Characterization ofend4+, a gene required for endocytosis inSchizosaccharomyces pombe

2A peptide from ERBV-1 efficiently separates endogenous protein domains in the fission yeast Schizosaccharomyces pombe

2A peptides are widely used for polycistronic gene expression from vectors. In contrast, the separation of endogenous genes via 2A peptides has been largely unexplored. We show that in fission yeast Schizosaccharomyces pombe , the "cleaving" efficiency of the 2A peptide from ERBV-1 (Equine rhinitis B virus 1) range from ~70% to ~99% for End4 at different insertion sites. Our results suggest a high "cleaving" efficiency as well as considerable variation for using 2A peptide to separate endogenous protein domains in fission yeast. Verification of the "cleaving" efficiency of 2A peptides is advised for its application.

Isolated THATCH domain of End4 is unable to bind F-actin independently in the fission yeast Schizosaccharomyces pombe

Clathrin mediated endocytosis (CME) in the fission yeast Schizosaccharomyces pombe critically depends on the connection between the lipid membrane and F-actin. The fission yeast endocytic protein End4 (homologous to Sla2 in budding yeast and HIP1R in human) contains a N-terminal domain that binds to PIP2 on the membrane, and a C-terminal THATCH domain that is postulated to be a binding partner of F-actin in vivo. Purified THATCH domain of the budding yeast Sla2, however, shows low affinity to F-actin in vitro. We tested if isolated THATCH domain still has low affinity to F-actin in vivo, using TEV protease (TEVp)-mediated protein cleaving to separate the THATCH domain from the rest of End4. Our results indicate that the isolated THATCH domain of End4 is unable to bind F-actin independently in vivo, consistent with the low affinity of the THATCH domain to F-actin measured from in vitro binding assays.

Sterol biosensor reveals LAM-family Ltc1-dependent sterol flow to endosomes upon Arp2/3 inhibition

Sterols are crucial components of biological membranes, which are synthetized in the ER and accumulate in the plasma membrane (PM). Here, by applying a genetically encoded sterol biosensor (D4H), we visualize a sterol flow between PM and endosomes in the fission yeast Schizosaccharomyces pombe. Using time-lapse and correlative light-electron microscopy, we found that inhibition of Arp2/3-dependent F-actin assembly promotes the reversible relocalization of D4H from the PM to internal sterol-rich compartments (STRIC) labeled by synaptobrevin Syb1. Retrograde sterol internalization to STRIC is independent of endocytosis or an intact Golgi, but depends on Ltc1, a LAM/StARkin-family protein localized to ER-PM contact sites. The PM in ltc1Δ cells over-accumulates sterols and upon Arp2/3 inhibition forms extended ER-interacting invaginations, indicating that sterol transfer contributes to PM size homeostasis. Anterograde sterol movement from STRIC is independent of canonical vesicular trafficking but requires Arp2/3, suggesting a novel role for this complex. Thus, transfer routes orthogonal to vesicular trafficking govern the flow of sterols in the cell.

The ancient claudin Dni2 facilitates yeast cell fusion by compartmentalizing Dni1 into a membrane subdomain

Dni1 and Dni2 facilitate cell fusion during mating. Here, we show that these proteins are interdependent for their localization in a plasma membrane subdomain, which we have termed the mating fusion domain. Dni1 compartmentation in the domain is required for cell fusion. The contribution of actin, sterol-dependent membrane organization, and Dni2 to this compartmentation was analysed, and the results showed that Dni2 plays the most relevant role in the process. In turn, the Dni2 exit from the endoplasmic reticulum depends on Dni1. These proteins share the presence of a cysteine motif in their first extracellular loop related to the claudin GLWxxC(8-10 aa)C signature motif. Structure-function analyses show that mutating each Dni1 conserved cysteine has mild effects, and that only simultaneous elimination of several cysteines leads to a mating defect. On the contrary, eliminating each single cysteine and the C-terminal tail in Dni2 abrogates Dni1 compartmentation and cell fusion. Sequence alignments show that claudin trans-membrane helixes bear small-XXX-small motifs at conserved positions. The fourth Dni2 trans-membrane helix tends to form homo-oligomers in Escherichia plasma membrane, and two concatenated small-XXX-small motifs are required for efficient oligomerization and for Dni2 export from the yeast endoplasmic reticulum. Together, our results strongly suggest that Dni2 is an ancient claudin that blocks Dni1 diffusion from the intercellular region where two plasma membranes are in close proximity, and that this function is required for Dni1 to facilitate cell fusion.

Gradients of phosphatidylserine contribute to plasma membrane charge localization and cell polarity in fission yeast

Surface charges at the inner leaflet of the plasma membrane may contribute to regulate the surface recruitment of key signaling factors. Phosphatidylserine (PS) is an abundant charged lipid that may regulate charge distribution in different cell types. Here we characterize the subcellular distribution and function of PS in the rod-shaped, polarized fission yeast. We find that PS preferably accumulates at cell tips and defines a gradient of negative charges along the cell surface. This polarization depends on actin-mediated endocytosis and contributes to the subcellular partitioning of charged polarity-regulating Rho GTPases like Rho1 or Cdc42 in a protein charge-dependent manner. Cells depleted of PS have altered cell dimensions and fail to properly regulate growth from the second end, suggesting a role for PS and membrane charge in polarized cell growth.

CAG Expansions Are Genetically Stable and Form Nontoxic Aggregates in Cells Lacking Endogenous Polyglutamine Proteins

Proteins containing polyglutamine (polyQ) regions are found in almost all eukaryotes, albeit with various frequencies. In humans, proteins such as huntingtin (Htt) with abnormally expanded polyQ regions cause neurodegenerative diseases such as Huntington's disease (HD). To study how the presence of endogenous polyQ aggregation modulates polyQ aggregation and toxicity, we expressed polyQ expanded Htt fragments (polyQ Htt) in Schizosaccharomyces pombe In stark contrast to other unicellular fungi, such as Saccharomyces cerevisiae, S. pombe is uniquely devoid of proteins with more than 10 Q repeats. We found that polyQ Htt forms aggregates within S. pombe cells only with exceedingly long polyQ expansions. Surprisingly, despite the presence of polyQ Htt aggregates in both the cytoplasm and nucleus, no significant growth defect was observed in S. pombe cells. Further, PCR analysis showed that the repetitive polyQ-encoding DNA region remained constant following transformation and after multiple divisions in S. pombe, in contrast to the genetic instability of polyQ DNA sequences in other organisms. These results demonstrate that cells with a low content of polyQ or other aggregation-prone proteins can show a striking resilience with respect to polyQ toxicity and that genetic instability of repetitive DNA sequences may have played an important role in the evolutionary emergence and exclusion of polyQ expansion proteins in different organisms.

IMPORTANCE

Polyglutamine (polyQ) proteins encoded by repetitive CAG DNA sequences serve a variety of normal biological functions. Yet some proteins with abnormally expanded polyQ regions cause neurodegeneration through unknown mechanisms. To study how distinct cellular environments modulate polyQ aggregation and toxicity, we expressed CAG-expanded huntingtin fragments in Schizosaccharomyces pombe In stark contrast to many other eukaryotes, S. pombe is uniquely devoid of proteins containing long polyQ tracts. Our results show that S. pombe cells, despite their low content of endogenous polyQ proteins, exhibit striking and unexpected resilience with respect to polyQ toxicity and that genetic instability of repetitive DNA sequences may have played an important role in the emergence and expansion of polyQ domains in eukaryotic evolution.

Rga6 is a Fission Yeast Rho GAP Involved in Cdc42 Regulation of Polarized Growth

Active Cdc42 is essential for the establishment of polarized growth. This GTPase is negatively regulated by the GTPase-activating proteins (GAPs), which are important for the spatial specificity of Cdc42 function. Rga4 is the only GAP described as negative regulator of fission yeast Cdc42. We report here that Rga6 is another fission yeast Cdc42 GAP which shares some functions with Rga4. Cells lacking Rga6 are viable but slightly shorter and broader than wild type, and cells lacking Rga6 and Rga4 simultaneously are rounded. In these cells, active Cdc42 is observed all around the membrane. These additive effects indicate that both GAPs collaborate in the spatial regulation of active Cdc42. Rga6 localizes to the plasma membrane forming clusters different from those formed by Rga4. A polybasic region at the Rga6 C-terminus is responsible for its membrane localization. Rga6-GFP fluorescence decreases considerably at the growing tips, and this decrease is dependent on the actin cables. Notably, in the absence of Rga6 the amplitude of active Cdc42 oscillations at the tips decreases, and less GTP-Cdc42 accumulates at the new end of the cells. We propose here that Rga6 collaborates with Rga4 to spatially restrict active Cdc42 at the cell tips and maintain cell dimensions.

The AP-2 complex is required for proper temporal and spatial dynamics of endocytic patches in fission yeast

In metazoans the AP-2 complex has a well-defined role in clathrin-mediated endocytosis. By contrast, its direct role in endocytosis in unicellular eukaryotes has been questioned. Here, we report co- immunoprecipitation between the fission yeast AP-2 component Apl3p and clathrin, as well as the genetic interactions between apl3Δ and clc1 and sla2Δ/end4Δ mutants. Furthermore, a double clc1 apl3Δ mutant was found to be defective in FM4-64 uptake. In an otherwise wild-type strain, apl3Δ cells exhibit altered dynamics of the endocytic sites, with a heterogeneous and extended lifetime of early and late markers at the patches. Additionally, around 50% of the endocytic patches exhibit abnormal spatial dynamics, with immobile patches and patches that bounce backwards to the cell surface, showing a pervasive effect of the absence of AP-2. These alterations in the endocytic machinery result in abnormal cell wall synthesis and morphogenesis. Our results complement those found in budding yeast and confirm that a direct role of AP-2 in endocytosis has been conserved throughout evolution.

A new phosphate-starvation response in fission yeast requires the endocytic function of myosin I

Endocytosis is essential for uptake of many substances into the cell, but how it links to nutritional signalling is poorly understood. Here, we show a new role for endocytosis in regulating the response to low phosphate in Schizosaccharomyces pombe. Loss of function of myosin I (Myo1), Sla2/End4 or Arp2, proteins involved in the early steps of endocytosis, led to increased proliferation in low-phosphate medium compared to controls. We show that once cells are deprived of phosphate they undergo a quiescence response that is dependent on the endocytic function of Myo1. Transcriptomic analysis revealed a wide perturbation of gene expression with induction of stress-regulated genes upon phosphate starvation in wild-type but not Δmyo1 cells. Thus, endocytosis plays a pivotal role in mediating the cellular response to nutrients, bridging the external environment and internal molecular functions of the cell.

Highlighted Article: The endocytic machinery, including the type 1 myosin Myo1, is required to establish a quiescence response to low-phosphate stress in fission yeast.

A formin-nucleated actin aster concentrates cell wall hydrolases for cell fusion in fission yeast

The formin Fus1 nucleates a novel actin structure in fission yeast, named the actin fusion focus, which consists of an aster of actin filaments whose barbed ends are focalized at a membrane proximal site and serves to focalize cell wall hydrolase delivery for cell fusion.

Cell–cell fusion is essential for fertilization. For fusion of walled cells, the cell wall must be degraded at a precise location but maintained in surrounding regions to protect against lysis. In fission yeast cells, the formin Fus1, which nucleates linear actin filaments, is essential for this process. In this paper, we show that this formin organizes a specific actin structure—the actin fusion focus. Structured illumination microscopy and live-cell imaging of Fus1, actin, and type V myosins revealed an aster of actin filaments whose barbed ends are focalized near the plasma membrane. Focalization requires Fus1 and type V myosins and happens asynchronously always in the M cell first. Type V myosins are essential for fusion and concentrate cell wall hydrolases, but not cell wall synthases, at the fusion focus. Thus, the fusion focus focalizes cell wall dissolution within a broader cell wall synthesis zone to shift from cell growth to cell fusion.

Spontaneous Cdc42 polarization independent of GDI-mediated extraction and actin-based trafficking

The small Rho-family GTPase Cdc42 is critical for cell polarization and polarizes spontaneously in absence of upstream spatial cues. Spontaneous polarization is thought to require dynamic Cdc42 recycling through Guanine nucleotide Dissociation Inhibitor (GDI)-mediated membrane extraction and vesicle trafficking. Here, we describe a functional fluorescent Cdc42 allele in fission yeast, which demonstrates Cdc42 dynamics and polarization independent of these pathways. Furthermore, an engineered Cdc42 allele targeted to the membrane independently of these recycling pathways by an amphipathic helix is viable and polarizes spontaneously to multiple sites in fission and budding yeasts. We show that Cdc42 is highly mobile at the membrane and accumulates at sites of activity, where it displays slower mobility. By contrast, a near-immobile transmembrane domain-containing Cdc42 allele supports viability and polarized activity, but does not accumulate at sites of activity. We propose that Cdc42 activation, enhanced by positive feedback, leads to its local accumulation by capture of fast-diffusing inactive molecules.

This study of fission yeast reveals that the active and inactive forms of the small GTPase Cdc42 have different rates of lateral diffusion in the membrane, providing insights into how it becomes spontaneously polarized, thereby determining the polarity of the cell.

Cell polarization is a critical feature of most cells that underlies their functional organization. A central polarity factor called Cdc42, a small GTPase targeted to the plasma membrane by prenylation, promotes cell polarization in its active GTP-bound form. Cdc42 is a key polarity factor because it accumulates at presumptive sites of polarity, which previous work suggested involves Cdc42 recycling on and off the plasma membrane. In addition, its activity can spontaneously polarize cells in a single location by self-enhancing positive feedback mechanisms, even in the absence of any pre-localized landmarks. In this study, we constructed the first functional fluorescently tagged allele of Cdc42 that replaces the endogenous genomic copy in Schizosaccharomyces pombe. This allowed measurements of Cdc42 dynamics at the plasma membrane by live microscopy. Unexpectedly, this approach revealed that Cdc42 primarily moves through lateral diffusion, rather than on and off the plasma membrane. Engineered Cdc42 alleles with alternative membrane-targeting mechanisms demonstrated that Cdc42 activity, indeed, polarizes in the absence of known pathways that recycle Cdc42 on and off the membrane. We further show that the active form, Cdc42-GTP, is less mobile than Cdc42-GDP. We thus propose that Cdc42 polarization occurs as a consequence of its local activation—either through self-enhanced feedback or in response to upstream cues—by a reduction in the active Cdc42 diffusion rate.

Oscillatory AAA+ ATPase Knk1 constitutes a novel morphogenetic pathway in fission yeast

Cellular morphogenesis relies partly on cell polarization by the cytoskeleton. In the fission yeast Schizosaccharomyces pombe, it is well established that microtubules (MTs) deliver the spatial cue Tea1, a kelch repeat protein, to the tip regions to direct the growth machinery at the cell tips driving the linear extension of the rod-shaped organism to maintain a straight long axis. Here, we report the characterization of Knk1 (kink), a previously unidentified member of the superfamily of ATPases associated with various cellular activities (AAA(+)), whose deletion causes a unique morphological defect characterized by the formation of kinks close to cell tips. Through genetic analysis, we place Knk1 into a novel pathway controlling cell shape independently of MTs and Tea1. Knk1 localizes at cell tips. Its localization is mediated by the Knk1 N terminus and is enhanced upon ATP binding to the C-terminal ATPase domain. Furthermore, Knk1 tip recruitment is regulated by SRC-like adaptor 2 (Sla2) and cell division cycle 42 (Cdc42) independently of Sla2's role in endocytosis. Finally, we discovered that Knk1 shows an anticorrelated oscillatory behavior between the two cell tips at a periodicity that is different from the reported oscillatory Cdc42 dynamics.

Eng2 is a component of a dynamic protein complex required for endocytic uptake in fission yeast

Eng2 is a glucanase required for spore release, although it is also expressed during vegetative growth, suggesting that it might play other cellular functions. Its homology to the Saccharomyces cerevisiae Acf2 protein, previously shown to promote actin polymerization at endocytic sites in vitro, prompted us to investigate its role in endocytosis. Interestingly, depletion of Eng2 caused profound defects in endocytic uptake, which were not due to the absence of its glucanase activity. Analysis of the dynamics of endocytic proteins by fluorescence microscopy in the eng2Δ strain unveiled a previously undescribed phenotype, in which assembly of the Arp2/3 complex appeared uncoupled from the internalization of the endocytic coat and resulted in a fission defect. Strikingly also, we found that Eng2-GFP dynamics did not match the pattern of other endocytic proteins. Eng2-GFP localized to bright cytosolic spots that moved around the cellular poles and occasionally contacted assembling endocytic patches just before recruitment of Wsp1, the Schizosaccharomyces pombe WASP. Interestingly, Csh3-YFP, a WASP-interacting protein, interacted with Eng2 by co-immunoprecipitation and was recruited to Eng2 in bright cytosolic spots. Altogether, our work defines a novel endocytic functional module, which probably couples the endocytic coat to the actin module.

An α-Amylase Homologue,aah3, Encodes a GPI-Anchored Membrane Protein Required for Cell Wall Integrity and Morphogenesis inSchizosaccharomyces pombe

Glycosylphosphatidylinositol (GPI)-anchored proteins are essential for normal cellular morphogenesis and have an additional role in mediating cross-linking of glycoproteins to cell wall glucan in yeast cells. Although many GPI-anchored proteins have been characterized in Saccharomyces cerevisiae, none have been reported for well-characterized GPI-anchored proteins in Schizosaccharomyces pombe to date. Among the putative GPI-anchored proteins in S. pombe, four alpha-amylase homologs (Aah1p-Aah4p) have putative signal sequences and C-terminal GPI anchor addition signals. Disruption of aah3(+) resulted in a morphological defect and hypersensitivity to cell wall-degrading enzymes. Biochemical analysis showed that Aah3p is an N-glycosylated, GPI-anchored membrane protein localized in the membrane and cell wall fractions. Conjugation and sporulation were not affected by the aah3(+) deletion, but the ascal wall of aah3Delta cells was easily lysed by hydrolases. Expression of aah3 alleles in which the conserved aspartic acid and glutamic acid residues required for hydrolase activity were replaced with alanine residues failed to rescue the morphological and ascal wall defects of aah3Delta cells. Taken together, these results indicate that Aah3p is a GPI-anchored protein and is required for cell and ascal wall integrity in S. pombe.

Mitochondrial ABC Transporter Atm1p Is Required for Protection against Oxidative Stress and Vacuolar Functions inSchizosaccharomyces pombe

A potential correlation between mitochondrial and vacuolar functions is known to exit in yeast. Fission yeast atm1(+), SPAC15A10.01, encodes a putative half-type ABC transporter with an N-terminal mitochondrial-targeting signal. In an attempt to evaluate the possible involvement of mitochondrion in vacuole function, a functional analysis of atm1(+) was performed by gene disruption. Growth of the atm1 mutant was inhibited in the presence of oxidizing agents, and S. cerevisiae Atm1p was found to complement this growth defect. atm1Delta cells exhibited defects in fluid-phase endocytosis and vacuolar fusion under hypotonic stress. GFP-tagged Atm1p was observed to be localized in the mitochondria. These data strongly suggest that fission yeast Atm1p was not only involved in protection against oxidative stress, but also played a role in vacuolar functions.

The small GTPase Rab5 homologue Ypt5 regulates cell morphology, sexual development, ion-stress response and vacuolar formation in fission yeast

Inner-membrane transport is critical to cell function. Rab family GTPases play an important role in vesicle transport. In mammalian cells, Rab5 is reported to be involved in the regulation of endosome formation, phagocytosis and chromosome alignment. Here, we examined the role of the fission yeast Rab5 homologue Ypt5 using a point mutant allele. Mutant cells displayed abnormal cell morphology, mating, sporulation, endocytosis, vacuole fusion and responses to ion stress. Our data strongly suggest that fission yeast Rab5 is involved in the regulation of various types of cellular functions.

Regulation of cell wall synthesis by the clathrin light chain is essential for viability in Schizosaccharomyces pombe

The regulation of cell wall synthesis by the clathrin light chain has been addressed. Schizosaccharomyces pombe clc1Δ mutant was inviable in the absence of osmotic stabilization; when grown in sorbitol-supplemented medium clc1Δ cells grew slowly, formed aggregates, and had strong defects in morphology. Additionally, clc1Δ cells exhibited an altered cell wall composition. A mutant that allowed modulating the amount of Clc1p was created to analyze in more detail the dependence of cell wall synthesis on clathrin. A 40% reduction in the amount of Clc1p did not affect acid phosphatase secretion and bulk lipid internalization. Under these conditions, β(1,3)glucan synthase activity and cell wall synthesis were reduced. Also, the delivery of glucan synthases to the cell surface, and the secretion of the Eng1p glucanase were defective. These results suggest that the defects in the cell wall observed in the conditional mutant were due to a defective secretion of enzymes involved in the synthesis/remodelling of this structure, rather than to their endocytosis. Our results show that a reduction in the amount of clathrin that has minor effects on general vesicle trafficking has a strong impact on cell wall synthesis, and suggest that this is the reason for the lethality of clc1Δ cells in the absence of osmotic stabilization.

Actin filament severing by cofilin dismantles actin patches and produces mother filaments for new patches

BACKGROUND

Yeast cells depend on Arp2/3 complex to assemble actin filaments at sites of endocytosis, but the source of the initial filaments required to activate Arp2/3 complex is not known.

RESULTS

We tested the proposal that cofilin severs actin filaments during endocytosis in fission yeast cells using a mutant cofilin defective in severing. We used quantitative fluorescence microscopy to track mGFP-tagged proteins, including early endocytic adaptor proteins, activators of Arp2/3 complex, and actin filaments. Consistent with the hypothesis, actin patches disassembled far more slowly in cells depending on severing-deficient cofilin than in wild-type cells. Even more interesting, actin patches assembled slowly in these cofilin mutant cells. Adaptor proteins End4p and Pan1p accumulated and persisted at endocytic sites more than ten times longer than in wild-type cells, followed by slow but persistent recruitment of activators of Arp2/3 complex, including WASP and myosin-I. Mutations revealed that actin filament binding sites on adaptor proteins Pan1p and End4p contribute to initiating actin polymerization in actin patches.

CONCLUSIONS

We propose a "sever, diffuse, and trigger" model for the nucleation of actin filaments at sites of endocytosis, whereby cofilin generates actin filament fragments that diffuse through the cytoplasm, bind adaptor proteins at nascent sites of endocytosis, and serve as mother filaments to initiate the autocatalytic assembly of the branched actin filament network of each new patch. This hypothesis explains the source of the "mother filaments" that are absolutely required for Arp2/3 complex to nucleate actin polymerization.

Cytokinesis-based constraints on polarized cell growth in fission yeast

The rod-shaped fission yeast Schizosaccharomyces pombe, which undergoes cycles of monopolar-to-bipolar tip growth, is an attractive organism for studying cell-cycle regulation of polarity establishment. While previous research has described factors mediating this process from interphase cell tips, we found that division site signaling also impacts the re-establishment of bipolar cell growth in the ensuing cell cycle. Complete loss or targeted disruption of the non-essential cytokinesis protein Fic1 at the division site, but not at interphase cell tips, resulted in many cells failing to grow at new ends created by cell division. This appeared due to faulty disassembly and abnormal persistence of the cell division machinery at new ends of fic1Δ cells. Moreover, additional mutants defective in the final stages of cytokinesis exhibited analogous growth polarity defects, supporting that robust completion of cell division contributes to new end-growth competency. To test this model, we genetically manipulated S. pombe cells to undergo new end take-off immediately after cell division. Intriguingly, such cells elongated constitutively at new ends unless cytokinesis was perturbed. Thus, cell division imposes constraints that partially override positive controls on growth. We posit that such constraints facilitate invasive fungal growth, as cytokinesis mutants displaying bipolar growth defects formed numerous pseudohyphae. Collectively, these data highlight a role for previous cell cycles in defining a cell's capacity to polarize at specific sites, and they additionally provide insight into how a unicellular yeast can transition into a quasi-multicellular state.

Endocytosis is essential for dynamic translocation of a syntaxin 1 orthologue during fission yeast meiosis

Fission yeast sporulation seems to accompany a dynamic alteration of membrane traffic pathways in which the destination of secretory vesicles changes from the plasma membrane to the developing spore membrane. Evidence shows that endocytosis is responsible for this alteration in traffic pathways via the relocalization of syntaxin 1.

Syntaxin is a component of the target soluble N-ethylmaleimide–sensitive factor attachment protein receptor complex, which is responsible for fusion of membrane vesicles at the target membrane. The fission yeast syntaxin 1 orthologue Psy1 is essential for both vegetative growth and spore formation. During meiosis, Psy1 disappears from the plasma membrane (PM) and dramatically relocalizes on the nascent forespore membrane, which becomes the PM of the spore. Here we report the molecular details and biological significance of Psy1 relocalization. We find that, immediately after meiosis I, Psy1 is selectively internalized by endocytosis. In addition, a meiosis-specific signal induced by the transcription factor Mei4 seems to trigger this internalization. The internalization of many PM proteins is facilitated coincident with the initiation of meiosis, whereas Pma1, a P-type ATPase, persists on the PM even during the progression of meiosis II. Ergosterol on the PM is also important for the internalization of PM proteins in general during meiosis. We consider that during meiosis in Schizosaccharomyces pombe cells, the characteristics of endocytosis change, thereby facilitating internalization of Psy1 and accomplishing sporulation.

Genome-wide screening for genes associated with FK506 sensitivity in fission yeast

We have been studying calcineurin signal transduction pathway in fission yeast Schizosaccharomyces pombe (S. pombe) by developing a genetic screen for mutants that show hypersensitivity to the immunosuppressive calcineurin inhibitor FK506 (tacrolimus). In the present study, to identify nonessential genes that are functionally related to the calcineurin signaling pathway, we performed a genome-wide screen of 3004 haploid deletion strains and confirmed 72 deletion strains to be FK506 sensitive. These 72 genes are classified into nine functional groups to include membrane trafficking (16 genes), signal transduction (10 genes), ubiquitination (8 genes), chromatin remodeling (6 genes), cytokinesis (4 genes), ribosomal protein (3 genes), RNA binding protein (3 genes), and a variety of other known functions (17 genes) or still unknown functions (5 genes) in the biological system. In our previous screening of FK506-sensitive mutants we isolated several membrane-trafficking mutants showing defective cell wall integrity. Here, we further examined the vacuolar fusion, the v-SNARE synaptobrevin Syb1 localization, and the sensitivity to the β-glucan synthase inhibitor micafungin in these 72 FK506-sensitive strains. Results showed that 25 deletion strains exhibited abnormal vacuole fusion, 19 deletion strains exhibited Syb1 mislocalization, and 14 deletion strains exhibited both abnormal vacuole fusion and Syb1 mislocalization, while 42 deletion strains showed both normal vacuole fusion and Syb1 localization. Likewise, 16 deletion strains showed sensitivity to micafungin. Altogether, our present study indicates that calcineurin mediates a plethora of physiological processes in fission yeast, and that calcineurin is extensively involved in cross-talk between signaling pathways.

Schizosaccharomyces pombe Pep12p is required for vacuolar protein transport and vacuolar homotypic fusion

In eukaryotic cells, SNARE proteins are essential for intracellular vesicle trafficking. Several SNARE proteins are required for vacuolar protein transport and vacuolar biogenesis in Saccharomyces cerevisiae. Previously we demonstrated that one of the fission yeast SNARE proteins, Pep12p, is not required for vacuolar fusion process in Schizosaccharomyces pombe. We have re-examined the function of S. pombe Pep12p using the newly created pep12(+) deletion strain. Deletion of the fission yeast pep12(+) gene results in pleiotropic phenotypes consistent with the absence of normal vacuoles, including missorting of vacuolar carboxypeptidase Y-and various ion- and drug-sensitivities. GFP-Pep12 fusion protein is mostly localized at the vacuolar membrane and the prevacuolar compartment. The S. pombe pep12Δ mutation phenocopies that of vps33Δ, suggesting that both Pep12p and Vps33p act at the same membrane fusion step in S. pombe, and both mutations cause vacuolar deficiency.

Characterization of Mug33 reveals complementary roles for actin cable-dependent transport and exocyst regulators in fission yeast exocytosis

Although endocytosis and exocytosis have been extensively studied in budding yeast, there have been relatively few investigations of these complex processes in the fission yeast Schizosaccharomyces pombe. Here we identify and characterize fission yeast Mug33, a novel Tea1-interacting protein, and show that Mug33 is involved in exocytosis. Mug33 is a Sur7/PalI-family transmembrane protein that localizes to the plasma membrane at the cell tips and to cytoplasmic tubulovesicular elements (TVEs). A subset of Mug33 TVEs make long-range movements along actin cables, co-translocating with subunits of the exocyst complex. TVE movement depends on the type V myosin Myo52. Although mug33Δ mutants are viable, with only a mild cell-polarity phenotype, mug33Δ myo52Δ double mutants are synthetically lethal. Combining mug33 Δ with deletion of the formin For3 (for3Δ) leads to synthetic temperature-sensitive growth and strongly reduced levels of exocytosis. Interestingly, mutants in non-essential genes involved in exocyst function behave in a manner similar to mug33Δ when combined with myo52Δ and for3Δ. By contrast, combining mug33Δ with mutants in non-essential exocyst genes has only minor effects on growth. We propose that Mug33 contributes to exocyst function and that actin cable-dependent vesicle transport and exocyst function have complementary roles in promoting efficient exocytosis in fission yeast.

Kin1 is a plasma membrane-associated kinase that regulates the cell surface in fission yeast

Cell morphogenesis is a complex process that depends on cytoskeleton and membrane organization, intracellular signalling and vesicular trafficking. The rod shape of the fission yeast Schizosaccharomyces pombe and the availability of powerful genetic tools make this species an excellent model to study cell morphology. Here we have investigated the function of the conserved Kin1 kinase. Kin1-GFP associates dynamically with the plasma membrane at sites of active cell surface remodelling and is present in the membrane fraction. Kin1Δ null cells show severe defects in cell wall structure and are unable to maintain a rod shape. To explore Kin1 primary function, we constructed an ATP analogue-sensitive allele kin1-as1. Kin1 inhibition primarily promotes delocalization of plasma membrane-associated markers of actively growing cell surface regions. Kin1 itself is depolarized and its mobility is strongly reduced. Subsequently, amorphous cell wall material accumulates at the cell surface, a phenotype that is dependent on vesicular trafficking, and the cell wall integrity mitogen-activated protein kinase pathway is activated. Deletion of cell wall integrity mitogen-activated protein kinase components reduces kin1Δ hypersensitivity to stresses such as those induced by Calcofluor white and SDS. We propose that Kin1 is required for a tight link between the plasma membrane and the cell wall.

Three's company: the fission yeast actin cytoskeleton

How the actin cytoskeleton assembles into different structures to drive diverse cellular processes is a fundamental cell biological question. In addition to orchestrating the appropriate combination of regulators and actin-binding proteins, different actin-based structures must insulate themselves from one another to maintain specificity within a crowded cytoplasm. Actin specification is particularly challenging in complex eukaryotes where a multitude of protein isoforms and actin structures operate within the same cell. Fission yeast Schizosaccharomyces pombe possesses a single actin isoform that functions in three distinct structures throughout the cell cycle. In this review we explore recent studies in fission yeast that help unravel how different actin structures operate in cells.

Autophagy in the fission yeast Schizosaccharomyces pombe

Autophagy is a non-selective degradation process in eukaryotic cells. The genome sequence of the fission yeast Schizosaccharomyces pombe has revealed that many of the genes required for autophagy are common between the fission yeast and budding yeast, suggesting that the basic machinery of autophagy is conserved between these species. Autophagy in fission yeast is specifically induced by nitrogen starvation based on monitoring a GFP-Atg8p marker. Upon nitrogen starvation, fission yeast cells exit the vegetative cell cycle and initiate sexual differentiation to produce spores. Most of the nitrogen used for de novo protein synthesis during sporulation derives from the autophagic protein degradation system. This review focuses on the recent advances in the role of autophagy in fission yeast.

Deletion mutants of AP-1 adaptin subunits display distinct phenotypes in fission yeast

Adaptins are subunits of the heterotetrameric (beta/mu/gamma/sigma) adaptor protein (AP) complexes that are involved in clathrin-mediated membrane trafficking. Here, we show that in Schizosaccharomyces pombe the deletion strains of each individual subunit of the AP-1 complex [Apl2 (beta), Apl4 (gamma), Apm1 (mu) and Aps1 (sigma)] caused distinct phenotypes on growth sensitivity to temperature or drugs. We also show that the Deltaapm1 and Deltaapl2 mutants displayed similar but more severe phenotypes than those of Deltaaps1 or Deltaapl4 mutants. Furthermore, the Deltaapl2Deltaaps1 and Deltaapl2Deltaapl4 double mutants displayed synthetic growth defects, whereas the Deltaaps1Deltaapl4 and Deltaapl2Deltaapm1 double mutants did not. In pull-down assay, Apm1 binds Apl2 even in the absence of Aps1 and Apl4, and Apl4 binds Aps1 even in the absence of Apm1 and Apl2. Consistently, the deletion of any subunit generally caused the disassociation of the heterotetrameric complex from endosomes, although some subunits weakly localized to endosomes. In addition, the deletion of individual subunits caused similar endosomal accumulation of v-SNARE synaptobrevin Syb1. Altogether, results suggest that the four subunits are all essential for the heterotetrameric complex formation and for the AP-1 function in exit transport from endosomes.

Genetic control of cellular quiescence in S. pombe

Transition from proliferation to quiescence brings about extensive changes in cellular behavior and structure. However, the genes that are crucial for establishing and/or maintaining quiescence are largely unknown. The fission yeast Schizosaccharomyces pombe is an excellent model in which to study this problem, because it becomes quiescent under nitrogen starvation. Here, we characterize 610 temperature-sensitive mutants, and identify 33 genes that are required for entry into and maintenance of quiescence. These genes cover a broad range of cellular functions in the cytoplasm, membrane and nucleus. They encode proteins for stress-responsive and cell-cycle kinase signaling pathways, for actin-bound and osmo-controlling endosome formation, for RNA transcription, splicing and ribosome biogenesis, for chromatin silencing, for biosynthesis of lipids and ATP, for cell-wall and membrane morphogenesis, and for protein trafficking and vesicle fusion. We specifically highlight Fcp1, a CTD phosphatase of RNA polymerase II, which differentially affects the transcription of genes that are involved in quiescence and proliferation. We propose that the transcriptional role of Fcp1 is central in differentiating quiescence from proliferation.

The fission yeast SEL1 domain protein Cfh3p: a novel regulator of the glucan synthase Bgs1p whose function is more relevant under stress conditions

In Schizosaccharomyces pombe, Bgs1/Cps1p is a beta(1,3)-glucan synthase required for linear beta(1,3)-glucan synthesis and primary septum formation. Here, we have studied the regulation of Bgs1p by Cfh3/Chr4p, a member of a family of conserved adaptor proteins, which resembles the chitin synthase regulator Chs4p from Saccharomyces cerevisiae and Candida albicans. cfh3Delta cells showed a genetic interaction with cps1-191, and Cfh3p co-immunoprecipitated with Bgs1/Cps1p. In the absence of cfh3(+), cells were more sensitive to digestion by glucanases, and both Calcofluor staining and glucan synthesis were reduced. We found that in a wild-type strain, beta(1,3)-glucan synthesis was reduced under stress conditions. In the cfh3Delta, cps1-191, and cfh3Delta cps1-191 strains, beta(1,3)-glucan synthesis was further reduced, and growth was impaired under stress conditions, suggesting that Cfh3p and Bgs1p might play a role in ensuring growth in unfavorable environments. In a cfh3Delta mutant, Bgs1p was delocalized when the cells were distressed, but a blockade in endocytosis prevented this delocalization. Finally, we found that the SEL1 repeats are required for Cfh3p function. These results show that Cfh3p is a regulatory protein for Bgs1p and that its function is particularly necessary when the cells are undergoing stress.

Mug27 is a meiosis-specific protein kinase that functions in fission yeast meiosis II and sporulation

Several meiosis-specific proteins of Schizosaccharomyces pombe play essential roles in meiotic progression. We report here that a novel meiosis-specific protein kinase, Mug27 (also known as Ppk35), is required for proper spore formation. This kinase is expressed by the mug27(+) gene, which is abruptly transcribed after horsetail movement. This transcription is maintained until the second meiotic division. Green fluorescent protein (GFP)-tagged Mug27 appears at the start of prometaphase I, localizes to the spindle pole body (SPB) and then translocates to the forespore membrane (FSM) at late anaphase II. In the mug27Delta strain, smaller spores are produced compared with those of the mug27(+) strain. Moreover, spore viability was reduced by half or more compared with that of the mug27(+) strain. The protein-kinase activity of Mug27 appears to be important for its function: the putative kinase-dead Mug27 mutant had similar phenotypes to mug27Delta. Our results here indicate that the Mug27 kinase localizes at the SPB and regulates FSM formation and sporulation.

btn1 affects endocytosis, polarization of sterol-rich membrane domains and polarized growth in Schizosaccharomyces pombe

btn1, the Schizosaccharomyces pombe orthologue of the human Batten disease gene CLN3, exerts multiple cellular effects. As well as a role in vacuole pH homoeostasis, we now show that Btn1p is essential for growth at high temperatures. Its absence results in progressive defects at 37°C that culminate in total depolarized growth and cell lysis. These defects are preceded by a progressive failure to correctly polarize sterol-rich domains after cytokinesis and are accompanied by loss of Myo1p localization. Furthermore, we found that in Sz. pombe, sterol spreading is linked to defective formation/polarization of F-actin patches and disruption of endocytosis and that these processes are aberrant in btn1Δ cells. Consistent with a role for Btn1p in polarized growth, Btn1p has an altered location at 37°C and is retained in actin-dependent endomembrane structures near the cell poles or septum.

Essential roles of class E Vps proteins for sorting into multivesicular bodies in Schizosaccharomyces pombe

The multivesicular body (MVB) sorting pathway is required for a number of biological processes, including downregulation of cell-surface proteins and protein sorting into the vacuolar lumen. The function of this pathway requires endosomal sorting complexes required for transport (ESCRT) composed of class E vacuolar protein sorting (Vps) proteins in Saccharomyces cerevisiae, many of which are conserved in Schizosaccharomyces pombe. Of these, sst4/vps27 (homologous to VPS27) and sst6 (similar to VPS23) have been identified as suppressors of sterility in ste12Delta (sst), although their functions have not been uncovered to date. In this report, these two sst genes are shown to be required for vacuolar sorting of carboxypeptidase Y (CPY) and an MVB marker, the ubiquitin-GFP-carboxypeptidase S (Ub-GFP-CPS) fusion protein, despite the lack of the ubiquitin E2 variant domain in Sst6p. Disruption mutants of a variety of other class E vps homologues also had defects in sorting of CPY and Ub-GFP-CPS. Sch. pombe has a mammalian AMSH homologue, sst2. Phenotypic analyses suggested that Sst2p is a class E Vps protein. Taken together, these results suggest that sorting into multivesicular bodies is dependent on class E Vps proteins, including Sst2p, in Sch. pombe.

Schizosaccharomyces pombe protein phosphatase 1 in mitosis, endocytosis and a partnership with Wsh3/Tea4 to control polarised growth

PP1 holoenzymes are composed of a small number of catalytic subunits and an array of regulatory, targeting, subunits. The Schizosaccharomyces pombe genome encodes two highly related catalytic subunits, Dis2 and Sds21. The gene for either protein can be individually deleted, however, simultaneous deletion of both is lethal. We fused enhanced green fluorescent protein (EGFP) coding sequences to the 5' end of the endogenous sds21(+) and dis2(+) genes. Dis2.NEGFP accumulated in nuclei, associated with centromeres, foci at cell tips and endocytic vesicles. This actin-dependent endocytosis occurred between nuclei and growing tips and was polarised towards growing tips. When dis2(+) was present, Sds21.NEGFP was predominantly a nuclear protein, greatly enriched in the nucleolus. When dis2(+) was deleted, Sds21.NEGFP levels increased and Sds21.NEGFP was then clearly detected at centromeres, endocytic vesicles and cell tips. Dis2.NEGFP was recruited to cell tips by the formin binding, stress pathway scaffold Wsh3 (also known as Tea4). Wsh3/Tea4 modulates polarised tip growth in unperturbed cell cycles and governs polarised growth following osmotic stress. Mutating the PP1 recruiting RVXF motif in Wsh3/Tea4 blocked PP1 binding, altered cell cycle regulated growth to induce branching, induced branching from existing tips in response to stress, and blocked the induction of actin filaments that would otherwise arise from Wsh3/Tea4 overproduction.

Geranylgeranyl diphosphate synthase in fission yeast is a heteromer of farnesyl diphosphate synthase (FPS), Fps1, and an FPS-like protein, Spo9, essential for sporulation

Both farnesyl diphosphate synthase (FPS) and geranylgeranyl diphosphate synthase (GGPS) are key enzymes in the synthesis of various isoprenoid-containing compounds and proteins. Here, we describe two novel Schizosaccharomyces pombe genes, fps1(+) and spo9(+), whose products are similar to FPS in primary structure, but whose functions differ from one another. Fps1 is essential for vegetative growth, whereas, a spo9 null mutant exhibits temperature-sensitive growth. Expression of fps1(+), but not spo9(+), suppresses the lethality of a Saccharomyces cerevisiae FPS-deficient mutant and also restores ubiquinone synthesis in an Escherichia coli ispA mutant, which lacks FPS activity, indicating that S. pombe Fps1 in fact functions as an FPS. In contrast to a typical FPS gene, no apparent GGPS homologues have been found in the S. pombe genome. Interestingly, although neither fps1(+) nor spo9(+) expression alone in E. coli confers clear GGPS activity, coexpression of both genes induces such activity. Moreover, the GGPS activity is significantly reduced in the spo9 mutant. In addition, the spo9 mutation perturbs the membrane association of a geranylgeranylated protein, but not that of a farnesylated protein. Yeast two-hybrid and coimmunoprecipitation analyses indicate that Fps1 and Spo9 physically interact. Thus, neither Fps1 nor Spo9 alone functions as a GGPS, but the two proteins together form a complex with GGPS activity. Because spo9 was originally identified as a sporulation-deficient mutant, we show here that expansion of the forespore membrane is severely inhibited in spo9Delta cells. Electron microscopy revealed significant accumulation membrane vesicles in spo9Delta cells. We suggest that lack of GGPS activity in a spo9 mutant results in impaired protein prenylation in certain proteins responsible for secretory function, thereby inhibiting forespore membrane formation.

Loss of a GPI-anchored membrane protein Aah3p causes a defect in vacuolar protein sorting in Schizosaccharomyces pombe

Schizosaccharomyces pombe has four alpha-amylase homologs (Aah1p-Aah4p) with a glycosylphosphatidylinositol (GPI) modification site at the C-terminal end. Disruption mutants of aah genes were tested for mislocalization of vacuolar carboxypeptidase Y (CPY), and aah3Delta was found to secrete CPY. The conversion rate from pro- to mature CPY was greatly impaired in aah3Delta, and fluorescence microscopy inidicated that a sorting receptor for CPY, Vps10p, mislocalized to the vacuolar membrane. These results indicate that aah3Delta had a defect in the retrograde transport of Vps10p, and that Aah3p is the first S. pombe specific protein required for vacuolar protein sorting.

Genetic and functional interaction between Ryh1 and Ypt3: two Rab GTPases that function in S. pombe secretory pathway

We have previously isolated ypt3-i5 mutant and showed that Ypt3 GTPase functions in the fission yeast secretory pathway. Here, the same genetic screen led to the isolation of ryh1-i6, a mutant allele of the ryh1+ gene encoding a homolog of Rab6. The ryh1-i6 mutant showed phenotypes that support its role in retrograde traffic from endosome to the Golgi. Interestingly, ryh1+ gene deletion was synthetically lethal with ypt3-i5 mutation. Consistently, the over-expression of the GDP-conformational mutant, Ryh1T25 N, inhibited the growth of ypt3-i5 mutant but had no effect on that of wild-type cells. Furthermore, the over-expression of the Ryh1T25N mutant inhibited the acid phosphatase glycosylation and exacerbated the cell wall integrity of ypt3-i5 mutant, but had no effect on those of wild-type cells. GFP-Ryh1 and GFP-Ypt3 both localized at the Golgi/endosome, but showed distinct subcellular localizations. The localization of GFP-Ryh1 in ypt3-i5 mutant and that of GFP-Ypt3 in ryh1-i6 mutant were distinct from those in wild-type cells. In addition, Ryh1 as well as Ypt3 were shown to be involved in acid phosphatase secretion. These results suggest that Ryh1 is involved in the secretory pathway and may have a potential overlapping function with Ypt3 in addition to its role in recycling.

A role for fission yeast Rab GTPase Ypt7p in sporulation

Ypt7p, a fission yeast (Schizosaccharomyces pombe) homologue of Rab7 GTPase, mediates fusion of endosomes to vacuoles and homotypic vacuole fusion. Here, we report that Ypt7p plays important roles in sporulation. Most ypt7Delta asci produced less than four spores, which were apparently immature and germinated at low frequency. Furthermore, ypt7Delta cells were defective in development of the forespore membranes. Vacuoles in sporulating cells were found to undergo extensive homotypic vacuole fusion to form a few large compartments occupying the entire cytoplasm of asci. This extensive vacuole fusion depended on Ypt7p.

btn1, theSchizosaccharomyces pombehomologue of the human Batten disease geneCLN3, regulates vacuole homeostasis

We have cloned the Schizosaccharomyces pombe homologue of the human Batten disease gene, CLN3. This gene, btn1, encodes a predicted transmembrane protein that is 30% identical and 48% similar to its human counterpart. Cells deleted for btn1 were viable but had enlarged and more alkaline vacuoles. Conversely overexpression of Btn1p reduced both vacuole diameter and pH. Thus Btn1p regulates vacuole homeostasis. The vacuolar defects of btn1Δ cells were rescued by heterologous expression of CLN3, proving that Btn1p and CLN3 are functional homologues. The disease severity of Batten disease-causing mutations (G187A, E295K and V330F), when expressed in btn1 appeared to correlate with their effect on vacuolar pH, suggesting that elevated lysosomal pH contributes to the disease process. In fission yeast, both Btn1p and CLN3 trafficked to the vacuole membrane via early endocytic and pre-vacuolar compartments, and localisation of Btn1p to the vacuole membrane was dependent on the Ras GTPase Ypt7p. Importantly, vacuoles in cells deleted for both ypt7 and btn1 were larger and more alkaline than those of cells deleted for ypt7 alone, indicating that Btn1p has a functional role prior to reaching the vacuole. Consistently, btn1 and vma1, the gene encoding subunit A of the V1 portion of vATPase, showed conditional synthetic lethality, and in cells deleted for vma1 (a subunit of the vacuolar ATPase) Btn1p was essential for septum deposition during cytokinesis.

Interactions of WASp, myosin-I, and verprolin with Arp2/3 complex during actin patch assembly in fission yeast

Yeast actin patches are dynamic structures that form at the sites of cell growth and are thought to play a role in endocytosis. We used biochemical analysis and live cell imaging to investigate actin patch assembly in fission yeast Schizosaccharomyces pombe. Patch assembly proceeds via two parallel pathways: one dependent on WASp Wsp1p and verprolin Vrp1p converges with another dependent on class 1 myosin Myo1p to activate the actin-related protein 2/3 (Arp2/3) complex. Wsp1p activates Arp2/3 complex via a conventional mechanism, resulting in branched filaments. Myo1p is a weaker Arp2/3 complex activator that makes unstable branches and is enhanced by verprolin. During patch assembly in vivo, Wsp1p and Vrp1p arrive first independent of Myo1p. Arp2/3 complex associates with nascent activator patches over 6–9 s while remaining stationary. After reaching a maximum concentration, Arp2/3 complex patches move centripetally as activator proteins dissociate. Genetic dependencies of patch formation suggest that patch formation involves cross talk between Myo1p and Wsp1p/Vrp1p pathways.

The novel fission yeast protein Pal1p interacts with Hip1-related Sla2p/End4p and is involved in cellular morphogenesis

The establishment and maintenance of characteristic cellular morphologies is a fundamental property of all cells. Here we describe Schizosaccharomyces pombe Pal1p, a protein important for maintenance of cylindrical cellular morphology. Pal1p is a novel membrane-associated protein that localizes to the growing tips of interphase cells and to the division site in cells undergoing cytokinesis in an F-actin- and microtubule-independent manner. Cells deleted for pal1 display morphological defects, characterized by the occurrence of spherical and pear-shaped cells with an abnormal cell wall. Pal1p physically interacts and displays overlapping localization with the Huntingtin-interacting-protein (Hip1)-related protein Sla2p/End4p, which is also required for establishment of cylindrical cellular morphology. Sla2p is important for efficient localization of Pal1p to the sites of polarized growth and appears to function upstream of Pal1p. Interestingly, spherical pal1Delta mutants polarize to establish a pearlike morphology before mitosis in a manner dependent on the kelch-repeat protein Tea1p and the cell cycle inhibitory kinase Wee1p. Thus, overlapping mechanisms involving Pal1p, Tea1p, and Sla2p contribute to the establishment of cylindrical cellular morphology, which is important for proper spatial regulation of cytokinesis.

End4/Sla2 is involved in establishment of a new growth zone inSchizosaccharomyces pombe

The rod-shaped Schizosaccharomyces pombe cell grows in a polarized fashion from opposing ends. Correct positioning of the growth zones is directed by the polarity marker Tea1 located at the cell ends where actin patches accumulate and cell growth takes place. We show that the S. pombe homologue of Saccharomyces cerevisiae SLA2, a protein involved in cortical actin organization and endocytosis, provides a link between the polarity marker and the growth machinery. In wild-type fission yeast cells, this homologue End4/Sla2 is enriched at cell ends during interphase and localizes to a medial ring at cell division, mirroring the actin localization pattern throughout the cell cycle. Proper localization relies on membrane trafficking and is independent of both the actin and microtubule cytoskeletons. End4/Sla2 is required for the establishment of new polarised growth zones, and deletion of its C-terminal talin-like domain prevents the establishment of a new growth zone after cell fission. We propose that End4/Sla2 acts downstream of the polarity marker Tea1 and is implicated in the recruitment of the actin cytoskeleton to bring about polarised cell growth.