The secretory pathway mediates localization of the cell polarity regulator Aip3p/Bud6p

A positive feedback loop involving the Spa2 SHD domain contributes to focal polarization

Focal polarization is necessary for finely arranged cell-cell interactions. The yeast mating projection, with its punctate polarisome, is a good model system for this process. We explored the critical role of the polarisome scaffold protein Spa2 during yeast mating with a hypothesis motivated by mathematical modeling and tested by in vivo experiments. Our simulations predicted that two positive feedback loops generate focal polarization, including a novel feedback pathway involving the N-terminal domain of Spa2. We characterized the latter using loss-of-function and gain-of-function mutants. The N-terminal region contains a Spa2 Homology Domain (SHD) which is conserved from yeast to humans, and when mutated largely reproduced the spa2Δ phenotype. Our work together with published data show that the SHD domain recruits Msb3/4 that stimulates Sec4-mediated transport of Bud6 to the polarisome. There, Bud6 activates Bni1-catalyzed actin cable formation, recruiting more Spa2 and completing the positive feedback loop. We demonstrate that disrupting this loop at any point results in morphological defects. Gain-of-function perturbations partially restored focal polarization in a spa2 loss-of-function mutant without restoring localization of upstream components, thus supporting the pathway order. Thus, we have collected data consistent with a novel positive feedback loop that contributes to focal polarization during pheromone-induced polarization in yeast.

Bil2 Is a Novel Inhibitor of the Yeast Formin Bnr1 Required for Proper Actin Cable Organization and Polarized Secretion

Cell growth in budding yeast depends on rapid and on-going assembly and turnover of polarized actin cables, which direct intracellular transport of post-Golgi vesicles to the bud tip. Saccharomyces cerevisiae actin cables are polymerized by two formins, Bni1 and Bnr1. Bni1 assembles cables in the bud, while Bnr1 is anchored to the bud neck and assembles cables that specifically extend filling the mother cell. Here, we report a formin regulatory role for YGL015c, a previously uncharacterized open reading frame, which we have named Bud6 Interacting Ligand 2 (BIL2). bil2Δ cells display defects in actin cable architecture and partially-impaired secretory vesicle transport. Bil2 inhibits Bnr1-mediated actin filament nucleation in vitro, yet has no effect on the rate of Bnr1-mediated filament elongation. This activity profile for Bil2 resembles that of another yeast formin regulator, the F-BAR protein Hof1, and we find that bil2Δ with hof1Δ are synthetic lethal. Unlike Hof1, which localizes exclusively to the bud neck, GFP-Bil2 localizes to the cytosol, secretory vesicles, and sites of polarized cell growth. Further, we provide evidence that Hof1 and Bil2 inhibitory effects on Bnr1 are overcome by distinct mechanisms. Together, our results suggest that Bil2 and Hof1 perform distinct yet genetically complementary roles in inhibiting the actin nucleation activity of Bnr1 to control actin cable assembly and polarized secretion.

YFR016c/Aip5 is part of an actin nucleation complex in yeast

The polarisome comprises a network of proteins that organizes polar growth in yeast and filamentous fungi. The yeast formin Bni1 and the actin nucleation-promoting factor Bud6 are subunits of the polarisome that together catalyze the formation of actin cables below the tip of yeast cells. We identified YFR016c (Aip5) as an interaction partner of Bud6 and the polarisome scaffold Spa2. Yeast cells lacking Aip5 display a reduced number of actin cables. Aip5 binds with its N-terminal region to Spa2 and with its C-terminal region to Bud6. Both interactions collaborate to localize Aip5 at bud tip and neck, and are required to stimulate the formation of actin cables. Our experiments characterize Aip5 as a novel subunit of a complex that regulates the number of actin filaments at sites of polar growth.

Integrated control of formin-mediated actin assembly by a stationary inhibitor and a mobile activator

This study shows that in vivo actin nucleation by the yeast formin Bnr1 is controlled through the coordinated effects of two distinct regulators, a stationary inhibitor (the F-BAR protein Hof1) and a mobile activator (Bud6), establishing a positive feedback loop for precise spatial and temporal control of actin assembly.

Formins are essential actin assembly factors whose activities are controlled by a diverse array of binding partners. Until now, most formin ligands have been studied on an individual basis, leaving open the question of how multiple inputs are integrated to regulate formins in vivo. Here, we show that the F-BAR domain of Saccharomyces cerevisiae Hof1 interacts with the FH2 domain of the formin Bnr1 and blocks actin nucleation. Electron microscopy of the Hof1–Bnr1 complex reveals a novel dumbbell-shaped structure, with the tips of the F-BAR holding two FH2 dimers apart. Deletion of Hof1’s F-BAR domain in vivo results in disorganized actin cables and secretory defects. The formin-binding protein Bud6 strongly alleviates Hof1 inhibition in vitro, and bud6Δ suppresses hof1Δ defects in vivo. Whereas Hof1 stably resides at the bud neck, we show that Bud6 is delivered to the neck on secretory vesicles. We propose that Hof1 and Bud6 functions are intertwined as a stationary inhibitor and a mobile activator, respectively.

Adipocyte differentiation-related protein promotes lipid accumulation in goat mammary epithelial cells

Milk fat originates from the secretion of cytosolic lipid droplets (CLD) synthesized within mammary epithelial cells. Adipocyte differentiation-related protein (ADRP; gene symbol PLIN2) is a CLD-binding protein that is crucial for synthesis of mature CLD. Our hypothesis was that ADRP regulates CLD production and metabolism in goat mammary epithelial cells (GMEC) and thus plays a role in determining milk fat content. To understand the role of ADRP in ruminant milk fat metabolism, ADRP (PLIN2) was overexpressed or knocked down in GMEC using an adenovirus system. Immunocytochemical staining revealed that ADRP localized to the surface of CLD. Supplementation with oleic acid (OA) enhanced its colocalization with CLD surface and enhanced lipid accumulation. Overexpression of ADRP increased lipid accumulation and the concentration of triacylglycerol in GMEC. In contrast, morphological examination revealed that knockdown of ADRP decreased lipid accumulation even when OA was supplemented. This response was confirmed by the reduction in mass of cellular TG when ADRP was knocked down. The fact that knockdown of ADRP did not completely eliminate lipid accumulation at a morphological level in GMEC without OA suggests that some other compensatory factors may also aid in the process of CLD formation. The ADRP reversed the decrease of CLD accumulation induced by adipose triglyceride lipase. This is highly suggestive of ADRP promoting triacylglycerol stability within CLD by preventing access to adipose triglyceride lipase. Collectively, these data provide direct in vitro evidence that ADRP plays a key role in CLD formation and stability in GMEC.

A sphingolipid-dependent diffusion barrier confines ER stress to the yeast mother cell

In many cell types, lateral diffusion barriers compartmentalize the plasma membrane and, at least in budding yeast, the endoplasmic reticulum (ER). However, the molecular nature of these barriers, their mode of action and their cellular functions are unclear. Here, we show that misfolded proteins of the ER remain confined into the mother compartment of budding yeast cells. Confinement required the formation of a lateral diffusion barrier in the form of a distinct domain of the ER-membrane at the bud neck, in a septin-, Bud1 GTPase- and sphingolipid-dependent manner. The sphingolipids, but not Bud1, also contributed to barrier formation in the outer membrane of the dividing nucleus. Barrier-dependent confinement of ER stress into the mother cell promoted aging. Together, our data clarify the physical nature of lateral diffusion barriers in the ER and establish the role of such barriers in the asymmetric segregation of proteotoxic misfolded proteins during cell division and aging.

DOI:http://dx.doi.org/10.7554/eLife.01883.001

Cell division isn't always about splitting a cell into two identical parts. The diversity of many of our own cells relies on asymmetric cell divisions. The yeast used to make bread rely on a process called ‘budding’ that involves a small daughter cell emerging from the surface of the mother cell. Mother cells can only produce around 20–50 daughter cells before dying from old age. However, their daughters are always born rejuvenated, and not aged like their mothers.

Budding involves part of the plasma membrane that surrounds the mother cell being pinched off to produce the daughter cell. This part of the membrane contains diffusion barriers that prevent various factors—including factors that cause aging—from entering the daughter cell. The barriers are known to contain several layers, but the details of how they work were not understood.

Inside the budding cell, the membrane of the endoplasmic reticulum (ER) also contains lateral diffusion barriers. The ER is the structure in the cell responsible for folding newly made proteins correctly. Any misfolded, toxic proteins are kept in the ER to be refolded or destroyed. However, if there are too many misfolded proteins, the ER gets stressed and triggers a mechanism that in extreme cases causes the cell to self-destruct.

Clay, Caudron et al. have now shown that ER stress causes yeast cells to age. Moreover, when the ER is stressed, the ER diffusion barrier prevents the stress that causes aging entering the daughter cells.

Clay, Caudron et al. also established that the diffusion barrier in the ER is made up of three layers. A layer of fatty molecules called sphingolipids is found at the bottom of the barrier, and such a layer is also present in other diffusion barriers. This could therefore act as the skeleton on which diffusion barriers form. Further investigation of this layer should provide a better understanding of how diffusion barriers work.

DOI:http://dx.doi.org/10.7554/eLife.01883.002

Cell-cycle regulation of formin-mediated actin cable assembly

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

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

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

Structure of the formin-interaction domain of the actin nucleation-promoting factor Bud6

Formin proteins and their associated factors cooperate to assemble unbranched actin filaments in diverse cellular structures. The Saccharomyces cerevisiae formin Bni1 and its associated nucleation-promoting factor (NPF) Bud6 generate actin cables and mediate polarized cell growth. Bud6 binds to both the tail of the formin and G-actin, thereby recruiting monomeric actin to the formin to create a nucleation seed. Here, we structurally and functionally dissect the nucleation-promoting C-terminal region of Bud6 into a Bni1-binding "core" domain and a G-actin binding "flank" domain. The ∼2-Å resolution crystal structure of the Bud6 core domain reveals an elongated dimeric rod with a unique fold resembling a triple-helical coiled-coil. Binding and actin-assembly assays show that conserved residues on the surface of this domain mediate binding to Bni1 and are required for NPF activity. We find that the Bni1 dimer binds two Bud6 dimers and that the Bud6 flank binds a single G-actin molecule. These findings suggest a model in which a Bni1/Bud6 complex with a 2:4 subunit stoichiometry assembles a nucleation seed with Bud6 coordinating up to four actin subunits.

Mechanism for astral microtubule capture by cortical Bud6p priming spindle polarity in S. cerevisiae

BACKGROUND

Budding yeast is a unique model to dissect spindle orientation in a cell dividing asymmetrically. In yeast, this process begins with the capture of pole-derived astral microtubules (MTs) by the polarity determinant Bud6p at the cortex of the bud in G(1). Bud6p couples MT growth and shrinkage with spindle pole movement relative to the contact site. This activity resides in N-terminal sequences away from a domain linked to actin organization. Kip3p (kinesin-8), a MT depolymerase, may be implicated, but other molecular details are essentially unknown.

RESULTS

We show that Bud6p and Kip3p play antagonistic roles in controlling the length of MTs contacting the bud. The stabilizing role of Bud6p required the plus-end-tracking protein Bim1p (yeast EB1). Bim1p bound Bud6p N terminus, an interaction that proved essential for cortical capture of MTs in vivo. Moreover, Bud6p influenced Kip3p dynamic distribution through its effect on MT stability during cortical contacts via Bim1p. Coupling between Kip3p-driven depolymerization and shrinkage at the cell cortex required Bud6p, Bim1p, and dynein, a minus-end-directed motor helping tether the receding plus ends to the cell cortex. Validating these findings, live imaging of the interplay between dynein and Kip3p demonstrated that both motors decorated single astral MTs with dynein persisting at the plus end in association with the site of cortical contact during shrinkage at the cell cortex.

CONCLUSIONS

Astral MT shrinkage linked to Bud6p involves its direct interaction with Bim1p and the concerted action of two MT motors-Kip3p and dynein.

Secretory pathway-dependent localization of the Saccharomyces cerevisiae Rho GTPase-activating protein Rgd1p at growth sites

Establishment and maintenance of cell polarity in eukaryotes depends upon the regulation of Rho GTPases. In Saccharomyces cerevisiae, the Rho GTPase activating protein (RhoGAP) Rgd1p stimulates the GTPase activities of Rho3p and Rho4p, which are involved in bud growth and cytokinesis, respectively. Consistent with the distribution of Rho3p and Rho4p, Rgd1p is found mostly in areas of polarized growth during cell cycle progression. Rgd1p was mislocalized in mutants specifically altered for Golgi apparatus-based phosphatidylinositol 4-P [PtdIns(4)P] synthesis and for PtdIns(4,5)P(2) production at the plasma membrane. Analysis of Rgd1p distribution in different membrane-trafficking mutants suggested that Rgd1p was delivered to growth sites via the secretory pathway. Rgd1p may associate with post-Golgi vesicles by binding to PtdIns(4)P and then be transported by secretory vesicles to the plasma membrane. In agreement, we show that Rgd1p coimmunoprecipitated and localized with markers specific to secretory vesicles and cofractionated with a plasma membrane marker. Moreover, in vivo imaging revealed that Rgd1p was transported in an anterograde manner from the mother cell to the daughter cell in a vectoral manner. Our data indicate that secretory vesicles are involved in the delivery of RhoGAP Rgd1p to the bud tip and bud neck.

Mechanism and cellular function of Bud6 as an actin nucleation-promoting factor

Bud6 functions as an actin nucleation–promoting factor (NPF) for Bni1; thus formins can depend on NPFs like the Arp2/3 complex. Unexpected parallels exist between Bud6 and WASp. Bud6 is the first nonmetazoan example of formins pairing with actin monomer–binding proteins to stimulate nucleation, akin to Spire-Capu and APC-mDia1

Formins are a conserved family of actin assembly–promoting factors with diverse biological roles, but how their activities are regulated in vivo is not well understood. In Saccharomyces cerevisiae, the formins Bni1 and Bnr1 are required for the assembly of actin cables and polarized cell growth. Proper cable assembly further requires Bud6. Previously it was shown that Bud6 enhances Bni1-mediated actin assembly in vitro, but the biochemical mechanism and in vivo role of this activity were left unclear. Here we demonstrate that Bud6 specifically stimulates the nucleation rather than the elongation phase of Bni1-mediated actin assembly, defining Bud6 as a nucleation-promoting factor (NPF) and distinguishing its effects from those of profilin. We generated alleles of Bud6 that uncouple its interactions with Bni1 and G-actin and found that both interactions are critical for NPF activity. Our data indicate that Bud6 promotes filament nucleation by recruiting actin monomers to Bni1. Genetic analysis of the same alleles showed that Bud6 regulation of formin activity is critical for normal levels of actin cable assembly in vivo. Our results raise important mechanistic parallels between Bud6 and WASP, as well as between Bud6 and other NPFs that interact with formins such as Spire.

Array comparative genomic hybridization analysis of Trichoderma reesei strains with enhanced cellulase production properties

Background

Trichoderma reesei is the main industrial producer of cellulases and hemicellulases that are used to depolymerize biomass in a variety of biotechnical applications. Many of the production strains currently in use have been generated by classical mutagenesis. In this study we characterized genomic alterations in high-producing mutants of T. reesei by high-resolution array comparative genomic hybridization (aCGH). Our aim was to obtain genome-wide information which could be utilized for better understanding of the mechanisms underlying efficient cellulase production, and would enable targeted genetic engineering for improved production of proteins in general.

Results

We carried out an aCGH analysis of four high-producing strains (QM9123, QM9414, NG14 and Rut-C30) using the natural isolate QM6a as a reference. In QM9123 and QM9414 we detected a total of 44 previously undocumented mutation sites including deletions, chromosomal translocation breakpoints and single nucleotide mutations. In NG14 and Rut-C30 we detected 126 mutations of which 17 were new mutations not documented previously. Among these new mutations are the first chromosomal translocation breakpoints identified in NG14 and Rut-C30. We studied the effects of two deletions identified in Rut-C30 (a deletion of 85 kb in the scaffold 15 and a deletion in a gene encoding a transcription factor) on cellulase production by constructing knock-out strains in the QM6a background. Neither the 85 kb deletion nor the deletion of the transcription factor affected cellulase production.

Conclusions

aCGH analysis identified dozens of mutations in each strain analyzed. The resolution was at the level of single nucleotide mutation. High-density aCGH is a powerful tool for genome-wide analysis of organisms with small genomes e.g. fungi, especially in studies where a large set of interesting strains is analyzed.

Actin-mediated delivery of astral microtubules instructs Kar9p asymmetric loading to the bud-ward spindle pole

We show that Kar9p polarity is instructed by a feedback loop that requires astral microtubules, actin cables, and Myo2p-based transport to enforce Kar9p loading to the bud-ward pole. This novel mechanism also provides the basis for a model unifying Kar9p polarity and the stereotyped pattern of spindle pole inheritance known to occur in yeast.

In Saccharomyces cerevisiae, Kar9p, one player in spindle alignment, guides the bud-ward spindle pole by linking astral microtubule plus ends to Myo2p-based transport along actin cables generated by the formins Bni1p and Bnr1p and the polarity determinant Bud6p. Initially, Kar9p labels both poles but progressively singles out the bud-ward pole. Here, we show that this polarization requires cell polarity determinants, actin cables, and microtubules. Indeed, in a bud6Δ bni1Δ mutant or upon direct depolymerization of actin cables Kar9p symmetry increased. Furthermore, symmetry was selectively induced by myo2 alleles, preventing Kar9p binding to the Myo2p cargo domain. Kar9p polarity was rebuilt after transient disruption of microtubules, dependent on cell polarity and actin cables. Symmetry breaking also occurred after transient depolymerization of actin cables, with Kar9p increasing at the spindle pole engaging in repeated cycles of Kar9p-mediated transport. Kar9p returning to the spindle pole on shrinking astral microtubules may contribute toward this bias. Thus, Myo2p transport along actin cables may support a feedback loop by which delivery of astral microtubule plus ends sustains Kar9p polarized recruitment to the bud-ward spindle pole. Our findings also explain the link between Kar9p polarity and the choice setting aside the old spindle pole for daughter-bound fate.

The polarisome is required for segregation and retrograde transport of protein aggregates

The paradigm sirtuin, Sir2p, of budding yeast is required for establishing cellular age asymmetry, which includes the retention of damaged and aggregated proteins in mother cells. By establishing the global genetic interaction network of SIR2 we identified the polarisome, the formin Bni1p, and myosin motor protein Myo2p as essential components of the machinery segregating protein aggregates during mitotic cytokinesis. Moreover, we found that daughter cells can clear themselves of damage by a polarisome- and tropomyosin-dependent polarized flow of aggregates into the mother cell compartment. The role of Sir2p in cytoskeletal functions and polarity is linked to the CCT chaperonin in sir2Delta cells being compromised in folding actin. We discuss the findings in view of recent models hypothesizing that polarity may have evolved to avoid clonal senescence by establishing an aging (soma-like) and rejuvenated (germ-like) lineage.

A biochemical genomics screen for substrates of Ste20p kinase enables the in silico prediction of novel substrates

The Ste20/PAK family is involved in many cellular processes, including the regulation of actin-based cytoskeletal dynamics and the activation of MAPK signaling pathways. Despite its numerous roles, few of its substrates have been identified. To better characterize the roles of the yeast Ste20p kinase, we developed an in vitro biochemical genomics screen to identify its substrates. When applied to 539 purified yeast proteins, the screen reported 14 targets of Ste20p phosphorylation. We used the data resulting from our screen to build an in silico predictor to identify Ste20p substrates on a proteome-wide basis. Since kinase-substrate specificity is often mediated by additional binding events at sites distal to the phosphorylation site, the predictor uses the presence/absence of multiple sequence motifs to evaluate potential substrates. Statistical validation estimates a threefold improvement in substrate recovery over random predictions, despite the lack of a single dominant motif that can characterize Ste20p phosphorylation. The set of predicted substrates significantly overrepresents elements of the genetic and physical interaction networks surrounding Ste20p, suggesting that some of the predicted substrates are in vivo targets. We validated this combined experimental and computational approach for identifying kinase substrates by confirming the in vitro phosphorylation of polarisome components Bni1p and Bud6p, thus suggesting a mechanism by which Ste20p effects polarized growth.

Septins and the lateral compartmentalization of eukaryotic membranes

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

Growth-speed-correlated localization of exocyst and polarisome components in growth zones of Ashbya gossypii hyphal tips

We use the fungus Ashbya gossypii to investigate how its polar growth machinery is organized to achieve sustained hyphal growth. In slowly elongating hyphae exocyst, cell polarity and polarisome proteins permanently localize as cortical cap at hyphal tips, thus defining the zone of secretory vesicle fusion. In tenfold faster growing hyphae, this zone is only slightly enlarged demonstrating a capacity of hyphal growth zones to increase rates of vesicle processing to reach higher speeds. Concomitant with this increase, vesicles accumulate as spheroid associated with the tip cortex, indicating that a Spitzenkörper forms in fast hyphae. We also found spheroid-like accumulations for the exocyst components AgSec3, AgSec5, AgExo70 and the polarisome components AgSpa2, AgBni1 and AgPea2 (but not AgBud6 or cell polarity factors such as AgCdc42 or AgBem1). The localization of AgSpa2, AgPea2 and AgBni1 depend on each other but only marginally on AgBud6, as concluded from a set of deletions. Our data define three conditions to achieve fast growth at hyphal tips: permanent presence of the polarity machinery in a confined cortical area, organized accumulation of vesicles and a subset of polarity components close to this area, and spatial separation of the zones of exocytosis (tip front) and endocytosis (tip rim).

Dissecting the involvement of formins in Bud6p-mediated cortical capture of microtubules in S. cerevisiae

In S. cerevisiae, spindle orientation is linked to the inheritance of the `old' spindle pole by the bud. A player in this asymmetric commitment, Bud6p, promotes cortical capture of astral microtubules. Additionally, Bud6p stimulates actin cable formation though the formin Bni1p. A relationship with the second formin, Bnr1p, is unclear. Another player is Kar9p, a protein that guides microtubules along actin cables organised by formins. Here, we ask whether formins mediate Bud6p-dependent microtubule capture beyond any links to Kar9p and actin. We found that both formins control Bud6p localisation. bni1 mutations advanced recruitment of Bud6p at the bud neck, ahead of spindle assembly, whereas bnr1Δ reduced Bud6p association with the bud neck. Accordingly, bni1 or bnr1 mutations redirected microtubule capture to or away from the bud neck, respectively. Furthermore, a Bni1p truncation that can form actin cables independently of Bud6p could not bypass a bud6Δ for microtubule capture. Conversely, Bud61-565p, a truncation insufficient for correct actin organisation via formins, supported microtubule capture. Finally, Bud6p or Bud61-565p associated with microtubules in vitro. Thus, surprisingly, Bud6p may promote microtubule capture independently of its links to actin organisation, whereas formins would contribute to the program of Bud6p-dependent microtubule-cortex interactions by controlling Bud6p localisation.

Spa2 is required for morphogenesis but it is dispensable for pathogenicity in the phytopathogenic fungus Ustilago maydis

The increasing evidence linking regulation of polar growth and pathogenicity in fungi has elicited a significant effort devoted to produce a better understanding of mechanisms determining polarization in pathogenic fungi. Here we characterize in the phytopathogenic basidiomycete Ustilago maydis, the Spa2 protein, a well-known component of polarisome, firstly described in Saccharomyces cerevisiae. U. maydis display a dimorphic switch between budding growth of hapoid cells and filamentous growth of the dikaryon. During yeast growth, a GFP-tagged Spa2 protein localized to distinct growth sites in a cell cycle-specific manner, while during hyphal growth is persistently located to hyphal tips. Deletion of spa2 gene produces rounder budding cells and thicker filaments than wild-type cells, suggesting a role of Spa2 for the determination of the growth area in U. maydis. We also address the connections between Spa2 and the actin- and microtubule-cytoskeleton. We found that the absence of Spa2 does not affect cytoskeleton organization and strikingly, interference with actin filament or microtubule formation does not affect the polar localization of Spa2. In contrast, defects in the small GTPase Rac1 seems to affect the ability of Spa2 to locate to precise sites at the tip cell. Finally, to our surprise, we found that cells defectives in Spa2 function were as pathogenic as wild-type cells.

Mechanism and function of formins in the control of actin assembly

Formins are a widely expressed family of proteins that govern cell shape, adhesion, cytokinesis, and morphogenesis by remodeling the actin and microtubule cytoskeletons. These large multidomain proteins associate with a variety of other cellular factors and directly nucleate actin polymerization through a novel mechanism. The signature formin homology 2 (FH2) domain initiates filament assembly and remains persistently associated with the fast-growing barbed end, enabling rapid insertion of actin subunits while protecting the end from capping proteins. On the basis of structural and mechanistic work, an integrated model is presented for FH2 processive motion. The adjacent FH1 domain recruits profilin-actin complexes and accelerates filament elongation. The most predominantly expressed formins in animals and fungi are autoinhibited through intramolecular interactions and appear to be activated by Rho GTPases and additional factors. Other classes of formins lack the autoinhibitory and/or Rho-binding domains and thus are likely to be controlled by alternative mechanisms.

Stable preanaphase spindle positioning requires Bud6p and an apparent interaction between the spindle pole bodies and the neck

Faithful partitioning of genetic material during cell division requires accurate spatial and temporal positioning of nuclei within dividing cells. In Saccharomyces cerevisiae, nuclear positioning is regulated by an elegant interplay between components of the actin and microtubule cytoskeletons. Regulators of this process include Bud6p (also referred to as the actin-interacting protein Aip3p) and Kar9p, which function to promote contacts between cytoplasmic microtubule ends and actin-delimited cortical attachment points. Here, we present the previously undetected association of Bud6p with the cytoplasmic face of yeast spindle pole bodies, the functional equivalent of metazoan centrosomes. Cells lacking Bud6p show exaggerated movements of the nucleus between mother and daughter cells and display reduced amounts of time a given spindle pole body spends in close association with the neck region of budding cells. Furthermore, overexpression of BUD6 greatly enhances interactions between the spindle pole body and mother-bud neck in a spindle alignment-defective dynactin mutant. These results suggest that association of either spindle pole body with neck components, rather than simply entry of a spindle pole body into the daughter cell, provides a positive signal for the progression of mitosis. We propose that Bud6p, through its localization at both spindle pole bodies and at the mother-bud neck, supports this positive signal and provides a regulatory mechanism to prevent excessive oscillations of preanaphase nuclei, thus reducing the likelihood of mitotic delays and nuclear missegregation.

Yeast formins Bni1 and Bnr1 utilize different modes of cortical interaction during the assembly of actin cables

The budding yeast formins Bni1 and Bnr1 control the assembly of actin cables. These formins exhibit distinct patterns of localization and polymerize two different populations of cables: Bni1 in the bud and Bnr1 in the mother cell. We generated a functional Bni1-3GFP that improved the visualization of Bni1 in vivo at endogenous levels. Bni1 exists as speckles in the cytoplasm, some of which colocalize on actin cables. These Bni1 speckles display linear, retrograde-directed movements. Loss of polymerized actin or specifically actin cables abolished retrograde movement, and resulted in depletion of Bni1 speckles from the cytoplasm, with enhanced targeting of Bni1 to the bud tip. Mutations that impair the actin assembly activity of Bni1 abolished the movement of Bni1 speckles, even when actin cables were present. In contrast, Bnr1-GFP or 3GFP-Bnr1 did not detectably associate with actin cables and was not observed as cytoplasmic speckles. Finally, fluorescence recovery after photobleaching demonstrated that Bni1 was very dynamic, exchanging between polarized sites and the cytoplasm, whereas Bnr1 was confined to the bud neck and did not exchange with a cytoplasmic pool. In summary, our results indicate that formins can have distinct modes of cortical interaction during actin cable assembly.

Functional characterization of Aspergillus nidulans homologues of Saccharomyces cerevisiae Spa2 and Bud6

The importance of polarized growth for fungi has elicited significant effort directed at better understanding underlying mechanisms of polarization, with a focus on yeast systems. At sites of tip growth, multiple protein complexes assemble and coordinate to ensure that incoming building material reaches the appropriate destination sites, and polarized growth is maintained. One of these complexes is the polarisome that consists of Spa2, Bud6, Pea2, and Bni1 in Saccharomyces cerevisiae. Filamentous hyphae differ in their development and life style from yeasts and likely regulate polarized growth in a different way. This is expected to reflect on the composition and presence of protein complexes that assemble at the hyphal tip. In this study we searched for polarisome homologues in the model filamentous fungus Aspergillus nidulans and characterized the S. cerevisiae Spa2 and Bud6 homologues, SpaA and BudA. Compared to the S. cerevisiae Spa2, SpaA lacks domain II but has three additional domains that are conserved within filamentous fungi. Gene replacement strains and localization studies show that SpaA functions exclusively at the hyphal tip, while BudA functions at sites of septum formation and possibly at hyphal tips. We show that SpaA is not required for the assembly or maintenance of the Spitzenkörper. We propose that the core function of the polarisome in polarized growth is maintained but with different contributions of polarisome components to the process.

Cdc42-dependent localization of polarisome component Spa2 to the incipient bud site is independent of the GDP/GTP exchange factor Cdc24

Cdc42, a member of the Rho subfamily of small GTPases, is highly conserved in both sequence and function across eukaryotic species. In budding yeast, Cdc42 triggers polarized growth necessary for bud emergence via rearrangement of the actin cytoskeleton. It has been shown that the role of Cdc42 in bud emergence requires both Cdc28-Cln (G1) kinase and the passage through START. In this report, we show that Cdc42 also serves an essential function in the establishment of bud site prior to START by catalyzing the translocation of bud-site components such as Spa2 to the cell cortex. Our analysis of various conditional alleles of CDC42 suggests that these two functions (bud site establishment and bud emergence) are genetically separable. Surprisingly, the role of Cdc42 in the cortical localization of Spa2 appears to be independent of its well known GTP/GDP exchange factor Cdc24. We also provide evidence that this role of Cdc42 requires the function of the COPI coatomer complex.

Lethal giant larvae proteins interact with the exocyst complex and are involved in polarized exocytosis

The tumor suppressor lethal giant larvae (Lgl) plays a critical role in epithelial cell polarization. However, the molecular mechanism by which Lgl carries out its functions is unclear. In this study, we report that the yeast Lgl proteins Sro7p and Sro77p directly interact with Exo84p, which is a component of the exocyst complex that is essential for targeting vesicles to specific sites of the plasma membrane for exocytosis, and that this interaction is important for post-Golgi secretion. Genetic analyses demonstrate a molecular pathway from Rab and Rho GTPases through the exocyst and Lgl to SNAREs, which mediate membrane fusion. We also found that overexpression of Lgl and t-SNARE proteins not only improves exocytosis but also rescues polarity defects in exocyst mutants. We propose that, although Lgl is broadly distributed in the cells, its localized interaction with the exocyst and kinetic activation are important for the establishment and reenforcement of cell polarity.

Cortical capture of microtubules and spindle polarity in budding yeast - where's the catch?

In asymmetric divisions, the mitotic spindle must align according to the cell polarity axis. This is achieved through targeting astral microtubules emanating from each spindle pole to opposite cell cortex compartments. Saccharomyces cerevisiae is a powerful genetic model for dissection of this complex process. Intense research in this yeast has rendered detailed models for a program linking actin organization and spindle orientation along the mother-bud axis. This program requires the separate contributions of Kar9p, a protein guiding microtubules along polarized actin cables, and the polarity determinant Bud6p/Aip3 that marks sites for cortical capture at the bud tip and bud neck. In an added layer of complexity, cyclin-dependent kinase (Cdk) differentially regulates spindle pole function to dictate asymmetric spindle pole fate. Asymmetric contacts established by the spindle poles impart a further layer of extrinsic asymmetry restricting recruitment of Kar9p to the pole destined for the daughter cell. As a result, astral microtubules from a single pole are guided to the bud compartment after spindle assembly. Finally, Cdk might also translocate along astral microtubules in association with Kar9p to modulate microtubule-cortex interactions following spindle alignment. Insertion of the mitotic spindle into the bud neck is driven by the microtubule motor dynein. This process relies on the combined action of microtubule-plus-end-tracking proteins and kinesins that control the cell-cycle-dependent abundance of dynein at microtubule plus ends. Thus, this actin-independent pathway for spindle orientation might also be influenced by Cdk.

Septin-dependent compartmentalization of the endoplasmic reticulum during yeast polarized growth

Polarized cells frequently use diffusion barriers to separate plasma membrane domains. It is unknown whether diffusion barriers also compartmentalize intracellular organelles. We used photobleaching techniques to characterize protein diffusion in the yeast endoplasmic reticulum (ER). Although a soluble protein diffused rapidly throughout the ER lumen, diffusion of ER membrane proteins was restricted at the bud neck. Ultrastructural studies and fluorescence microscopy revealed the presence of a ring of smooth ER at the bud neck. This ER domain and the restriction of diffusion for ER membrane proteins through the bud neck depended on septin function. The membrane-associated protein Bud6 localized to the bud neck in a septin-dependent manner and was required to restrict the diffusion of ER membrane proteins. Our results indicate that Bud6 acts downstream of septins to assemble a fence in the ER membrane at the bud neck. Thus, in polarized yeast cells, diffusion barriers compartmentalize the ER and the plasma membrane along parallel lines.

Cell polarity protein Spa2P associates with proteins involved in actin function in Saccharomyces cerevisiae

Spa2p is a nonessential protein that regulates yeast cell polarity. It localizes early to the presumptive bud site and remains at sites of growth throughout the cell cycle. To understand how Spa2p localization is regulated and to gain insight into its molecular function in cell polarity, we used a coimmunoprecipitation strategy followed by tandem mass spectrometry analysis to identify proteins that associate with Spa2p in vivo. We identified Myo1p, Myo2p, Pan1p, and the protein encoded by YFR016c as proteins that interact with Spa2p. Strikingly, all of these proteins are involved in cell polarity and/or actin function. Here we focus on the functional significance of the interactions of Spa2p with Myo2p and Myo1p. We find that localization of Spa2GFP to sites of polarized growth depends on functional Myo2p but not on Myo1p. We also find that Spa2p, like Myo2p, cosediments with F-actin in an ATP-sensitive manner. We hypothesize that Spa2p associates with actin via a direct or indirect interaction with Myo2p and that Spa2p may be involved in mediating polarized localization of polarity proteins via Myo2p. In addition, we observe an enhanced cell-separation defect in a myo1spa2 strain at 37 degrees C. This provides further evidence that Spa2p is involved in cytokinesis and cell wall morphogenesis.

Differential activities and regulation of Saccharomyces cerevisiae formin proteins Bni1 and Bnr1 by Bud6

Formins are conserved proteins that nucleate actin assembly and tightly associate with the fast growing barbed ends of actin filaments to allow insertional growth. Most organisms express multiple formins, but it has been unclear whether they have similar or distinct activities and how they may be regulated differentially. We isolated and compared the activities of carboxyl-terminal fragments of the only two formins expressed in Saccharomyces cerevisiae, Bni1 and Bnr1. Bnr1 was an order of magnitude more potent than Bni1 in actin nucleation and processive capping, and unlike Bni1, Bnr1 bundled actin filaments. Profilin bound directly to Bni1 and Bnr1 and regulated their activities similarly. However, the cell polarity factor Bud6/Aip3 specifically bound to and stimulated Bni1, but not Bnr1. This was unexpected, since previous two-hybrid studies suggested Bud6 interacts with both formins. We mapped Bud6 binding activity to specific residues in the carboxyl terminus of Bni1 that are adjacent to its diaphanous autoregulatory domain (DAD). Fusion of the carboxyl terminus of Bni1 to Bnr1 conferred Bud6 stimulation to a Bnr1-Bni1 chimera. Thus, Bud6 differentially stimulates Bni1 and not Bnr1. We found that Bud6 is up-regulated during bud growth, when it is delivered to the bud tip on Bni1-nucleated actin cables. We propose that Bud6 stimulation of Bni1 promotes robust cable formation, which in turn delivers more Bud6 to the bud tip, reinforcing polarized cell growth through a positive feedback loop.

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.

Cyclical regulation of the exocyst and cell polarity determinants for polarized cell growth

Polarized exocytosis is important for morphogenesis and cell growth. The exocyst is a multiprotein complex implicated in tethering secretory vesicles at specific sites of the plasma membrane for exocytosis. In the budding yeast, the exocyst is localized to sites of bud emergence or the tips of small daughter cells, where it mediates secretion and cell surface expansion. To understand how exocytosis is spatially controlled, we systematically analyzed the localization of Sec15p, a member of the exocyst complex and downstream effector of the rab protein Sec4p, in various mutants. We found that the polarized localization of Sec15p relies on functional upstream membrane traffic, activated rab protein Sec4p, and its guanine exchange factor Sec2p. The initial targeting of both Sec4p and Sec15p to the bud tip depends on polarized actin cable. However, different recycling mechanisms for rab and Sec15p may account for the different kinetics of polarization for these two proteins. We also found that Sec3p and Sec15p, though both members of the exocyst complex, rely on distinctive targeting mechanisms for their localization. The assembly of the exocyst may integrate various cellular signals to ensure that exocytosis is tightly controlled. Key regulators of cell polarity such as Cdc42p are important for the recruitment of the exocyst to the budding site. Conversely, we found that the proper localization of these cell polarity regulators themselves also requires a functional exocytosis pathway. We further report that Bem1p, a protein essential for the recruitment of signaling molecules for the establishment of cell polarity, interacts with the exocyst complex. We propose that a cyclical regulatory network contributes to the establishment and maintenance of polarized cell growth in yeast.

Stable and dynamic axes of polarity use distinct formin isoforms in budding yeast

Bud growth in yeast is guided by myosin-driven delivery of secretory vesicles from the mother cell to the bud. We find transport occurs along two sets of actin cables assembled by two formin isoforms. The Bnr1p formin assembles cables that radiate from the bud neck into the mother, providing a stable mother-bud axis. These cables also depend on septins at the neck and are required for efficient transport from the mother to the bud. The Bni1p formin assembles cables that line the bud cortex and target vesicles to varying locations in the bud. Loss of these cables results in morphological defects as vesicles accumulate at the neck. Assembly of these cables depends on continued polarized secretion, suggesting vesicular transport provides a positive feedback signal for Bni1p activation, possibly by rho-proteins. By coupling different formin isoforms to unique cortical landmarks, yeast uses common cytoskeletal elements to maintain stable and dynamic axes in the same cell.

Regulation of a formin complex by the microtubule plus end protein tea1p

The plus ends of microtubules have been speculated to regulate the actin cytoskeleton for the proper positioning of sites of cell polarization and cytokinesis. In the fission yeast Schizosaccharomyces pombe, interphase microtubules and the kelch repeat protein tea1p regulate polarized cell growth. Here, we show that tea1p is directly deposited at cell tips by microtubule plus ends. Tea1p associates in large “polarisome” complexes with bud6p and for3p, a formin that assembles actin cables. Tea1p also interacts in a separate complex with the CLIP-170 protein tip1p, a microtubule plus end–binding protein that anchors tea1p to the microtubule plus end. Localization experiments suggest that tea1p and bud6p regulate formin distribution and actin cable assembly. Although single mutants still polarize, for3Δbud6Δtea1Δ triple-mutant cells lack polarity, indicating that these proteins contribute overlapping functions in cell polarization. Thus, these experiments begin to elucidate how microtubules contribute to the proper spatial regulation of actin assembly and polarized cell growth.

Cdc50p, a protein required for polarized growth, associates with the Drs2p P-type ATPase implicated in phospholipid translocation in Saccharomyces cerevisiae

Cdc50p, a transmembrane protein localized to the late endosome, is required for polarized cell growth in yeast. Genetic studies suggest that CDC50 performs a function similar to DRS2, which encodes a P-type ATPase of the aminophospholipid translocase (APT) subfamily. At low temperatures, drs2Delta mutant cells exhibited depolarization of cortical actin patches and mislocalization of polarity regulators, such as Bni1p and Gic1p, in a manner similar to the cdc50Delta mutant. Both Cdc50p and Drs2p were localized to the trans-Golgi network and late endosome. Cdc50p was coimmunoprecipitated with Drs2p from membrane protein extracts. In cdc50Delta mutant cells, Drs2p resided on the endoplasmic reticulum (ER), whereas Cdc50p was found on the ER membrane in drs2Delta cells, suggesting that the association on the ER membrane is required for transport of the Cdc50p-Drs2p complex to the trans-Golgi network. Lem3/Ros3p, a homolog of Cdc50p, was coimmunoprecipitated with another APT, Dnf1p; Lem3p was required for exit of Dnf1p out of the ER. Both Cdc50p-Drs2p and Lem3p-Dnf1p were confined to the plasma membrane upon blockade of endocytosis, suggesting that these proteins cycle between the exocytic and endocytic pathways, likely performing redundant functions. Thus, phospholipid asymmetry plays an important role in the establishment of cell polarity; the Cdc50p/Lem3p family likely constitute potential subunits specific to unique P-type ATPases of the APT subfamily.

Polarized distribution of intracellular components by class V myosins in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has three classes of myosins corresponding to three actin structures: class I myosin for endocytic actin structure, actin patches; class II myosin for contraction of the actomyosin contractile ring around the bud neck; and class V myosin for transport along a cable-like actin structure (actin cables), extending toward the growing cortex. Myo2p and Myo4p constitute respective class V myosins as the heavy chain and, like class V myosins in other organisms, function as actin-based motors for polarized distribution of organelles and intracellular molecules. Proper distribution of organelles is essential for autonomously replicating organelles that cannot be reproduced de novo, and is also quite important for other organelles to ensure their efficient segregation and proper positioning, even though they can be newly synthesized, such as those derived from endoplasmic reticulum. In the budding yeast, microtubule-based motors play limited roles in the distribution. Instead, the actin-based motor myosins, especially Myo2p, play a major role. Studies on Myo2p have revealed a wide variety of Myo2p cargo and Myo2p-interacting proteins and have established that Myo2p interacts with cargo and transfers it along actin cables. Moreover, recent findings suggest that Myo2p has another way to distribute cargo in that Myo2p conveys the attaching cargo along the actin track. Thus, the myosin have "dual paths" for distribution of a cargo. This dual path mechanism is proposed in the last section of this review.

The PXL1 gene of Saccharomyces cerevisiae encodes a paxillin-like protein functioning in polarized cell growth

The Saccharomyces cerevisiae open reading frame YKR090w encodes a predicted protein displaying similarity in organization to paxillin, a scaffolding protein that organizes signaling and actin cytoskeletal regulating activities in many higher eucaryotic cell types. We found that YKR090w functions in a manner analogous to paxillin as a mediator of polarized cell growth; thus, we have named this gene PXL1 (Paxillin-like protein 1). Analyses of pxl1Delta strains show that PXL1 is required for the selection and maintenance of polarized growth sites during vegetative growth and mating. Genetic analyses of strains lacking both PXL1 and the Rho GAP BEM2 demonstrate that such cells display pronounced growth defects in response to different conditions causing Rho1 pathway activation. PXL1 also displays genetic interactions with the Rho1 effector FKS1. Pxl1p may therefore function as a modulator of Rho-GTPase signaling. A GFP::Pxl1 fusion protein localizes to sites of polarized cell growth. Experiments mapping the localization determinants of Pxl1p demonstrate the existence of localization mechanisms conserved between paxillin and Pxl1p and indicate an evolutionarily ancient and conserved role for LIM domain proteins in acting to modulate cell signaling and cytoskeletal organization during polarized growth.

The GAP activity of Msb3p and Msb4p for the Rab GTPase Sec4p is required for efficient exocytosis and actin organization

Polarized growth in Saccharomyces cerevisiae is thought to occur by the transport of post-Golgi vesicles along actin cables to the daughter cell, and the subsequent fusion of the vesicles with the plasma membrane. Previously, we have shown that Msb3p and Msb4p genetically interact with Cdc42p and display a GTPase-activating protein (GAP) activity toward a number of Rab GTPases in vitro. We show here that Msb3p and Msb4p regulate exocytosis by functioning as GAPs for Sec4p in vivo. Cells lacking the GAP activity of Msb3p and Msb4p displayed secretory defects, including the accumulation of vesicles of 80-100 nm in diameter. Interestingly, the GAP activity of Msb3p and Msb4p was also required for efficient polarization of the actin patches and for the suppression of the actin-organization defects in cdc42 mutants. Using a strain defective in polarized secretion and actin-patch organization, we showed that a change in actin-patch organization could be a consequence of the fusion of mistargeted vesicles with the plasma membrane.

Localization of VIP36 in the post-Golgi secretory pathway also of rat parotid acinar cells

VIP36 (36-kD vesicular integral membrane protein), originally purified from Madin-Darby canine kidney (MDCK) epithelial cells, belongs to a family of animal lectins and may act as a cargo receptor. To understand its role in secretory processes, we performed morphological analysis of the rat parotid gland. Immunoelectron microscopy provided evidence that endogenous VIP36 is localized in the trans-Golgi network, on immature granules, and on mature secretory granules in acinar cells. Double-staining immunofluorescence experiments confirmed that VIP36 and amylase co-localized in the apical regions of the acinar cells. This is the first study to demonstrate that endogenous VIP36 is involved in the post-Golgi secretory pathway, suggesting that VIP36 plays a role in trafficking and sorting of secretory and/or membrane proteins during granule formation.

Actin recovery and bud emergence in osmotically stressed cells requires the conserved actin interacting mitogen-activated protein kinase kinase kinase Ssk2p/MTK1 and the scaffold protein Spa2p

Osmotic stress causes actin cytoskeleton disassembly, a cell cycle arrest, and activation of the high osmolarity growth mitogen-activated protein kinase pathway. A previous study showed that Ssk2p, a mitogen-activated protein kinase kinase kinase of the high osmolarity growth pathway, promotes actin cytoskeleton recovery to the neck of late cell cycle, osmotically stressed yeast cells. Data presented herein examined the role of Ssk2p in actin recovery early in the cell cycle. We found that actin recovery at all stages of the cell cycle is not controlled by Ssk1p, the known activator of Ssk2p, but required a polarized distribution of Ssk2p as well as its actin-interacting and kinase activity. Stress-induced localization of Ssk2p to the neck required the septin Shs1p, whereas localization to the bud cortex depended on the polarity scaffold protein Spa2p. spa2delta cells, like ssk2delta cells, were defective for actin recovery from osmotic stress. These spa2delta defects could be suppressed by overexpression of catalytically active Ssk2p. Furthermore, Spa2p could be precipitated by GST-Ssk2p from extracts of osmotically stressed cells. The Ssk2p mediated actin recovery pathway seems to be conserved; MTK1, a human mitogen-activated protein kinase kinase kinase of the p38 stress response pathway and Ssk2p homolog, was also able to localize at polarized growth sites, form a complex with actin and Spa2p, and complement actin recovery defects in osmotically stressed ssk2delta and spa2delta yeast cells. We hypothesize that osmotic stress-induced actin disassembly leads to the formation of an Ssk2p-actin complex and the polarized localization of Ssk2p. Polarized Ssk2p associates with the scaffold protein Spa2p in the bud and Shs1p in the neck, allowing Ssk2p to regulate substrates involved in polarized actin assembly.

Yeasts make their mark

Budding and fission yeast serve as genetic model organisms for the study of the molecular mechanisms of cell polarity in single cells. Similar to other polarized eukaryotic cells, yeast cells have polarity programmes that regulate where they grow and divide. Here, we describe recent advances in defining the proteins that establish cell polarity and the numerous molecular interactions that may link these factors to the actin cytoskeleton. As many of these components are identified, a comprehensive understanding of complex pathways is beginning to emerge.

Cdc50p, a conserved endosomal membrane protein, controls polarized growth in Saccharomyces cerevisiae

During the cell cycle of the yeast Saccharomyces cerevisiae, the actin cytoskeleton and the growth of cell surface are polarized, mediating bud emergence, bud growth, and cytokinesis. We identified CDC50 as a multicopy suppressor of the myo3 myo5-360 temperature-sensitive mutant, which is defective in organization of cortical actin patches. The cdc50 null mutant showed cold-sensitive cell cycle arrest with a small bud as reported previously. Cortical actin patches and Myo5p, which are normally localized to polarization sites, were depolarized in the cdc50 mutant. Furthermore, actin cables disappeared, and Bni1p and Gic1p, effectors of the Cdc42p small GTPase, were mislocalized in the cdc50 mutant. As predicted by its amino acid sequence, Cdc50p appears to be a transmembrane protein because it was solubilized from the membranes by detergent treatment. Cdc50p colocalized with Vps21p in endosomal compartments and was also localized to the class E compartment in the vps27 mutant. The cdc50 mutant showed defects in a late stage of endocytosis but not in the internalization step. It showed, however, only modest defects in vacuolar protein sorting. Our results indicate that Cdc50p is a novel endosomal protein that regulates polarized cell growth.

Spatial regulation of the guanine nucleotide exchange factor Lte1 in Saccharomyces cerevisiae

In budding yeast, activation of the small Ras-like GTPase Tem1 triggers exit from mitosis and cytokinesis. Tem1 is regulated by Bub2/Bfa1, a two-component GTPase-activating protein (GAP), and by Lte1, a putative guanine nucleotide exchange factor. Lte1 is confined to the bud cortex, and its spatial separation from Tem1 at the spindle pole body (SPB) is important to prevent untimely exit from mitosis. The pathways contributing to Lte1 asymmetry have not been elucidated. Here we show that establishment of Lte1 at the cortex occurs by an actin-independent mechanism, which requires activation of Cdc28/Cln kinase at START and Cdc42, a key regulator of cell polarity and cytoskeletal organisation. This defines a novel role for Cdc42 in late mitotic events. In turn, dissociation of Lte1 from the cortex in telophase depends on activation of the Cdc14 phosphatase. Ectopic expression of Cdc14 at metaphase results in premature dephosphorylation of Lte1 coincident with its release from the cortex. In vitro phosphatase assays confirm that Lte1 is a direct substrate for Cdc14. Our results suggest that the asymmetry in Lte1 localisation is imposed by Cdc28-dependent phosphorylation. Finally, we report a mutational analysis undertaken to investigate intrinsic Lte1 determinants for localisation. Our data suggest that an intrameric interaction between the N-and C-terminal regions of Lte1 is important for cortex association.

Mlc1p promotes septum closure during cytokinesis via the IQ motifs of the vesicle motor Myo2p

Little is known about the molecular machinery that directs secretory vesicles to the site of cell separation during cytokinesis. We show that in Saccharomyces cerevisiae, the class V myosin Myo2p and the Rab/Ypt Sec4p, that are required for vesicle polarization processes at all stages of the cell cycle, form a complex with each other and with a myosin light chain, Mlc1p, that is required for actomyosin ring assembly and cytokinesis. Mlc1p travels on secretory vesicles and forms a complex(es) with Myo2p and/or Sec4p. Its functional interaction with Myo2p is essential during cytokinesis to target secretory vesicles to fill the mother bud neck. The role of Mlc1p in actomyosin ring assembly instead is dispensable for this process. Therefore, in yeast, as recently shown in mammals, class V myosins associate with vesicles via the formation of a complex with Rab/Ypt proteins. Further more, myosin light chains, via their ability to be transported by secretory vesicles and to interact with class V myosin IQ motifs, can regulate vesicle polarization processes at a specific location and stage of the cell cycle.

Fission yeast Aip3p (spAip3p) is required for an alternative actin-directed polarity program

Aip3p is an actin-interacting protein that regulates cell polarity in budding yeast. The Schizosaccharomyces pombe-sequencing project recently led to the identification of a homologue of Aip3p that we have named spAip3p. Our results confirm that spAip3p is a true functional homologue of Aip3p. When expressed in budding yeast, spAip3p localizes similarly to Aip3p during the cell cycle and complements the cell polarity defects of an aip3Delta strain. Two-hybrid analysis shows that spAip3p interacts with actin similarly to Aip3p. In fission yeast, spAip3p localizes to both cell ends during interphase and later organizes into two rings at the site of cytokinesis. spAip3p localization to cell ends is dependent on microtubule cytoskeleton, its localization to the cell middle is dependent on actin cytoskeleton, and both patterns of localization require an operative secretory pathway. Overexpression of spAip3p disrupts the actin cytoskeleton and cell polarity, leading to morphologically aberrant cells. Fission yeast, which normally rely on the microtubule cytoskeleton to establish their polarity axis, can use the actin cytoskeleton in the absence of microtubule function to establish a new polarity axis, leading to the formation of branched cells. spAip3p localizes to, and is required for, branch formation, confirming its role in actin-directed polarized cell growth in both Schizosaccharomyces pombe and Saccharomyces cerevisiae.

Cell polarity: a tale of two Ts

The microtubule cytoskeleton plays an important role in cell polarity. Central to this process in fission yeast is tea1p, a marker of polarized cell growth that is delivered to the cell surface in a microtubule-dependent fashion. Recent studies suggest that the actin-binding protein bud6p may be a tea1p effector.

Role of bud6p and tea1p in the interaction between actin and microtubules for the establishment of cell polarity in fission yeast

BACKGROUND

In many cell types, microtubules are thought to direct the spatial distribution of F-actin in cell polarity. Schizosaccharomyces pombe cells exhibit a regulated program of polarized cell growth: after cell division, they grow first in a monopolar manner at the old end, and in G2 phase, initiate growth at the previous cell division site (the new end). The role of microtubule ends in cell polarity is highlighted by the finding that the cell polarity factor, tea1p, is present on microtubule plus ends and cell tips [1].

RESULTS

Here, we characterize S. pombe bud6p/fat1p, a homolog of S. cerevisiae Bud6/Aip3. bud6Delta mutant cells have a specific defect in the efficient initiation of growth at the new end and like tea1Delta cells, form T-shaped cells in a cdc11 background. Bud6-GFP localizes to both cell tips and the cytokinesis ring. Maintenance of cell tip localization is dependent upon actin but not microtubules. Bud6-GFP localization is tea1p dependent, and tea1p localization is not bud6p dependent. tea1Delta and bud6Delta cells generally grow in a monopolar manner but exhibit different growth patterns. tea1(Delta)bud6Delta mutants resemble tea1Delta mutants. Tea1p and bud6p coimmunoprecipitate and comigrate in large complexes.

CONCLUSIONS

Our studies show that tea1p (a microtubule end-associated factor) and bud6p (an actin-associated factor) function in a common pathway, with bud6p downstream of tea1p. To our knowledge, bud6p is the first protein shown to interact physically with tea1p. These studies delineate a pathway for how microtubule plus ends function to polarize the actin cytoskeleton through actin-associated polarity factors.

Ras regulates the polarity of the yeast actin cytoskeleton through the stress response pathway

Polarized growth in yeast requires cooperation between the polarized actin cytoskeleton and delivery of post-Golgi secretory vesicles. We have previously reported that loss of the major tropomyosin isoform, Tpm1p, results in cells sensitive to perturbations in cell polarity. To identify components that bridge these processes, we sought mutations with both a conditional defect in secretion and a partial defect in polarity. Thus, we set up a genetic screen for mutations that conferred a conditional growth defect, showed synthetic lethality with tpm1Delta, and simultaneously became denser at the restrictive temperature, a hallmark of secretion-defective cells. Of the 10 complementation groups recovered, the group with the largest number of independent isolates was functionally null alleles of RAS2. Consistent with this, ras2Delta and tpm1Delta are synthetically lethal at 35 degrees C. We show that ras2Delta confers temperature-sensitive growth and temperature-dependent depolarization of the actin cytoskeleton. Furthermore, we show that at elevated temperatures ras2Delta cells are partially defective in endocytosis and show a delocalization of two key polarity markers, Myo2p and Cdc42p. However, the conditional enhanced density phenotype of ras2Delta cells is not a defect in secretion. All the phenotypes of ras2Delta cells can be fully suppressed by expression of yeast RAS1 or RAS2 genes, human Ha-ras, or the double disruption of the stress response genes msn2Deltamsn4Delta. Although the best characterized pathway of Ras function in yeast involves activation of the cAMP-dependent protein kinase A pathway, activation of the protein kinase A pathway does not fully suppress the actin polarity defects, suggesting that there is an additional pathway from Ras2p to Msn2/4p. Thus, Ras2p regulates cytoskeletal polarity in yeast under conditions of mild temperature stress through the stress response pathway.

Cdc28-Clb mitotic kinase negatively regulates bud site assembly in the budding yeast

In the budding yeast Saccharomyces cerevisiae, a prospective mother normally commences the formation of a daughter (the bud) only in the G(1) phase of the cell division cycle. This suggests a strict temporal regulation of the processes that initiate the formation of a new bud. Using cortical localization of bud site components Spa2 and Bni1 as an indicator of bud site assembly, we show that cells assemble a bud site following inactivation of the Cdc28-Clb mitotic kinase but prior to START. Interestingly, an untimely inactivation of the mitotic kinase is sufficient to drive cells to assemble a new bud site inappropriately in G(2) or M phases. The induction of Cdc28/Clb kinase activity in G(1), on the other hand, dramatically reduces a cell's ability to construct an incipient bud site. Our findings strongly suggest that the Cdc28-Clb kinase plays a critical role in the mechanism that restricts the timing of bud formation to the G(1) phase of the cell cycle.