, randomly distributed sterol-rich membrane (SRM) domains form at the plasma membrane, independent of the cytoskeleton and cell growth. These domains provide platforms on which the growth and polarity machinery assembles. SRM domains are then polarized by the microtubule-dependent polarity factor tea1p, which prepares for monopolar growth initiation and later switching to bipolar growth. SRM polarization requires F-actin but not the F-actin organizing polarity factors for3p and bud6p. We conclude that SRMs are key to cell polarization.

The coordination of cell growth during fission yeast mating requires Ras1-GTP hydrolysis

The spatial and temporal control of polarity is fundamental to the survival of all organisms. Cells define their polarity using highly conserved mechanisms that frequently rely upon the action of small GTPases, such as Ras and Cdc42. Schizosaccharomyces pombe is an ideal system with which to study the control of cell polarity since it grows from defined tips using Cdc42-mediated actin remodeling. Here we have investigated the importance of Ras1-GTPase activity for the coordination of polarized cell growth during fission yeast mating. Following pheromone stimulation, Ras1 regulates both a MAPK cascade and the activity of Cdc42 to enable uni-directional cell growth towards a potential mating partner. Like all GTPases, when bound to GTP, Ras1 adopts an active conformation returning to an inactive state upon GTP-hydrolysis, a process accelerated through interaction with negative regulators such as GAPs. Here we show that, at low levels of pheromone stimulation, loss of negative regulation of Ras1 increases signal transduction via the MAPK cascade. However, at the higher concentrations observed during mating, hyperactive Ras1 mutations promote cell death. We demonstrate that these cells die due to their failure to coordinate active Cdc42 into a single growth zone resulting in disorganized actin deposition and unsustainable elongation from multiple tips. These results provide a striking demonstration that the deactivation stage of Ras signaling is fundamentally important in modulating cell polarity.

Pob1 ensures cylindrical cell shape by coupling two distinct rho signaling events during secretory vesicle targeting

Proper cell morphogenesis requires the co-ordination of cell polarity, cytoskeletal organization and vesicle trafficking. The Schizosaccharomyces pombe mutant pob1-664 has a curious lemon-like shape, the basis of which is not understood. Here, we found abundant vesicle accumulation in these cells, suggesting that Pob1 plays a role in vesicle trafficking. We identified Rho3 as a multicopy suppressor of this phenotype. Because Rho3 function is related to For3, an actin-polymerizing protein, and Sec8, a component of the exocyst complex, we analyzed their functional relationship with Pob1. Pob1 was essential for the formation of actin cables (by interacting with For3) and for the polarized localization of Sec8. Although neither For3 nor Sec8 is essential for polarized growth, their simultaneous disruption prevented tip growth and yielded a lemon-like cell morphology similar to pob1-664. Thus, Pob1 may ensure cylindrical cell shape of S. pombe by coupling actin-mediated vesicle transport and exocyst-mediated vesicle tethering during secretory vesicle targeting.

A global census of fission yeast deubiquitinating enzyme localization and interaction networks reveals distinct compartmentalization profiles and overlapping functions in endocytosis and polarity

Proteomic, localization, and enzymatic activity screens in fission yeast reveal how deubiquitinating enzyme localization and function are tuned.

Ubiquitination and deubiquitination are reciprocal processes that tune protein stability, function, and/or localization. The removal of ubiquitin and remodeling of ubiquitin chains is catalyzed by deubiquitinating enzymes (DUBs), which are cysteine proteases or metalloproteases. Although ubiquitination has been extensively studied for decades, the complexity of cellular roles for deubiquitinating enzymes has only recently been explored, and there are still several gaps in our understanding of when, where, and how these enzymes function to modulate the fate of polypeptides. To address these questions we performed a systematic analysis of the 20 Schizosaccharomyces pombe DUBs using confocal microscopy, proteomics, and enzymatic activity assays. Our results reveal that S. pombe DUBs are present in almost all cell compartments, and the majority are part of stable protein complexes essential for their function. Interestingly, DUB partners identified by our study include the homolog of a putative tumor suppressor gene not previously linked to the ubiquitin pathway, and two conserved tryptophan-aspartate (WD) repeat proteins that regulate Ubp9, a DUB that we show participates in endocytosis, actin dynamics, and cell polarity. In order to understand how DUB activity affects these processes we constructed multiple DUB mutants and find that a quintuple deletion of ubp4 ubp5 ubp9 ubp15 sst2/amsh displays severe growth, polarity, and endocytosis defects. This mutant allowed the identification of two common substrates for five cytoplasmic DUBs. Through these studies, a common regulatory theme emerged in which DUB localization and/or activity is modulated by interacting partners. Despite apparently distinct cytoplasmic localization patterns, several DUBs cooperate in regulating endocytosis and cell polarity. These studies provide a framework for dissecting DUB signaling pathways in S. pombe and may shed light on DUB functions in metazoans.

The post-translational modification of proteins by conjugation of monomers or chains of ubiquitin is a regulatory mechanism for tuning protein stability, localization and function. Given these vital functions, ubiquitination has to be highly regulated so that protein degradation and cell signaling are controlled in space and time. Although the ubiquitin-conjugation machinery has been thoroughly studied, there are still several gaps in our understanding of when, where and how ubiquitin is removed by deubiquitinating enzymes (DUBs). To address these questions we performed a systematic analysis of the 20 DUBs in the fission yeast Schizosaccharomyces pombe using confocal microscopy, proteomics and enzymatic activity assays. We first showed that S. pombe DUBs are present in almost all cell compartments and that the majority are part of stable protein complexes essential for their function. Then, we constructed strains mutant for a number of the DUBs involved in the newly identified protein complexes and showed that five cytoplasmic DUBs have redundant roles in controlling endocytosis and cell polarity. We postulate that regulatory networks identified in our study might be conserved and hence shed light on DUB function in metazoans.

Cooperation between the septins and the actomyosin ring and role of a cell-integrity pathway during cell division in fission yeast

A major question about cytokinesis concerns the role of the septin proteins, which localize to the division site in all animal and fungal cells but are essential for cytokinesis only in some cell types. For example, in Schizosaccharomyces pombe, four septins localize to the division site, but deletion of the four genes produces only a modest delay in cell separation. To ask if the S. pombe septins function redundantly in cytokinesis, we conducted a synthetic-lethal screen in a septin-deficient strain and identified seven mutations. One mutation affects Cdc4, a myosin light chain that is an essential component of the cytokinetic actomyosin ring. Five others cause frequent cell lysis during cell separation and map to two loci. These mutations and their dosage suppressors define a signaling pathway (including Rho1 and a novel arrestin) for repairing cell-wall damage. The seventh mutation affects the poorly understood RNA-binding protein Scw1 and severely delays cell separation when combined either with a septin mutation or with a mutation affecting the septin-interacting, anillin-like protein Mid2, suggesting that Scw1 functions in a pathway parallel to that of the septins. Taken together, our results suggest that the S. pombe septins participate redundantly in one or more pathways that cooperate with the actomyosin ring during cytokinesis and that a septin defect causes septum defects that can be repaired effectively only when the cell-integrity pathway is intact.

Pob1 participates in the Cdc42 regulation of fission yeast actin cytoskeleton

Rho GTPases regulate the actin cytoskeleton in all eukaryotes. Fission yeast Cdc42 is involved in actin cable assembly and formin For3 regulation. We isolated cdc42-879 as a thermosensitive strain with actin cable and For3 localization defects. In a multicopy suppressor screening, we identified pob1(+) as suppressor of cdc42-879 thermosensitivity. Pob1 overexpression also partially restores actin cables and localization of For3 in the mutant strain. Pob1 interacts with Cdc42 and this GTPase regulates Pob1 localization and/or stability. The C-terminal pleckstrin homology (PH) domain of Pob1 is required for Cdc42 binding. Pob1 also binds to For3 through its N-terminal sterile alpha motif (SAM) domain and contributes to the formin localization at the cell tips. The previously described pob1-664 mutant strain (Mol. Biol. Cell. 10, 2745-2757, 1999), which carries a mutation in the PH domain, as well as pob1 mutant strains in which Pob1 lacks the N-terminal region (pob1DeltaN) or the SAM domain (pob1DeltaSAM), have cytoskeletal defects similar to that of cdc42-879 cells. Expression of constitutively active For3DAD* partially restores actin organization in cdc42-879, pob1-664, pob1DeltaN, and pob1DeltaSAM. Therefore, we propose that Pob1 is required for For3 localization to the tips and facilitates Cdc42-mediated relief of For3 autoinhibition to stimulate actin cable formation.

Genomic expression patterns in cell separation mutants of Schizosaccharomyces pombe defective in the genes sep10 ( + ) and sep15 ( + ) coding for the Mediator subunits Med31 and Med8

Cell division is controlled by a complex network involving regulated transcription of genes and postranslational modification of proteins. The aim of this study is to demonstrate that the Mediator complex, a general regulator of transcription, is involved in the regulation of the second phase (cell separation) of cell division of the fission yeast Schizosaccharomyces pombe. In previous studies we have found that the fission yeast cell separation genes sep10 ( + ) and sep15 ( + ) code for proteins (Med31 and Med8) associated with the Mediator complex. Here, we show by genome-wide gene expression profiling of mutants defective in these genes that both Med8 and Med31 control large, partially overlapping sets of genes scattered over the entire genome and involved in diverse biological functions. Six cell separation genes controlled by the transcription factors Sep1 and Ace2 are among the target genes. Since neither sep1 ( + ) nor ace2 ( + ) is affected in the mutant cells, we propose that the Med8 and Med31 proteins act as coactivators of the Sep1-Ace2-dependent cell separation genes. The results also indicate that the subunits of Mediator may contribute to the coordination of cellular processes by fine-tuning of the expression of larger sets of genes.

The SH3/PH domain protein AgBoi1/2 collaborates with the Rho-type GTPase AgRho3 to prevent nonpolar growth at hyphal tips of Ashbya gossypii

Unlike most other cells, hyphae of filamentous fungi permanently elongate and lack nonpolar growth phases. We identified AgBoi1/2p in the filamentous ascomycete Ashbya gossypii as a component required to prevent nonpolar growth at hyphal tips. Strains lacking AgBoi1/2p frequently show spherical enlargement at hyphal tips with concomitant depolarization of actin patches and loss of tip-located actin cables. These enlarged tips can repolarize and resume hyphal tip extension in the previous polarity axis. AgBoi1/2p permanently localizes to hyphal tips and transiently to sites of septation. Only the tip localization is important for sustained elongation of hyphae. In a yeast two-hybrid experiment, we identified the Rho-type GTPase AgRho3p as an interactor of AgBoi1/2p. AgRho3p is also required to prevent nonpolar growth at hyphal tips, and strains deleted for both AgBOI1/2 and AgRHO3 phenocopied the respective single-deletion strains, demonstrating that AgBoi1/2p and AgRho3p function in a common pathway. Monitoring the polarisome of growing hyphae using AgSpa2p fused to the green fluorescent protein as a marker, we found that polarisome disassembly precedes the onset of nonpolar growth in strains lacking AgBoi1/2p or AgRho3p. AgRho3p locked in its GTP-bound form interacts with the Rho-binding domain of the polarisome-associated formin AgBni1p, implying that AgRho3p has the capacity to directly activate formin-driven actin cable nucleation. We conclude that AgBoi1/2p and AgRho3p support polarisome-mediated actin cable formation at hyphal tips, thereby ensuring permanent polar tip growth.

The fission yeast MO25 protein functions in polar growth and cell separation

Proteins of the MO25 family are widely conserved but their function has not been characterized in detail. Human MO25 is a cofactor of LKB1, a conserved protein kinase with roles in cell polarity in nematodes, flies and mammalian cells. Furthermore, the budding yeast MO25 homologue, Hym1, is important for cell separation and morphogenesis. We have characterized Pmo25p, the MO25 homologue in the fission yeast Schizosaccharomyces pombe. Pmo25p is an essential protein required for polar growth; in its absence the actin cytoskeleton becomes depolarized and cells adopt a round morphology. In addition, pmo25 mutants are defective in cell separation. Both functions of Pmo25p appear to be mediated by the Orb6p-Mob2p kinase complex. Pmo25p shows no distinct localization during interphase, but it is recruited to one of the two spindle pole bodies during anaphase and to the division site during cytokinesis. The septation initiation network (SIN) regulates the localization of Pmo25p, suggesting that it regulates Pmo25p function during cell division.

The small GTPase Rho3 and the diaphanous/formin For3 function in polarized cell growth in fission yeast

We identified a novel Rho gene rho3(+) and studied its interaction with diaphanous/formin for3(+) in the fission yeast Schizosaccharomyces pombe. Both rho3 null cells and for3 null cells showed defects in organization of not only actin cytoskeleton but also cytoplasmic microtubules (MTs). rho3 for3 double null cells had defects that were more severe than each single null cell: polarized growth was deficient in the double null cells. Function of For3 needed the highly conserved FH1 and FH2 domains, an N-terminal region containing a Rho-binding domain, and the C-terminal region. For3 bound to active forms of both Rho3 and Cdc42 but not to that of Rho1. For3 was localized as dots to the ends of interphase cells and to the mid-region in dividing cells. This localization was probably dependent on its interaction with Rho proteins. Overexpression of For3 produced huge swollen cells containing depolarized F-actin patches and thick cytoplasmic MT bundles. In addition, overexpression of a constitutively active Rho3Q71L induced a strong defect in cytokinesis. In conclusion, we propose that the Rho3-For3 signaling system functions in the polarized cell growth of fission yeast by controlling both actin cytoskeleton and MTs.

Probing the importance and potential roles of the binding of the PH-domain protein Boi1 to acidic phospholipids

BACKGROUND

The related proteins Boi1 and Boi2, which appear to promote polarized growth in S. cerevisiae, both contain a PH (pleckstrin homology) and an SH3 (src homology 3) domain. Previously, we gained evidence that a PH domain-bearing segment of Boi1, which we call Boi1-PH, is sufficient and necessary for function. In the current study, we investigate the binding of Boi1's PH domain to the acidic phospholipids PIP2 (phosphatidylinositol-4,5-bisphosphate) and PS (phosphatidylserine).

RESULTS

Boi1-PH co-sediments with PS vesicles. It does so more readily when these vesicles contain a small amount of PIP2. Boi1-PH is degraded in yeast extracts in a manner that is stimulated by PIP2. Amino-acid substitutions that diminish binding to PIP2 and PS impair Boi1 function. Fusion to a myristoyl group-accepting sequence improves to different degrees the ability of these different mutant versions of Boi1-PH to function. Boi1 and Boi2 are localized to the periphery of buds during much of the budding cycle and to necks late in the cell cycle. Amino-acid substitutions that diminish binding to PIP2 and PS impair localization of Boi1 to the bud, but do not affect the localization of Boi1 to the neck. Conversely, a mutation in the SH3 domain prevents the localization of Boi1 to the neck, but does not impair localization to the bud.

CONCLUSIONS

Boi1's PH domain binds to acidic phospholipids, and this binding appears to be important for Boi1 function. The main role of binding to PS may simply be to promote the association of the PH domain with membrane. The higher-affinity binding to PIP2, which apparently promotes a conformational change in the PH domain, may play an important additional role. Boi1 and Boi2 are localized to sites of polarized growth. Whereas the SH3 domain is needed for localization of Boi1 to the neck, the phospholipid-binding portion of the PH domain is important for localization to the bud.

The Phox homology (PX) domain, a new player in phosphoinositide signalling

Phosphoinositides are key regulators of diverse cellular processes. The pleckstrin homology (PH) domain mediates the action of PtdIns(3,4)P(2), PtdIns(4,5)P(2) and PtdIns(3,4,5)P(3), while the FYVE domain relays the pulse of PtdIns3P. The recent establishment that the Phox homology (PX) domain interacts with PtdIns3P and other phosphoinositides suggests another mechanism by which phosphoinositides can regulate/integrate multiple cellular events via a spectrum of PX domain-containing proteins. Together with the recent discovery that the epsin N-terminal homologue (ENTH) domain interacts with PtdIns(4,5)P(2), it is becoming clear that phosphoinositides regulate diverse cellular events through interactions with several distinct structural motifs present in many different proteins.

Roles of the fission yeast formin for3p in cell polarity, actin cable formation and symmetric cell division

BACKGROUND

Both symmetric and asymmetric cell divisions are required for the generation of appropriate cell lineages during development. Wild-type Schizosaccharomyces pombe cells divide in a symmetric fashion to produce two similar rod-shaped daughter cells. Formins are proteins with conserved roles in cell polarity, cytokinesis, and the regulation of actin and microtubule cytoskeletons.

RESULTS

Here, we identify and characterize a new S. pombe formin, for3p. for3 Delta mutant cells divide in an asymmetric manner; a mother cell divides medially to produce one daughter cell that develops into a monopolar cell and one daughter that develops into a bipolar cell. Both daughter cells recapitulate similar asymmetric lineages themselves. Inheritance of the bipolar pattern correlates with inheritance of the recent birth scar, not with asymmetry in the spindle pole bodies. for3 Delta mutants lack interphase actin cables and have delocalized actin patch and myo52p (type V myosin) distributions. for3 Delta cells have normal microtubule dynamics and cortical interactions but have defects in microtubule organization and increased numbers of microtubule bundles. for3p-GFP is localized at both cell tips in an actin-dependent manner and at the cell division site.

CONCLUSIONS

for3p is a cell polarity factor required for interphase actin cable formation and microtubule organization. The for3 Delta phenotype suggests that cells are able to grow in a polarized manner even in the absence of functional actin cables and polarized distribution of actin patches. for3p and possibly actin cables are part of a regulatory network that ensures that cell divisions are symmetric.

The ubc2 gene of Ustilago maydis encodes a putative novel adaptor protein required for filamentous growth, pheromone response and virulence

The Basidiomycete fungus Ustilago maydis causes corn smut disease and alternates between a budding haploid saprophyte and a filamentous dikaryotic pathogen. Previous work demonstrated that haploid adenylate cyclase (uac1) mutants display a constitutively filamentous phenotype. Suppressor mutants of a uac1 disruption strain, named ubc for Ustilago bypass of cyclase, no longer require cAMP for the budding morphology. The ubc2 gene was isolated by complementation and is required for filamentous growth. The deduced amino acid sequence encoded by ubc2 shows localized homology to Sterile Alpha Motif (SAM), Ras Association (RA) and Src homology 3 (SH3) protein-protein interaction domains. A K78E missense mutation within the SAM domain, revealed a genetic interaction between ubc2 and ubc4, a pheromone-responsive MAP kinase kinase kinase. This indicates involvement of ubc2 in the pheromone-responsive MAP kinase cascade and ubc2 is required for pheromone-responsive morphogenesis. The ubc2 gene is a critical virulence factor. Thus, ubc2 encodes a putative novel adaptor protein that may act directly upstream of the pheromone-responsive MAP kinase cascade in U. maydis.

Characterization of a fission yeast mutant which displays defects in cell wall integrity and cytokinesis

The fission yeast cps6-153 mutant was originally isolated based on its hypersensitivity to the spindle poison isopropyl N-3-chlorophenyl carbamate (CIPC). The mutant also shows defects in both cell wall integrity and cytokinesis, resulting in the accumulation of unseparated cells with weakened cell walls. The arrested cells display a disoriented alignment of cytoplasmic microtubules. When the mutant cells are cultivated at high temperature (35 degrees C), both cell walls and septa become very thick. Electron microscopy revealed the disorganized structure of the thickened cell walls and septa, in which fibrillar components were not completely masked with an amorphous matrix. rad25+ was cloned from a genomic library by complementation of the mutant phenotypes, suggesting the involvement of Rad25p, one of two 14-3-3 proteins in S. pombe, in the pathway of cell wall integrity and cytokinesis.

The Schizosaccharomyces pombe spo20(+) gene encoding a homologue of Saccharomyces cerevisiae Sec14 plays an important role in forespore membrane formation

The Schizosaccharomyces pombe spo20-KC104 mutation was originally isolated in a screen for sporulation-deficient mutants, and the spo20-KC104 mutant exhibits temperature-sensitive growth. Herein, we report that S. pombe, spo20(+) is essential for fission yeast cell viability and is constitutively expressed throughout the life cycle. We also demonstrate that the spo20(+) gene product is structurally homologous to Saccharomyces cerevisiae Sec14, the major phosphatidylinositol transfer protein of budding yeast. This structural homology translates to a significant degree of functional relatedness because reciprocal complementation experiments demonstrate that each protein is able to fulfill the essential function of the other. Moreover, biochemical experiments show that, like Sec14, Spo20 is a phosphatidylinositol/phosphatidylcholine-transfer protein. That Spo20 is required for Golgi secretory function in vegetative cells is indicated by our demonstration that the spo20-KC104 mutant accumulates aberrant Golgi cisternae at restrictive temperatures. However, a second phenotype observed in Spo20-deficient fission yeast is arrest of cell division before completion of cell separation. Consistent with a direct role for Spo20 in controlling cell septation in vegetatively growing cells, localization experiments reveal that Spo20 preferentially localizes to the cell poles and to sites of septation of fission yeast cells. We also report that, when fission yeasts are challenged with nitrogen starvation, Spo20 translocates to the nucleus. This nuclear localization persists during conjugation and meiosis. On completion of meiosis, Spo20 translocates to forespore membranes, and it is the assembly of forespore membranes that is abnormal in spo20-KC104 cells. In such mutants, a considerable fraction of forming prespores fail to encapsulate the haploid nucleus. Our results indicate that Spo20 regulates the formation of specialized membrane structures in addition to its recognized role in regulating Golgi secretory function.

Identification and functional analysis of the gene for type I myosin in fission yeast

BACKGROUND

Type I myosin is highly conserved among eukaryotes, and apparently plays important roles in a number of cellular processes. In the budding yeast, two myosin I species have been identified and their role in F-actin assembly has been inferred.

RESULTS

We cloned the fission yeast myo1 gene, which apparently encoded a myosin I protein. Disruption of myo1 was not lethal, but it caused growth retardation at high and low temperatures, sensitivity to a high concentration of KCl, and aberrance in cell morphology associated with an abnormal distribution of F-actin patches. An abnormal deposition of cell wall materials was also seen. Homothallic myo1Delta cells could mate, but heterothallic myo1Delta cells were poor in conjugation. Myo1p was necessary for the encapsulation of spores. The tail domain of Myo1p was pivotal for its function. Calmodulin could bind to Myo1p through the IQ domain at the neck.

CONCLUSIONS

Myo1p appears to control the redistribution of F-actin patches during the cell cycle. Loss of Myo1p function is likely to slow down the actin assembly/disassembly process, which results in a failure of the actin cycle to catch up with other events in both the mitotic and meiotic cell cycles, including extension of the conjugation tubes.

The S. pombe orthologue of the S. cerevisiae mob1 gene is essential and functions in signalling the onset of septum formation

We have isolated the Schizosaccharomyces pombe orthologue of the Saccharomyces cerevisiae MOB1 gene in a screen designed to enrich for septation mutants. The gene is essential, and cells lacking it display a phenotype typical of septation signalling network mutants. mob1p is located on both spindle pole bodies throughout mitosis. In addition it is also co-localised with the medial ring later in mitosis, and flanks the septum as the medial ring contracts. We also demonstrate that mob1p can be precipitated from cells in a complex with the septation regulating kinase sid2p.