Assembly and function of the actin cytoskeleton of yeast: relationships between cables and patches

Identification of differentially expressed genes in yeast Saccharomyces cerevisiae cells with inactivated Mmf1p and Hmf1p, members of proteins family YERO57c/YJGF

We used differential display analysis of mRNA to investigate the differences between gene expression in wild-type (wt) yeast Saccharomyces cerevisiae cells and mutated ones with disrupted activity of genes MMF1 and HMF1, members of the YERO57c/YJGF family. Reverse transcription-polymerase chain reaction (RT-PCR) analysis was performed to determine the differences in the degree of expression of 14 specific transcripts in normal and mutated yeast cells. Obtained data demonstrate that disruption of genes encoding proteins Mmf1p, Hmf1p (or both of them) result in the correlative variation of expression level of the target 12 genes both in the cells with changed phenotype (mmf1 and mmf1 hmf1) and in the cells retaining w.t. shape and growth rate (wt cells, hmf1). Metabolic processes and cellular pathways have been indicated for Mmf1p and Hmf1p based on the different profiles of the expression of 14 genes in mmf1, hmf1 yeast S. cerevisiae cells.

Actin and septin ultrastructures at the budding yeast cell cortex

Budding yeast has been a powerful model organism for studies of the roles of actin in endocytosis and septins in cell division and in signaling. However, the depth of mechanistic understanding that can be obtained from such studies has been severely hindered by a lack of ultrastructural information about how actin and septins are organized at the cell cortex. To address this problem, we developed rapid-freeze and deep-etch techniques to image the yeast cell cortex in spheroplasted cells at high resolution. The cortical actin cytoskeleton assembles into conical or mound-like structures composed of short, cross-linked filaments. The Arp2/3 complex localizes near the apex of these structures, suggesting that actin patch assembly may be initiated from the apex. Mutants in cortical actin patch components with defined defects in endocytosis disrupted different stages of cortical actin patch assembly. Based on these results, we propose a model for actin function during endocytosis. In addition to actin structures, we found that septin-containing filaments assemble into two kinds of higher order structures at the cell cortex: rings and ordered gauzes. These images provide the first high-resolution views of septin organization in cells.

Capping protein: new insights into mechanism and regulation

Temporal and spatial control of the actin cytoskeleton are crucial for a range of eukaryotic cellular processes. Capping protein (CP), a ubiquitous highly conserved heterodimer, tightly caps the barbed (fast-growing) end of the actin filament and is an important component in the assembly of various actin structures, including the dynamic branched filament network at the leading edge of motile cells. New research into the molecular mechanism of how CP interacts with the actin filament in vitro and the function of CP in vivo, including discoveries of novel interactions of CP with other proteins, has greatly enhanced our understanding of the role of CP in regulating the actin cytoskeleton.

A role for yeast oxysterol-binding protein homologs in endocytosis and in the maintenance of intracellular sterol-lipid distribution

The seven yeast OSH genes (OSH1-OSH7) encode a family of orthologs of the mammalian oxysterol-binding protein (OSBP). The OSH genes share at least one essential overlapping function, potentially linked to the regulation of secretory trafficking and membrane lipid composition. To investigate the essential roles of the OSH genes, we constructed conditional OSH mutants and analyzed their cellular defects. Elimination of all OSH function altered intracellular sterol-lipid distribution, caused vacuolar fragmentation, and resulted in an accumulation of lipid droplets in the cytoplasm and within vacuolar fragments. Gradual depletion of Osh proteins also caused cell budding defects and abnormal cell wall deposition. In OSH mutant cells endocytosis was severely impaired, but protein transport to the vacuole and the plasma membrane was largely unaffected. Other mutants affecting sterol-lipid function and distribution, namely erg2Delta and arv1Delta, shared similar defects. These findings suggested that OSH genes, through effects on intracellular sterol distribution, establish a plasma membrane lipid composition that promotes endocytosis.

Coordinated regulation of actin filament turnover by a high-molecular-weight Srv2/CAP complex, cofilin, profilin, and Aip1

BACKGROUND

Dynamic remodeling of the actin cytoskeleton requires rapid turnover of actin filaments, which is regulated in part by the actin filament severing/depolymerization factor cofilin/ADF. Two factors that cooperate with cofilin are Srv2/CAP and Aip1. Human CAP enhances cofilin-mediated actin turnover in vitro, but its biophysical properties have not been defined, and there has been no in vivo evidence reported for its role in turnover. Xenopus Aip1 forms a cofilin-dependent cap at filament barbed ends. It has been unclear how these diverse activities are coordinated in vivo.

RESULTS

Purified native yeast Srv2/CAP forms a high molecular weight structure comprised solely of actin and Srv2. The complex is linked to actin filaments via the SH3 domain of Abp1. Srv2 complex catalytically accelerates cofilin-dependent actin turnover by releasing cofilin from ADP-actin monomers and enhances the ability of profilin to stimulate nucleotide exchange on ADP-actin. Yeast Aip1 forms a cofilin-dependent filament barbed end cap, disrupted by the cof1-19 mutant. Genetic analyses show that specific combinations of activities mediated by cofilin, Srv2, Aip1, and capping protein are required in vivo.

CONCLUSIONS

We define two genetically and biochemically separable functions for cofilin in actin turnover. One is formation of an Aip1-cofilin cap at filament barbed ends. The other is cofilin-mediated severing/depolymerization of filaments, accelerated indirectly by Srv2 complex. We show that the Srv2 complex is a large multimeric structure and functions as an intermediate in actin monomer processing, converting cofilin bound ADP-actin monomers to profilin bound ATP-actin monomers and recycling cofilin for new rounds of filament depolymerization.

A Role for GEA1 and GEA2 in the Organization of the Actin Cytoskeleton in Saccharomyces cerevisiae

Profilin is an actin monomer-binding protein implicated in the polymerization of actin filaments. In the budding yeast Saccharomyces cerevisiae, the pfy1-111 rho2Δ double mutant has severe growth and actin cytoskeletal defects. The GEA1 and GEA2 genes, which code for paralog guanosine exchange factors for Arf proteins, were identified as multicopy suppressors of the mutant phenotype. These two genes restored the polarized distribution of actin cortical patches and produced visible actin cables in both the pfy1-111 rho2Δ and pfy1Δ cells. Thus, overexpression of GEA1 or GEA2 bypassed the requirement for profilin in actin cable formation. In addition, gea1 gea2 double mutants showed defects in budding and in actin cytoskeleton organization, while overexpression of GEA1 or GEA2 led to the formation of supernumerary actin cable-like structures in a Bni1p/Bnr1p-dependent manner. The ADP-ribosylation factor Arf3p may be a target of Gea1p/Gea2p, since overexpression of ARF3 partially suppressed the profilin-deficient phenotype and a deletion of ARF3 exacerbated the phenotype of a pfy1-111 mutant. Gea1p, Gea2p, Arf1p, and Arf2p but not Arf3p are known to function in vesicular transport between the endoplasmic reticulum and the Golgi. In this work, we demonstrate a role for Gea1p, Gea2p, and Arf3p in the organization of the actin cytoskeleton.

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.

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.

The bacterial cytoskeleton: in vivo dynamics of the actin-like protein Mbl of Bacillus subtilis

Mbl is a bacterial actin homolog that controls cell morphogenesis in Bacillus subtilis. A functional GFP-Mbl fusion protein was used to examine the behavior of the helical cables formed by Mbl protein in live B. subtilis cells. The cables undergo dynamic changes during cell cycle progression. They are stable but not rigid while elongating in parallel with cell growth, and they require septum formation to divide/cleave. Fluorescence recovery after photobleaching (FRAP) analysis showed that the cables are continuously remodeled during cell elongation. Turnover occurs along the length of the helical Mbl filaments, with no obvious polarity and a recovery half-time of about 8 min. These findings have important implications for the nature of bacterial cell wall architecture and synthesis.

Quantitative analysis of actin patch movement in yeast

To investigate the mechanism of cortical actin patch movement in yeast, we implement a method for computer tracking the motion of the patches. Digital images from fluorescence microscope movies of living cells are fed into an image-processing program, which generates two-dimensional patch coordinates in the plane of focus for each movie frame via an algorithm based on detection of rapid intensity variations. The patch coordinates in neighboring frames are connected by a minimum-distance algorithm. The method is used to analyze control cells and cells treated with the actin-depolymerizing agent latrunculin. The motion of the patches in both cases, as analyzed by mean-square patch displacements, is found to be a random walk on average, with a much lower diffusion coefficient for the latrunculin-treated cells. The mean-squared patch travel distances for all of the latrunculin-treated cells are lower than those for all of the control cells. The patches move independently of one another. We develop a quantitative criterion for the presence of directed motion, and show that numerous patches in the control cells display directed motion to a very high degree of certainty. A small number of patches in the latrunculin-treated cells display directed motion.

F-actin ring formation and the role of F-actin cables in the fission yeastSchizosaccharomyces pombe

Cells of the fission yeast Schizosaccharomyces pombe divide by the contraction of the F-actin ring formed at the medial region of the cell. We investigated the process of F-actin ring formation in detail using optical sectioning and three-dimensional reconstruction fluorescence microscopy. In wild-type cells, formation of an aster-like structure composed of F-actin cables and accumulation of F-actin cables were recognized at the medial cortex of the cell during prophase to metaphase. The formation of the aster-like structure seemed to initiate from branching of the longitudinal F-actin cables at a site near the spindle pole bodies, which had been duplicated but not yet separated. A single cable extended from the aster and encircled the cell at the equator to form a primary F-actin ring during metaphase. During anaphase,the accumulated F-actin cables were linked to the primary F-actin ring, and then all of these structures seemed to be packed to form the F-actin ring. These observations suggest that formation of the aster-like structure and the accumulation of the F-actin cables at the medial region of the cell during metaphase may be required to initiate the F-actin ring formation. In the nda3 mutant, which has a mutation in ß-tubulin and has been thought to be arrested at prophase, an F-actin ring with accumulated F-actin cables similar to that of anaphase wild-type cells was formed at a restrictive temperature. Immediately after shifting to a permissive temperature, this structure changed into a tightly packed ring. This suggests that the F-actin ring formation progresses beyond prophase in the nda3 cells once the cells enter prophase. We further examined F-actin structures in both cdc12 and cdc15 early cytokinesis mutants. As a result,Cdc12 seemed to be required for the primary F-actin ring formation during prophase, whereas Cdc15 may be involved in both packing the F-actin cables to form the F-actin ring and rearrangement of the F-actin after anaphase. In spg1, cdc7 and sid2 septum initiation mutants, the F-actin ring seemed to be formed in order.

The yeast synaptojanin-like proteins control the cellular distribution of phosphatidylinositol (4,5)-bisphosphate

Phosphoinositides (PI) are synthesized and turned over by specific kinases, phosphatases, and lipases that ensure the proper localization of discrete PI isoforms at distinct membranes. We analyzed the role of the yeast synaptojanin-like proteins using a strain that expressed only a temperature-conditional allele of SJL2. Our analysis demonstrated that inactivation of the yeast synaptojanins leads to increased cellular levels of phosphatidylinositol (3,5)-bisphosphate and phosphatidylinositol (4,5)-bisphosphate (PtdIns(4,5)P(2)), accompanied by defects in actin organization, endocytosis, and clathrin-mediated sorting between the Golgi and endosomes. The phenotypes observed in synaptojanin-deficient cells correlated with accumulation of PtdIns(4,5)P(2), because these effects were rescued by mutations in MSS4 or a mutant form of Sjl2p that harbors only PI 5-phosphatase activity. We utilized green fluorescent protein-pleckstrin homology domain chimeras (termed FLAREs for fluorescent lipid-associated reporters) with distinct PI-binding specificities to visualize pools of PtdIns(4,5)P(2) and phosphatidylinositol 4-phosphate in yeast. PtdIns(4,5)P(2) localized to the plasma membrane in a manner dependent on Mss4p activity. On inactivation of the yeast synaptojanins, PtdIns(4,5)P(2) accumulated in intracellular compartments, as well as the cell surface. In contrast, phosphatidylinositol 4-phosphate generated by Pik1p localized in intracellular compartments. Taken together, our results demonstrate that the yeast synaptojanins control the localization of PtdIns(4,5)P(2) in vivo and provide further evidence for the compartmentalization of different PI species.

The Sur7p family defines novel cortical domains in Saccharomyces cerevisiae, affects sphingolipid metabolism, and is involved in sporulation

We have discovered a novel cortical patch structure in Saccharomyces cerevisiae defined by a family of integral plasma membrane proteins, including Sur7p, Ynl194p, and Ydl222p. Sur7p-family patches localized as cortical patches that were immobile and stable. These patches were polarized to regions of the cell with a mature cell wall; they were absent from small buds and the tips of many medium-sized buds. These patches were distinct from other known cortical structures. Digestion of the cell wall caused Sur7p patches to disassemble, indicating that Sur7p requires cell wall-dependent extracellular interactions for its localization as patches. sur7Delta, ydl222Delta, and ynl194Delta mutants had reduced sporulation efficiencies. SUR7 was originally described as a multicopy suppressor of rvs167, whose product is an actin patch component. This suppression is probably mediated by sphingolipids, since deletion of SUR7, YDL222, and YNL194 altered the sphingolipid content of the yeast plasma membrane, and other SUR genes suppress rvs167 via effects on sphingolipid synthesis. In particular, the sphingoid base length and number of hydroxyl groups in inositol phosphorylceramides were altered in sur7Delta, ydl222Delta, and yne194Delta strains.

Actin cable dynamics in budding yeast

Actin cables, bundles of actin filaments that align along the long axis of budding yeast, are crucial for establishment of cell polarity. We fused green fluorescent protein (GFP) to actin binding protein 140 (Abp140p) and visualized actin cable dynamics in living yeast. We detected two populations of actin cables: (i) bud-associated cables, which extend from the bud along the mother-bud axis, and (ii) randomly oriented cables, which are relatively short. Time-lapse imaging of Abp140p-GFP revealed an apparent increase in the length of bud-associated actin cables. Analysis of movement of Abp140p-GFP fiduciary marks on bud-associated cables and fluorescence loss in photobleaching experiments revealed that this apparent elongation occurs by assembly of new material at the end of the cable within the bud and movement of the opposite end of the cable toward the tip of the mother cell distal to the bud. The rate of extension of the tip of an elongating actin cable is 0.29 +/- 0.08 microm/s. Latrunculin A (Lat-A) treatment completely blocked this process. We also observed movement of randomly oriented cables around the cortex of cells at a rate of 0.59 +/- 0.14 microm/s. Mild treatment with Lat-A did not affect the velocity of movement of randomly oriented cables. However, Lat-A treatment did increase the number of randomly oriented, motile cables per cell. Our observations suggest that establishment of bud-associated actin cables during the cell cycle is accomplished not by realignment of existing cables but by assembly of new cables within the bud or bud neck, followed by elongation.

Yeast formins regulate cell polarity by controlling the assembly of actin cables

Formins are conserved Rho-GTPase effectors that communicate Rho-GTPase signals to the cytoskeleton. We found that formins were required for the assembly of one of the three budding yeast actin structures: polarized arrays of actin cables. A dominant-active formin induced the assembly of actin cables. The activation and localization of the formin Bni1p required components of the polarisome complex. These findings potentially define the cellular function of formins in budding yeast and explain their involvement in the generation of cell polarity. A requirement for formins in constructing specific actin structures might be the basis for the diverse activities of formins in development.

Movement of yeast 1,3-beta-glucan synthase is essential for uniform cell wall synthesis

BACKGROUND

The cell wall has an important role in maintaining cell shape. In the budding yeast Saccharomyces cerevisiae, the major filamentous component of the cell wall responsible for its rigidity is 1,3-beta-glucan and is synthesized by 1,3-beta-glucan synthase (GS), localized on the plasma membrane.

RESULTS

Observations of green fluorescent protein (GFP)-conjugated Fks1p, a catalytic subunit of GS, revealed that it is co-localized with cortical actin patches and moves on the cell surface at the sites of cell wall remodelling. Mutants with impaired actin patch movement show immobility of Fks1p-GFP spots, indicating that actin patch motility is required for the movement of Fks1p. Cells with immobilized Fks1p exhibit defective cell wall structure and function. The cell wall thickness of the mutants becomes irregular, eventually leading to cell lysis.

CONCLUSION

We propose that GS movement is necessary for proper cell wall remodelling.

Functions of Vrp1p in cytokinesis and actin patches are distinct and neither requires a WH2/V domain

Vrp1 (verprolin, End5) is a Saccharomyces cerevisiae actin-associated protein and is related to mammalian Wiskott-Aldrich syndrome protein (WASP)-interacting protein (WIP). Vrp1-deficient (vrp1 Delta) cells are inviable at high temperature, have partially depolarized cortical actin patches and have defects in both actomyosin ring-dependent and Hof1 (Cyk2)-dependent pathways of cytokinesis. We demonstrate here that N-Vrp1(1-364) and C-Vrp1(364-817) are each sufficient to restore viability, actomyosin ring constriction and Hof1 localization at 37 degrees C to vrp1 Delta. C-Vrp1, like Vrp1, partially co-localizes with cortical actin patches and restores actin patch polarization to vrp1 Delta. Cortical localization of C-Vrp1, but not Vrp1, requires Las17. N-Vrp1 exhibits diffuse cytoplasmic localization and functions in cytokinesis without efficiently restoring polarization of cortical actin patches. N-Vrp1 function is not abolished by mutations affecting the WASP homology 2 (WH2) [verprolin homology (V)] actin-binding domain. N-Vrp1 may function through the type I myosins and actin, while C-Vrp1 may function through both Las17 (Bee1) and type I myosins. The functions of Vrp1 in viability at 37 degrees C and cytokinesis do not require efficient localization to, and function in, the cortical actin cytoskeleton.

Interactions with PIP2, ADP-actin monomers, and capping protein regulate the activity and localization of yeast twinfilin

Twinfilin is a ubiquitous actin monomer-binding protein that regulates actin filament turnover in yeast and mammalian cells. To elucidate the mechanism by which twinfilin contributes to actin filament dynamics, we carried out an analysis of yeast twinfilin, and we show here that twinfilin is an abundant protein that localizes to cortical actin patches in wild-type yeast cells. Native gel assays demonstrate that twinfilin binds ADP-actin monomers with higher affinity than ATP-actin monomers. A mutant twinfilin that does not interact with actin monomers in vitro no longer localizes to cortical actin patches when expressed in yeast, suggesting that the ability to interact with actin monomers may be essential for the localization of twinfilin. The localization of twinfilin to the cortical actin cytoskeleton is also disrupted in yeast strains where either the CAP1 or CAP2 gene, encoding for the alpha and beta subunits of capping protein, is deleted. Purified twinfilin and capping protein form a complex on native gels. Twinfilin also interacts with phosphatidylinositol 4,5-bisphosphate (PI[4,5]P2), and its actin monomer-sequestering activity is inhibited by PI(4,5)P2. Based on these results, we propose a model for the biological role of twinfilin as a protein that localizes actin monomers to the sites of rapid filament assembly in cells.

The life cycle of actin patches in mating yeast

Actin patches are core components of the yeast actin cytoskeleton that undergo redistribution during establishment of cell polarity. Using 4D imaging, we observe the life cycle of actin patches in living yeast for the first time. We observe assembly of actin patches at sites of polarized growth, and disassembly of actin patches concomitant with movement away from those sites. The total lifetime of an actin patch is 10.9+/-4.2 seconds. These findings indicate that actin patches are labile structures, and that the localization of actin patches during establishment of cell polarity occurs by assembly of these structures at sites of polarized cell surface growth. These findings were confirmed and extended by analysis of myosin I proteins and their receptor, verprolin, proteins implicated in actin assembly in yeast. Deletion of type I myosins or their receptor has no effect on the velocity of actin patch movement. However, these mutants show a 65% reduction in number of patch movements and a three-fold increase in patch lifetime. Finally, the actin patch resident proteins Abp1p, fimbrin, and Arp2p show normal association with actin patches in myosin I and verprolin mutants. However, cofilin accumulates in abnormal ‘bars’ of G-actin in myo3(Δ),myo5(Δ) and vrp1(Δ) strains, and Las17p/Bee1p is not associated with actin patches in vrp1(Δ) strains. These findings imply a multi-step process for actin patch assembly. Early events in this process, including assembly of Abp1p, fimbrin and Arp2p with F-actin, can occur throughout the cell and do not require myosin I proteins or their receptor. Later events in this process are myosin I-dependent, and are required for assembly of actin patches at sites of polarized cell surface growth. http://www.biologists.com/JCS/movies/jcs1990.html

Molecular requirements for the internalisation step of endocytosis: insights from yeast

Molecular genetic studies of endocytosis using the unicellular eukaryote Saccharomyces cerevisiae (budding yeast) have led to the identification of many cellular components, both proteins and lipids, required for this process. While initially, many of these requirements (e.g. for actin, various actin-associated proteins, the ubiquitin conjugation system, and for ergosterol and sphingolipids) appeared to differ from known requirements for endocytosis in higher eukaryotes (e.g. clathrin, AP-2, dynamin), it now seems that endocytosis in higher and lower eukaryotes share many requirements. Often, what were initially identified as actin cytoskeleton-associated proteins in S. cerevisiae, are now revealing themselves as clathrin-coated pit- and vesicle-associated proteins in higher eukaryotes. So rather than delineating two endocytic pathways, one actin-based and one clathrin-based, the combined studies on higher and lower eukaryotes are revealing interesting interplay in both systems between the actin cytoskeleton, clathrin coats, and lipids in the formation of endocytic vesicles at the plasma membrane. Recent results from the yeast system show that the Arp2/3p complex, Wiskott-Aldrich syndrome protein (WASP), and WASP-interacting protein (WIP), proteins involved in the nucleation step of actin filament assembly, play a major role in the formation of endocytic vesicles. This discovery suggests models whereby endocytic vesicles may be actively pushed from the plasma membrane and into the cell by newly forming and rapidly extending actin filaments.

Role of actin polymerization and actin cables in actin-patch movement in Schizosaccharomyces pombe

Factors that are involved in actin polymerization, such as the Arp2/3 complex, have been found to be packaged into discrete, motile, actin-rich foci. Here we investigate the mechanism of actin-patch motility in S. pombe using a fusion of green fluorescent protein (GFP) to a coronin homologue, Crn1p. Actin patches are associated with cables and move with rates of 0.32 microm s(-1) primarily in an undirected manner at cell tips and also in a directed manner along actin cables, often away from cell tips. Patches move more slowly or stop when actin polymerization is attenuated by Latrunculin A or in arp3 and cdc3 (profilin) mutants. In a cdc8 (tropomyosin) mutant, actin cables are absent, and patches move with similar speed but in a non-directed manner. Patches are sites of Arp3-dependent F-actin polymerization in vitro. Rapid F-actin turnover rates in vivo indicate that patches and cables are maintained continuously by actin polymerization. Our studies give rise to a model in which actin patches are centres for actin polymerization that drive their own movement on actin cables using Arp2/3-based actin polymerization.

Generation of an isogenic collection of yeast actin mutants and identification of three interrelated phenotypes

A large collection of yeast actin mutations has been previously isolated and used in numerous studies of actin cytoskeletal function. However, the various mutations have been in congenic, rather than isogenic, backgrounds, making it difficult to compare the subtle phenotypes that are characteristic of these mutants. We have therefore placed 27 mutations in an isogenic background. We used a subset of these mutants to compare the degree to which different actin alleles are defective in sporulation, endocytosis, and growth on NaCl-containing media. We found that the three phenotypes are highly correlated. The correlations are specific and not merely a reflection of general growth defects, because the phenotypes are not correlated with growth rates under normal conditions. Significantly, those actin mutants exhibiting the most severe phenotypes in all three processes have altered residues that cluster to a small region of the actin crystal structure previously defined as the fimbrin (Sac6p)-binding site. We examined the relationship between endocytosis and growth on salt and found that shifting wild-type or actin mutant cells to high salt reduces the rate of alpha-factor internalization. These results suggest that actin mutants may be unable to grow on salt because of additive endocytic defects (due to mutation and salt).

Endocytosis and the development of cell polarity in yeast require a dynamic F-actin cytoskeleton

Studies using drugs that cause the disassembly of filamentous actin (F-actin) have demonstrated the importance of an intact actin cytoskeleton for polarised secretion by yeast cells [1,2]. To address the level of dynamic turnover needed for such processes, however, drugs or mutants that confer stabilising properties on F-actin are needed. Jasplakinolide is the only readily available drug that stabilises F-actin structures both in vivo and in vitro [3-6]. Yeast strains have been generated in which two of the ABC multidrug resistance transporter genes have been deleted, rendering normally jasplakinolide-resistant yeast cells sensitive to its effects. Treatment of these cells with jasplakinolide caused rapid and dramatic effects on the actin cytoskeleton, resulting in the accumulation of single large actin structures in cells. These structures, however, still contained components that are normally associated with cortical actin patches. A dynamic actin cytoskeleton was found to be critical for the generation of cell polarity and endocytosis.

The cortical protein Num1p is essential for dynein-dependent interactions of microtubules with the cortex

In budding yeast, the mitotic spindle moves into the neck between the mother and bud via dynein-dependent sliding of cytoplasmic microtubules along the cortex of the bud. How dynein and microtubules interact with the cortex is unknown. We found that cells lacking Num1p failed to exhibit dynein-dependent microtubule sliding in the bud, resulting in defective mitotic spindle movement and nuclear segregation. Num1p localized to the bud cortex, and that localization was independent of microtubules, dynein, or dynactin. These data are consistent with Num1p being an essential element of the cortical attachment mechanism for dynein-dependent sliding of microtubules in the bud.

The yeast inositol polyphosphate 5-phosphatases inp52p and inp53p translocate to actin patches following hyperosmotic stress: mechanism for regulating phosphatidylinositol 4,5-bisphosphate at plasma membrane invaginations

The Saccharomyces cerevisiae inositol polyphosphate 5-phosphatases (Inp51p, Inp52p, and Inp53p) each contain an N-terminal Sac1 domain, followed by a 5-phosphatase domain and a C-terminal proline-rich domain. Disruption of any two of these 5-phosphatases results in abnormal vacuolar and plasma membrane morphology. We have cloned and characterized the Sac1-containing 5-phosphatases Inp52p and Inp53p. Purified recombinant Inp52p lacking the Sac1 domain hydrolyzed phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] and PtdIns(3, 5)P(2). Inp52p and Inp53p were expressed in yeast as N-terminal fusion proteins with green fluorescent protein (GFP). In resting cells recombinant GFP-tagged 5-phosphatases were expressed diffusely throughout the cell but were excluded from the nucleus. Following hyperosmotic stress the GFP-tagged 5-phosphatases rapidly and transiently associated with actin patches, independent of actin, in both the mother and daughter cells of budding yeast as demonstrated by colocalization with rhodamine phalloidin. Both the Sac1 domain and proline-rich domains were able to independently mediate translocation of Inp52p to actin patches, following hyperosmotic stress, while the Inp53p proline-rich domain alone was sufficient for stress-mediated localization. Overexpression of Inp52p or Inp53p, but not catalytically inactive Inp52p, which lacked PtdIns(4,5)P(2) 5-phosphatase activity, resulted in a dramatic reduction in the repolarization time of actin patches following hyperosmotic stress. We propose that the osmotic-stress-induced translocation of Inp52p and Inp53p results in the localized regulation of PtdIns(3,5)P(2) and PtdIns(4,5)P(2) at actin patches and associated plasma membrane invaginations. This may provide a mechanism for regulating actin polymerization and cell growth as an acute adaptive response to hyperosmotic stress.

Suppression of the profilin-deficient phenotype by the RHO2 signaling pathway in Saccharomyces cerevisiae

Profilin plays an important role in actin organization in all eukaryotic cells through mechanisms that are still poorly understood. We had previously shown that Mid2p, a transmembrane protein and a potential cell wall sensor, is an effective multicopy suppressor of the profilin-deficient phenotype in Saccharomyces cerevisiae. To better understand the role of Mid2p in the organization of the actin cytoskeleton, we isolated five additional multicopy suppressors of pfy1Delta cells that are Rom1p, Rom2p, Rho2p, Smy1p, and the previously uncharacterized protein Syp1p. The problems of caffeine and NaCl sensitivity, growth defects at 30 degrees and 37 degrees, the accumulation of intracellular vesicular structures, and a random budding pattern in pfy1Delta cells are corrected by all the suppressors tested. This is accompanied by a partial repolarization of the cortical actin patches without the formation of visible actin cables. The overexpression of Mid2p, Rom2p, and Syp1p, but not the overexpression of Rho2p and Smy1p, results in an abnormally thick cell wall in wild-type and pfy1Delta cells. Since none of the suppressors, except Rho2p, can correct the phenotype of the pfy1-111/rho2Delta strain, we propose a model in which the suppressors act through the Rho2p signaling pathway to repolarize cortical actin patches.

Suppressors of mdm20 in yeast identify new alleles of ACT1 and TPM1 predicted to enhance actin-tropomyosin interactions

The actin cytoskeleton is required for many aspects of cell division in yeast, including mitochondrial partitioning into growing buds (mitochondrial inheritance). Yeast cells lacking MDM20 function display defects in both mitochondrial inheritance and actin organization, specifically, a lack of visible actin cables and enhanced sensitivity to Latrunculin A. mdm20 mutants also exhibit a temperature-sensitive growth phenotype, which we exploited to isolate second-site suppressor mutations. Nine dominant suppressors selected in an mdm20/mdm20 background rescue temperature-sensitive growth defects and mitochondrial inheritance defects and partially restore actin cables in haploid and diploid mdm20 strains. The suppressor mutations define new alleles of ACT1 and TPM1, which encode actin and the major form of tropomyosin in yeast, respectively. The ACT1 mutations cluster in a region of the actin protein predicted to contact tropomyosin, suggesting that they stabilize actin cables by enhancing actin-tropomyosin interactions. The characteristics of the mutant ACT1 and TPM1 alleles and their potential effects on protein structure and binding are discussed.

Cbk1p, a protein similar to the human myotonic dystrophy kinase, is essential for normal morphogenesis in Saccharomyces cerevisiae

We have studied the CBK1 gene of Saccharomyces cerevisiae, which encodes a conserved protein kinase similar to the human myotonic dystrophy kinase. We have shown that the subcellular localization of the protein, Cbk1p, varies in a cell cycle-dependent manner. Three phenotypes are associated with the inactivation of the CBK1 gene: large aggregates of cells, round rather than ellipsoidal cells and a change from a bipolar to a random budding pattern. Two-hybrid and extragenic suppressor studies have linked Cbk1p with the transcription factor Ace2p, which is responsible for the transcription of chitinase. Cbk1p is necessary for the activation of Ace2p and we have shown that the aggregation phenotype is due to a lack of chitinase expression. The random budding pattern and the round cell phenotype of the CBK1 deletion strain show that in addition to its role in regulating chitinase expression via Ace2p, Cbk1p is essential for a wild-type morphological development of the cell.

Interdependence of filamentous actin and microtubules for asymmetric cell division

Asymmetric cell divisions are crucial to the generation of cell fate diversity. They contribute to unequal distribution of cellular factors to the daughter cells. Asymmetric divisions are characterized by a 90 degrees rotation of the mitotic spindle. There is increasing evidence that a tight cooperation between cortical, filamentous actin and astral microtubules is indispensable for successful spindle rotation. Over the past years, the dynactin complex has emerged as a key candidate to mediate actin/microtubule interaction at the cortex. This review discusses our current understanding of how spindle rotation is accomplished by the interplay of filamentous actin and microtubules in a variety of experimental systems.

Alterations in the actin cytoskeleton of pollen tubes are induced by the self-incompatibility reaction in Papaver rhoeas

Self-incompatibility (SI) is a genetically controlled process used to prevent self-pollination. In Papaver rhoeas, the induction of SI is triggered by a Ca(2)+-dependent signaling pathway that results in the rapid and S allele-specific inhibition of pollen tube tip growth. Tip growth of cells is dependent on a functioning actin cytoskeleton. We have investigated the effect of self-incompatibility (S) proteins on the actin cytoskeleton in poppy pollen tubes. Here, we report that the actin cytoskeleton of incompatible pollen tubes is rapidly and dramatically rearranged during the SI response, not only in our in vitro SI system but also in vivo. We demonstrate that nonspecific inhibition of growth does not result in similar actin rearrangements. Because the SI-induced alterations are not observed if growth stops, this clearly demonstrates that these alterations are triggered by the SI signaling cascade rather than merely resulting from the consequent inhibition of growth. We establish a detailed time course of events and discuss the mechanisms that might be involved. Our data strongly implicate a role for the actin cytoskeleton as a target for signaling pathways involved in the SI response of P. rhoeas.

Determination of cell polarity in germinated spores and hyphal tips of the filamentous ascomycete Ashbya gossypii requires a rhoGAP homolog

In the filamentous ascomycete Ashbya gossypii, like in other filamentous fungi onset of growth in dormant spores occurs as an isotropic growth phase generating spherical germ cells. Thereafter, a switch to polarized growth results in the formation of the first hyphal tip. The initial steps of hyphal tip formation in filamentous fungi, therefore, resemble processes taking place prior to and during bud emergence of unicellular yeast-like fungi. We investigated whether phenotypic similarities between these distinct events extended to the molecular level. To this end we isolated and characterized the A. gossypii homolog of the Saccharomyces cerevisiae BEM2 gene which is part of a network of rho-GTPases and their regulators required for bud emergence and bud growth in yeast. Here we show that the AgBem2 protein contains a GAP- (GTPase activating protein) domain for rho-like GTPases at its carboxy terminus, and that this part of AgBem2p is required for complementation of an Agbem2 null strain. Germination of spores resulted in enlarged Agbem2 germ cells that were unable to generate the bipolar branching pattern found in wild-type germ cells. In addition, mutant hyphae were swollen due to defects in polarized cell growth indicated by the delocalized distribution of chitin and cortical actin patches. Surprisingly, the complete loss of cell polarity which lead to spherical hyphal tips was overcome by the establishment of new cell polarities and the formation of multiple new hyphal tips. In conclusion these results and other findings demonstrate that establishment of cell polarity, maintenance of cell polarity, and polarized hyphal growth in filamentous fungi require members of Ρ-GTPase modules.

Role of actin and Myo2p in polarized secretion and growth of Saccharomyces cerevisiae

We examined the role of the actin cytoskeleton in secretion in Saccharomyces cerevisiae with the use of several quantitative assays, including time-lapse video microscopy of cell surface growth in individual living cells. In latrunculin, which depolymerizes filamentous actin, cell surface growth was completely depolarized but still occurred, albeit at a reduced level. Thus, filamentous actin is necessary for polarized secretion but not for secretion per se. Consistent with this conclusion, latrunculin caused vesicles to accumulate at random positions throughout the cell. Cortical actin patches cluster at locations that correlate with sites of polarized secretion. However, we found that actin patch polarization is not necessary for polarized secretion because a mutant, bee1Delta(las17Delta), which completely lacks actin patch polarization, displayed polarized growth. In contrast, a mutant lacking actin cables, tpm1-2 tpm2Delta, had a severe defect in polarized growth. The yeast class V myosin Myo2p is hypothesized to mediate polarized secretion. A mutation in the motor domain of Myo2p, myo2-66, caused growth to be depolarized but with only a partial decrease in the level of overall growth. This effect is similar to that of latrunculin, suggesting that Myo2p interacts with filamentous actin. However, inhibition of Myo2p function by expression of its tail domain completely abolished growth.

Compartmentalization of the cell cortex by septins is required for maintenance of cell polarity in yeast

Formation and maintenance of specialized plasma membrane domains are crucial for many biological processes, such as cell polarization and signaling. During isotropic bud growth, the yeast cell periphery is divided into two domains: the bud surface, an active site of exocytosis and growth, and the relatively quiescent surface of the mother cell. We found that cells lacking septins at the bud neck failed to maintain the exocytosis and morphogenesis factors Spa2, Sec3, Sec5, and Myo2 in the bud during isotropic growth. Furthermore, we found that septins were required for proper regulation of actin patch stability; septin-defective cells permitted to enter isotropic growth lost actin and growth polarity. We propose that septins maintain cell polarity by specifying a boundary between cortical domains.

Dynein-dependent movements of the mitotic spindle in Saccharomyces cerevisiae Do not require filamentous actin

In budding yeast, the mitotic spindle is positioned in the neck between the mother and the bud so that both cells inherit one nucleus. The movement of the mitotic spindle into the neck can be divided into two phases: (1) Kip3p-dependent movement of the nucleus to the neck and alignment of the short spindle, followed by (2) dynein-dependent movement of the spindle into the neck and oscillation of the elongating spindle within the neck. Actin has been hypothesized to be involved in all these movements. To test this hypothesis, we disrupted the actin cytoskeleton with the use of mutations and latrunculin A (latrunculin). We assayed nuclear segregation in synchronized cell populations and observed spindle movements in individual living cells. In synchronized cell populations, no actin cytoskeletal mutant segregated nuclei as poorly as cells lacking dynein function. Furthermore, nuclei segregated efficiently in latrunculin-treated cells. Individual living cell analysis revealed that the preanaphase spindle was mispositioned and misaligned in latrunculin-treated cells and that astral microtubules were misoriented, confirming a role for filamentous actin in the early, Kip3p-dependent phase of spindle positioning. Surprisingly, mispositioned and misaligned mitotic spindles moved into the neck in the absence of filamentous actin, albeit less efficiently. Finally, dynein-dependent sliding of astral microtubules along the cortex and oscillation of the elongating mitotic spindle in the neck occurred in the absence of filamentous actin.

Polarization of cell growth in yeast

The actin cytoskeleton provides the structural basis for cell polarity in Saccharomyces cerevisiae as well as most other eukaryotes. In Part I of this two-part commentary, presented in the previous issue of Journal of Cell Science, we discussed the basis by which yeast establishes and maintains different states of polarity through Ρ GTPases and cyclin-dependent protein kinase signaling. Here we discuss how, in response to those signals, the actin cytoskeleton guides growth of the yeast cell. A polarized array of actin cables at the cell cortex is the primary structural determinant of polarity. Motors such as class V myosins use this array to transport secretory vesicles, mRNA and organelles towards growth sites, where they are anchored by a cap of cytoskeletal and regulatory proteins. Cortical actin patches enhance and maintain this polarity, probably through endocytic recycling, which allows reuse of materials and prevents continued growth at old sites. The dynamic arrangement of targeting and recycling provides flexibility for the precise control of morphogenesis.

An antiprion effect of the anticytoskeletal drug latrunculin A in yeast

Prions are infectious aggregation-prone isoforms of the normal proteins, supposedly able to seed aggregation of the normal cellular counterparts. In vitro, prion proteins form amyloid fibers, resembling cytoskeletal structures. Yeast prion [PSI], which is a cytoplasmically inherited aggregated isoform of the translation termination factor Sup35p (eRF3), serves as a useful model for studying mechanisms of prion diseases and other amyloidoses. The previously described interaction between Sup35p and cytoskeletal assembly protein Sla1p points to the possible relationships between prions and cytoskeletal networks. Although the Sup35PSI+ aggregates do not colocalize with actin patches, we have shown that yeast cells are efficiently cured of the [PSI] prion by prolonged incubation with latrunculin A, a drug disrupting the actin cytoskeleton. On the other hand, treatments with sodium azide or cycloheximide, agents blocking yeast protein synthesis and cell proliferation but not disrupting the cytoskeleton, do not cause a significant loss of [PSI]. Moreover, simultaneous treatment with sodium azide or cycloheximide blocks [PSI] curing by latrunculin A, indicating that prion loss in the presence of latrunculin A requires a continuation of protein synthesis during cytoskeleton disruption. The sodium azide treatment also decreases the toxic effect of latrunculin A. Latrunculin A influences neither the levels of total cellular Sup35p nor the levels of chaperone proteins, such as Hsp104 and Hsp70, which were previously shown to affect [PSI]. This makes an indirect effect of latrunculin A on [PSI] via induction of Hsps unlikely. Fluorescence microscopy detects changes in the structure and/or localization of the Sup35PSI+ aggregates in latrunculin A-treated cells. We conclude that the stable maintenance of the [PSI] prion aggregates in the protein-synthesizing yeast cells partly depends on an intact actin cytoskeleton, suggesting that anticytoskeletal treatments could be used to counteract some aggregation-related disorders.

Cell polarity in yeast

Subcellular asymmetry, cell polarity, is fundamental to the diverse specialized functions of eukaryotic cells. In yeast, cell polarization is essential to division and mating. As a result, this highly accessible experimental system serves as a paradigm for deciphering the molecular mechanisms underlying the generation of polarity. Beyond yeast, cell polarity is essential to the partitioning of cell fate in embryonic development, the generation of axons and their guidance during neuronal development, and the intimate communication between lymphocytes within the immune system. The polarization of yeast cells shares many features with that of these more complex examples, including regulation by both intrinsic and extrinsic cues, conserved regulatory molecules such as Cdc42 GTPase, and asymmetry of the cytoskeleton as its centerpiece. This review summarizes the molecular pathways governing the generation of cell polarity in yeast.

The Rho GTPase Rho3 has a direct role in exocytosis that is distinct from its role in actin polarity

Budding yeast grow asymmetrically by the polarized delivery of proteins and lipids to specific sites on the plasma membrane. This requires the coordinated polarization of the actin cytoskeleton and the secretory apparatus. We identified Rho3 on the basis of its genetic interactions with several late-acting secretory genes. Mutational analysis of the Rho3 effector domain reveals three distinct functions in cell polarity: regulation of actin polarity, transport of exocytic vesicles from the mother cell to the bud, and docking and fusion of vesicles with the plasma membrane. We provide evidence that the vesicle delivery function of Rho3 is mediated by the unconventional myosin Myo2 and that the docking and fusion function is mediated by the exocyst component Exo70. These data suggest that Rho3 acts as a key regulator of cell polarity and exocytosis, coordinating several distinct events for delivery of proteins to specific sites on the cell surface.

PAK-family kinases regulate cell and actin polarization throughout the cell cycle of Saccharomyces cerevisiae

During the cell cycle of the yeast Saccharomyces cerevisiae, the actin cytoskeleton and cell surface growth are polarized, mediating bud emergence, bud growth, and cytokinesis. We have determined whether p21-activated kinase (PAK)-family kinases regulate cell and actin polarization at one or several points during the yeast cell cycle. Inactivation of the PAK homologues Ste20 and Cla4 at various points in the cell cycle resulted in loss of cell and actin cytoskeletal polarity, but not in depolymerization of F-actin. Loss of PAK function in G1 depolarized the cortical actin cytoskeleton and blocked bud emergence, but allowed isotropic growth and led to defects in septin assembly, indicating that PAKs are effectors of the Rho-guanosine triphosphatase Cdc42. PAK inactivation in S/G2 resulted in depolarized growth of the mother and bud and a loss of actin polarity. Loss of PAK function in mitosis caused a defect in cytokinesis and a failure to polarize the cortical actin cytoskeleton to the mother-bud neck. Cla4-green fluorescent protein localized to sites where the cortical actin cytoskeleton and cell surface growth are polarized, independently of an intact actin cytoskeleton. Thus, PAK family kinases are primary regulators of cell and actin cytoskeletal polarity throughout most or all of the yeast cell cycle. PAK-family kinases in higher organisms may have similar functions.

Three-dimensional imaging by deconvolution microscopy

Deconvolution is a computational method used to reduce out-of-focus fluorescence in three-dimensional (3D) microscope images. It can be applied in principle to any type of microscope image but has most often been used to improve images from conventional fluorescence microscopes. Compared to other forms of 3D light microscopy, like confocal microscopy, the advantage of deconvolution microscopy is that it can be accomplished at very low light levels, thus enabling multiple focal-plane imaging of light-sensitive living specimens over long time periods. Here we discuss the principles of deconvolution microscopy, describe different computational approaches for deconvolution, and discuss interpretation of deconvolved images with a particular emphasis on what artifacts may arise.

Cell wall stress depolarizes cell growth via hyperactivation of RHO1

Cells sense and physiologically respond to environmental stress via signaling pathways. Saccharomyces cerevisiae cells respond to cell wall stress by transiently depolarizing the actin cytoskeleton. We report that cell wall stress also induces a transient depolarized distribution of the cell wall biosynthetic enzyme glucan synthase FKS1 and its regulatory subunit RHO1, possibly as a mechanism to repair general cell wall damage. The redistribution of FKS1 is dependent on the actin cytoskeleton. Depolarization of the actin cytoskeleton and FKS1 is mediated by the plasma membrane protein WSC1, the RHO1 GTPase switch, PKC1, and a yet-to-be defined PKC1 effector branch. WSC1 behaves like a signal transducer or a stress-specific actin landmark that both controls and responds to the actin cytoskeleton, similar to the bidirectional signaling between integrin receptors and the actin cytoskeleton in mammalian cells. The PKC1-activated mitogen-activated protein kinase cascade is not required for depolarization, but rather for repolarization of the actin cytoskeleton and FKS1. Thus, activated RHO1 can mediate both polarized and depolarized cell growth via the same effector, PKC1, suggesting that RHO1 may function as a rheostat rather than as a simple on-off switch.

The Saccharomyces cerevisiae homologue of human Wiskott-Aldrich syndrome protein Las17p interacts with the Arp2/3 complex

Yeast Las17 protein is homologous to the Wiskott-Aldrich Syndrome protein, which is implicated in severe immunodeficiency. Las17p/Bee1p has been shown to be important for actin patch assembly and actin polymerization. Here we show that Las17p interacts with the Arp2/3 complex. LAS17 is an allele-specific multicopy suppressor of ARP2 and ARP3 mutations; overexpression restores both actin patch organization and endocytosis defects in ARP2 temperature-sensitive (ts) cells. Six of seven ARP2 ts mutants and at least one ARP3 ts mutant are synthetically lethal with las17Delta ts confirming functional interaction with the Arp2/3 complex. Further characterization of las17Delta cells showed that receptor-mediated internalization of alpha factor by the Ste2 receptor is severely defective. The polarity of normal bipolar bud site selection is lost. Las17-gfp remains localized in cortical patches in vivo independently of polymerized actin and is required for the polarized localization of Arp2/3 as well as actin. Coimmunoprecipitation of Arp2p with Las17p indicates that Las17p interacts directly with the complex. Two hybrid results also suggest that Las17p interacts with actin, verprolin, Rvs167p and several other proteins including Src homology 3 (SH3) domain proteins, suggesting that Las17p may integrate signals from different regulatory cascades destined for the Arp2/3p complex and the actin cytoskeleton.

Identification and characterization of Schizosaccharomyces pombe asp1(+), a gene that interacts with mutations in the Arp2/3 complex and actin

The Arp2/3 complex is an essential component of the actin cytoskeleton in yeast and is required for the movement of actin patches. In an attempt to identify proteins that interact with this complex in the fission yeast Schizosaccharomyces pombe, we sought high-copy suppressors of the S. pombe arp3-c1 mutant, and have identified one, which we have termed asp1(+). The asp1(+) open reading frame (ORF) predicts a highly conserved protein of 921 amino acids with a molecular mass of 106 kD that does not contain motifs of known function. Neither asp1(+) nor its apparent Saccharomyces cerevisiae ortholog, VIP1, are essential genes. However, disruption of asp1(+) leads to altered morphology and growth properties at elevated temperatures and defects in polarized growth. The asp1 disruption strain also is hypersensitive to Ca+ ions and to low pH conditions. Although Asp1p is not stably associated with the Arp2/3 complex nor localized in any discrete structure within the cytoplasm, the asp1 disruption mutant was synthetically lethal with mutations in components of the Arp2/3 complex, arp3-c1 and sop2-1, as well as with a mutation in actin, act1-48. Moreover, the vip1 disruption strain showed a negative genetic interaction with a las17Delta strain. We conclude that Asp1p/Vip1p is important for the function of the cortical actin cytoskeleton.

New actin mutants allow further characterization of the nucleotide binding cleft and drug binding sites

We have generated 9 site-specific mutations in Saccharomyces cerevisiae actin. These mutants display a variety of phenotypes when expressed in vivo, including slow actin filament turnover, slow fluid-phase endocytosis, and defects in actin organization. Actin mutation D157E confers resistance to the actin-sequestering drug, latrunculin A. Latrunculin A inhibits nucleotide exchange on wild-type yeast actin but not on D157E actin, suggesting that this residue is part of the latrunculin A binding site. We have refined our earlier map of the phalloidin binding site on actin, demonstrating a requirement for residue G158 in addition to D179 and R177. The nine new actin mutants as well as a large collection of existing actin mutants were also used to identify the putative binding site of another actin binding drug, tolytoxin, on actin. The actin alleles that result in decreased sensitivity to this drug cluster at a site near the nucleotide-binding pocket. Actin purified from one of these mutants has a reduced affinity for tolytoxin. In addition, tolytoxin causes a 2.4-fold increase in the t1/2 of ATP exchange, further suggesting that this drug binds near the nucleotide-binding pocket of actin. We note that the binding sites for latrunculin A, phalloidin, and tolytoxin all map close to the actin nucleotide binding pocket.

Tropomodulin assembles early in myofibrillogenesis in chick skeletal muscle: evidence that thin filaments rearrange to form striated myofibrils

Actin filament lengths in muscle and nonmuscle cells are believed to depend on the regulated activity of capping proteins at both the fast growing (barbed) and slow growing (pointed) filament ends. In striated muscle, the pointed end capping protein, tropomodulin, has been shown to maintain the lengths of thin filaments in mature myofibrils. To determine whether tropomodulin might also be involved in thin filament assembly, we investigated the assembly of tropomodulin into myofibrils during differentiation of primary cultures of chick skeletal muscle cells. Our results show that tropomodulin is expressed early in differentiation and is associated with the earliest premyofibrils which contain overlapping and misaligned actin filaments. In addition, tropomodulin can be found in actin filament bundles at the distal tips of growing myotubes, where sarcomeric alpha-actinin is not always detected, suggesting that tropomodulin caps actin filament pointed ends even before the filaments are cross-linked into Z bodies by alpha-actinin. Tropomodulin staining exhibits an irregular punctate pattern along the length of premyofibrils that demonstrate a smooth phalloidin staining pattern for F-actin. Strikingly, the tropomodulin dots often appear to be located between the closely spaced, dot-like Z bodies that are stained for (α)-actinin. Thus, in the earliest premyofibrils, the pointed ends of the thin filaments are clustered and partially aligned with respect to the Z bodies (the location of the barbed filament ends). At later stages of differentiation, the tropomodulin dots become aligned into regular periodic striations concurrently with the appearance of striated phalloidin staining for F-actin and alignment of Z bodies into Z lines. Tropomodulin, together with the barbed end capping protein, CapZ, may function from the earliest stages of myofibrillogenesis to restrict the lengths of newly assembled thin filaments by capping their ends; thus, transitions from nonstriated to striated myofibrils in skeletal muscle are likely due principally to filament rearrangements rather than to filament polymerization or depolymerization. Rearrangements of actin filaments capped at their pointed and barbed ends may be a general mechanism by which cells restructure their actin cytoskeletal networks during cell growth and differentiation.

Actin polymerization: Where the WASP stings

How do extracellular signals induce actin polymerization, as required for many cellular responses? Key signal transducers, such as the small GTPases Cdc42 and Rac, have now been shown to link via proteins of the WASP family to the Arp2/3 complex, which nucleates actin polymerization.

Tropomyosin-containing actin cables direct the Myo2p-dependent polarized delivery of secretory vesicles in budding yeast

The actin cytoskeleton in budding yeast consists of cortical patches and cables, both of which polarize toward regions of cell growth. Tropomyosin localizes specifically to actin cables and not cortical patches. Upon shifting cells with conditionally defective tropomyosin to restrictive temperatures, actin cables disappear within 1 min and both the unconventional class V myosin Myo2p and the secretory vesicle-associated Rab GTPase Sec4p depolarize rapidly. Bud growth ceases and the mother cell grows isotropically. When returned to permissive temperatures, tropomyosin-containing cables reform within 1 min in polarized arrays. Cable reassembly permits rapid enrichment of Myo2p at the focus of nascent cables as well as the Myo2p- dependent recruitment of Sec4p and the exocyst protein Sec8p, and the initiation of bud emergence. With the loss of actin cables, cortical patches slowly assume an isotropic distribution within the cell and will repolarize only after restoration of cables. Therefore, actin cables respond to polarity cues independently of the overall distribution of cortical patches and are able to directly target the Myo2p-dependent delivery of secretory vesicles and polarization of growth.