Actin-binding verprolin is a polarity development protein required for the morphogenesis and function of the yeast actin cytoskeleton

Imaging the Actin Cytoskeleton in Live Budding Yeast Cells

Although budding yeast, Saccharomyces cerevisiae, is widely used as a model organism in biological research, studying cell biology in yeast was hindered due to its small size, rounded morphology, and cell wall. However, with improved techniques, researchers can acquire high-resolution images and carry out rapid multidimensional analysis of a yeast cell. As a result, imaging in yeast has emerged as an important tool to study cytoskeletal organization, function, and dynamics. This chapter describes techniques and approaches for visualizing the actin cytoskeleton in live yeast cells.

MoVrp1, a putative verprolin protein, is required for asexual development and infection in the rice blast fungus Magnaporthe oryzae

Endocytosis is a crucial cellular process in eukaryotic cells which involves clathrin and/or adaptor proteins, lipid kinases, phosphatases and the actin cytoskeleton. Verprolin proteins, such as Vrp1 in Saccharomyces cerevisiae, are conserved family proteins that regulate actin binding and endocytosis. Here, we identified and characterized MoVrp1 as the yeast Vrp1 homolog in Magnaporthe oryzae. Deletion of the MoVRP1 gene resulted in defects in vegetative growth, asexual development, and infection of the host plant. The ∆Movrp1 mutants also exhibited decreased extracellular peroxidase and laccase activities and showed defects in colony pigmentation, hyphal surface hydrophobicity, cell wall integrity, autophagy, endocytosis, and secretion of avirulent effector. Our studies provided new evidences that MoVrp1 involved in actin cytoskeleton is important for growth, morphogenesis, cellular trafficking, and fungal pathogenesis.

Live-Cell Imaging of Mitochondria and the Actin Cytoskeleton in Budding Yeast

Maintenance and regulation of proper mitochondrial dynamics and functions are necessary for cellular homeostasis. Numerous diseases, including neurodegeneration and muscle myopathies, and overall cellular aging are marked by declining mitochondrial function and subsequent loss of multiple other cellular functions. For these reasons, optimized protocols are needed for visualization and quantification of mitochondria and their function and fitness. In budding yeast, mitochondria are intimately associated with the actin cytoskeleton and utilize actin for their movement and inheritance. This chapter describes optimal approaches for labeling mitochondria and the actin cytoskeleton in living budding yeast cells, for imaging the labeled cells, and for analyzing the resulting images.

Identification and functional analysis of healing regulators in Drosophila

Wound healing is an essential homeostatic mechanism that maintains the epithelial barrier integrity after tissue damage. Although we know the overall steps in wound healing, many of the underlying molecular mechanisms remain unclear. Genetically amenable systems, such as wound healing in Drosophila imaginal discs, do not model all aspects of the repair process. However, they do allow the less understood aspects of the healing response to be explored, e.g., which signal(s) are responsible for initiating tissue remodeling? How is sealing of the epithelia achieved? Or, what inhibitory cues cancel the healing machinery upon completion? Answering these and other questions first requires the identification and functional analysis of wound specific genes. A variety of different microarray analyses of murine and humans have identified characteristic profiles of gene expression at the wound site, however, very few functional studies in healing regulation have been carried out. We developed an experimentally controlled method that is healing-permissive and that allows live imaging and biochemical analysis of cultured imaginal discs. We performed comparative genome-wide profiling between Drosophila imaginal cells actively involved in healing versus their non-engaged siblings. Sets of potential wound-specific genes were subsequently identified. Importantly, besides identifying and categorizing new genes, we functionally tested many of their gene products by genetic interference and overexpression in healing assays. This non-saturated analysis defines a relevant set of genes whose changes in expression level are functionally significant for proper tissue repair. Amongst these we identified the TCP1 chaperonin complex as a key regulator of the actin cytoskeleton essential for the wound healing response. There is promise that our newly identified wound-healing genes will guide future work in the more complex mammalian wound healing response.

Two major challenges in our understanding of epithelial repair and regeneration is the identification of the signals triggered after injury and the characterization of mechanisms initiated during tissue repair. From a clinical perspective, a key question that remains unanswered is “Why do some wounds fail to heal?” Considering the low genetic redundancy of Drosophila and its high degree of conservation of fundamental functions, the analysis of wound closure in imaginal discs, whose features are comparable to other post-injury events, seems to be a good model. To proceed to genomic studies, we developed a healing-permissive in vitro culture system for discs. Employing this method and microarray analysis, we aimed to identify relevant genes that are involved in healing. We compared cells that were actively involved in healing to those not involved, and identified a set of upregulated or downregulated genes. They were annotated, clustered by expression profiles, chromosomal locations, and presumptive functions. Most importantly, we functionally tested them in a healing assay. This led to the selection of a group of genes whose changes in expression level and functionality are significant for proper tissue repair. Data obtained from these analyses must facilitate the targeting of these genes in gene therapy or pharmacological studies in mammals.

Actin and endocytosis in budding yeast

Endocytosis, the process whereby the plasma membrane invaginates to form vesicles, is essential for bringing many substances into the cell and for membrane turnover. The mechanism driving clathrin-mediated endocytosis (CME) involves > 50 different protein components assembling at a single location on the plasma membrane in a temporally ordered and hierarchal pathway. These proteins perform precisely choreographed steps that promote receptor recognition and clustering, membrane remodeling, and force-generating actin-filament assembly and turnover to drive membrane invagination and vesicle scission. Many critical aspects of the CME mechanism are conserved from yeast to mammals and were first elucidated in yeast, demonstrating that it is a powerful system for studying endocytosis. In this review, we describe our current mechanistic understanding of each step in the process of yeast CME, and the essential roles played by actin polymerization at these sites, while providing a historical perspective of how the landscape has changed since the preceding version of the YeastBook was published 17 years ago (1997). Finally, we discuss the key unresolved issues and where future studies might be headed.

Role of endocytosis in localization and maintenance of the spatial markers for bud-site selection in yeast

The yeast Saccharomyces cerevisiae normally selects bud sites (and hence axes of cell polarization) in one of two distinct patterns, the axial pattern of haploid cells and the bipolar pattern of diploid cells. These patterns depend on distinct sets of cortical-marker proteins that transmit positional information through a common signaling pathway based on a Ras-type GTPase. It has been reported previously that various proteins of the endocytic pathway may be involved in determining the bipolar pattern but not the axial pattern. To explore this question systematically, we constructed and analyzed congenic haploid and diploid deletion mutants for 14 genes encoding proteins that are involved in endocytosis. The mutants displayed a wide range of severities in their overall endocytosis defects, as judged by their growth rates and abilities to take up the lipophilic dye FM 4-64. Consistent with the previous reports, none of the mutants displayed a significant defect in axial budding, but they displayed defects in bipolar budding that were roughly correlated with the severities of their overall endocytosis defects. Both the details of the mutant budding patterns and direct examination of GFP-tagged marker proteins suggested that both initial formation and maintenance of the normally persistent bipolar marks depend on endocytosis, as well as polarized exocytosis, in actively growing cells. Interestingly, maintenance of the bipolar marks in non-growing cells did not appear to require normal levels of endocytosis. In some cases, there was a striking lack of correlation between the overall severities of the general-endocytosis defect and the bud-site selection defect, suggesting that various endocytosis proteins may differ in their importance for the uptake of various plasma-membrane targets.

There is more than one way to model an elephant. Experiment-driven modeling of the actin cytoskeleton

Mathematical modeling has established its value for investigating the interplay of biochemical and mechanical mechanisms underlying actin-based motility. Because of the complex nature of actin dynamics and its regulation, many of these models are phenomenological or conceptual, providing a general understanding of the physics at play. But the wealth of carefully measured kinetic data on the interactions of many of the players in actin biochemistry cries out for the creation of more detailed and accurate models that could permit investigators to dissect interdependent roles of individual molecular components. Moreover, no human mind can assimilate all of the mechanisms underlying complex protein networks; so an additional benefit of a detailed kinetic model is that the numerous binding proteins, signaling mechanisms, and biochemical reactions can be computationally organized in a fully explicit, accessible, visualizable, and reusable structure. In this review, we will focus on how comprehensive and adaptable modeling allows investigators to explain experimental observations and develop testable hypotheses on the intracellular dynamics of the actin cytoskeleton.

Actin dosage lethality screening in yeast mediated by selective ploidy ablation reveals links to urmylation/wobble codon recognition and chromosome stability

The actin cytoskeleton exists in a dynamic equilibrium with monomeric and filamentous states of its subunit protein actin. The spatial and temporal regulation of actin dynamics is critical to the many functions of actin. Actin levels are remarkably constant, suggesting that cells have evolved to function within a narrow range of actin concentrations. Here we report the results of screens in which we have increased actin levels in strains deleted for the ~4800 nonessential yeast genes using a technical advance called selective ploidy ablation. We detected 83 synthetic dosage interactions with actin, 78 resulted in reduced growth, whereas in 5 cases overexpression of actin suppressed the growth defects caused by the deleted genes. The genes were highly enriched in several classes, including transfer RNA wobble uridine modification, chromosome stability and segregation, cell growth, and cell division. We show that actin overexpression sequesters a limited pool of eEF1A, a bifunctional protein involved in aminoacyl-transfer RNA recruitment to the ribosome and actin filament cross-linking. Surprisingly, the largest class of genes is involved in chromosome stability and segregation. We show that actin mutants have chromosome segregation defects, suggesting a possible role in chromosome structure and function. Monomeric actin is a core component of the INO80 and SWR chromatin remodeling complexes and the NuA4 histone modification complex, and our results suggest these complexes may be sensitive to actin stoichiometry. We propose that the resulting effects on chromatin structure can lead to synergistic effects on chromosome stability in strains lacking genes important for chromosome maintenance.

Phospholipid flippases Lem3p-Dnf1p and Lem3p-Dnf2p are involved in the sorting of the tryptophan permease Tat2p in yeast

The type 4 P-type ATPases are flippases that generate phospholipid asymmetry in membranes. In budding yeast, heteromeric flippases, including Lem3p-Dnf1p and Lem3p-Dnf2p, translocate phospholipids to the cytoplasmic leaflet of membranes. Here, we report that Lem3p-Dnf1/2p are involved in transport of the tryptophan permease Tat2p to the plasma membrane. The lem3Δ mutant exhibited a tryptophan requirement due to the mislocalization of Tat2p to intracellular membranes. Tat2p was relocalized to the plasma membrane when trans-Golgi network (TGN)-to-endosome transport was inhibited. Inhibition of ubiquitination by mutations in ubiquitination machinery also rerouted Tat2p to the plasma membrane. Lem3p-Dnf1/2p are localized to endosomal/TGN membranes in addition to the plasma membrane. Endocytosis mutants, in which Lem3p-Dnf1/2p are sequestered to the plasma membrane, also exhibited the ubiquitination-dependent missorting of Tat2p. These results suggest that Tat2p is ubiquitinated at the TGN and missorted to the vacuolar pathway in the lem3Δ mutant. The NH(2)-terminal cytoplasmic region of Tat2p containing ubiquitination acceptor lysines interacted with liposomes containing acidic phospholipids, including phosphatidylserine. This interaction was abrogated by alanine substitution mutations in the basic amino acids downstream of the ubiquitination sites. Interestingly, a mutant Tat2p containing these substitutions was missorted in a ubiquitination-dependent manner. We propose the following model based on these results; Tat2p is not ubiquitinated when the NH(2)-terminal region is bound to membrane phospholipids, but if it dissociates from the membrane due to a low level of phosphatidylserine caused by perturbation of phospholipid asymmetry in the lem3Δ mutant, Tat2p is ubiquitinated and then transported from the TGN to the vacuole.

Cell polarization and cytokinesis in budding yeast

Asymmetric cell division, which includes cell polarization and cytokinesis, is essential for generating cell diversity during development. The budding yeast Saccharomyces cerevisiae reproduces by asymmetric cell division, and has thus served as an attractive model for unraveling the general principles of eukaryotic cell polarization and cytokinesis. Polarity development requires G-protein signaling, cytoskeletal polarization, and exocytosis, whereas cytokinesis requires concerted actions of a contractile actomyosin ring and targeted membrane deposition. In this chapter, we discuss the mechanics and spatial control of polarity development and cytokinesis, emphasizing the key concepts, mechanisms, and emerging questions in the field.

Connecdenn 3/DENND1C binds actin linking Rab35 activation to the actin cytoskeleton

The small GTPase Rab35 regulates endosomal membrane trafficking but also recruits effectors that modulate actin assembly and organization. Differentially expressed in normal and neoplastic cells (DENN)-domain proteins are a newly identified class of Rab guanine-nucleotide exchange factors (GEFs) that are grouped into eight families, each activating a common Rab. The members of one family, connecdenn 1-3/DENND1A-C, are all GEFs for Rab35. Why Rab35 requires multiple GEFs is unknown. We demonstrate that connecdenn 3 uses a unique C-terminal motif, a feature not found in connecdenn 1 or 2, to directly bind actin. This interaction couples Rab35 activation to the actin cytoskeleton, resulting in dramatic changes in cell shape, notably the formation of protrusive membrane extensions. These alterations are specific to Rab35 activated by connecdenn 3 and require both the actin-binding motif and N-terminal DENN domain, which harbors the GEF activity. It was previously demonstrated that activated Rab35 recruits the actin-bundling protein fascin to actin, but the relevant GEF for this activity was unknown. We demonstrate that connecdenn 3 and Rab35 colocalize with fascin and actin filaments, suggesting that connecdenn 3 is the relevant GEF. Thus, whereas connecdenn 1 and 2 activate Rab35 for endosomal trafficking, connecdenn 3 uniquely activates Rab35 for its role in actin regulation.

Vrp1p-Las17p interaction is critical for actin patch polarization but is not essential for growth or fluid phase endocytosis in S. cerevisiae

Vrp1p (yeast WIP) forms a protein complex with Las17p (yeast WASP), however the physiological significance of the interaction has not been fully characterized. Vrp1p residues, (788)MPKPR(792) are essential for Vrp1p-Las17p interaction. While C-Vrp1p(364-817) complements all the defects of the vrp1Δ strain, C-Vrp1p(364-817)(5A) ((788)AAAAA(792)) does not complement any of the defects, due to its inability to localize to cortical patches. Targeting C-Vrp1p(364-817)(5A) to membranes using CAAX motif (C-Vrp1p(364-817)(5A)-CAAX) rescued the growth and endocytosis defect but not the actin patch polarization defect of vrp1Δ. Vrp1p can localize to cortical patches, either by binding to Las17p through LBD (Las17 Binding Domain, Vrp1p(760-817)) or independent of Las17p through residues in N-Vrp1p(1-364). Unlike Vrp1p, Vrp1p(5A) localizes poorly to cortical patches and complements all the defects of vrp1Δ strain except actin patch polarization at elevated temperature. N-Vrp1p(1-364) complements all the defects of vrp1Δ strain except the actin patch polarization defect while N-Vrp1p(1-364)-LBD fusion protein complements all the defects. Thus our results show that while both Vrp1p and Las17p are essential for many cellular processes, the two proteins do not necessarily have to bind to each other to carry out these cellular functions. However, Las17p-Vrp1p interaction is essential for actin patch polarization at elevated temperature.

The mammalian verprolin, WIRE induces filopodia independent of N-WASP through IRSp53

The mammalian verprolin family of proteins, WIP (WASP Interacting Protein), CR16 (Corticoid Regulated) and WIRE (WIp-RElated) regulate the actin cytoskeleton through WASP/N-WASP (Wiskott Aldrich Syndrome Protein and Neural-WASP). In order to characterize the WASP/N-WASP-independent function of WIRE, we screened and identified IRSp53 (Insulin Receptor Substrate) as a WIRE interacting protein. Expression of IRSp53 with WIRE in N-WASP(-/-) mouse fibroblast cells induced filopodia while co-expression of IRSp53 with WIP did not. The induction of filopodia is dependent on WIRE-IRSp53 interaction as mutation in the SH3 domain of IRSp53 abolished WIRE-IRSp53 interaction as well as the ability to induce filopodia. Similarly, the Verprolin (V)-domain of WIRE is critical for IRSp53-WIRE interaction and for filopodia formation. The interaction between WIRE and IRSp53 is regulated by Cdc42 as mutations which abolish Cdc42-IRSp53 interaction lead to loss of IRSp53-WIRE interaction as shown by pull down assay. The plasma membrane localization of IRSp53 is dependent on Cdc42 and WIRE. Expression of Cdc42(G12V) (active mutant) with WIRE-IRSp53 caused significant increase in the number of filopodia per cell. Thus our results show that Cdc42 regulates the activity of IRSp53 by regulating the IRSp53-WIRE interaction as well as localization of the complex to plasma membrane to generate filopodia.

Las17p-Vrp1p but not Las17p-Arp2/3 interaction is important for actin patch polarization in yeast

The actin cytoskeleton plays a central role in many important cellular processes such as cell polarization, cell division and endocytosis. The dynamic changes to the actin cytoskeleton that accompany these processes are regulated by actin-associated proteins Wiskott-Aldrich Syndrome Protein (WASP) (known as Las17p in yeast) and WASP-Interacting Protein (WIP) (known as Vrp1p in yeast). Both yeast and human WASP bind to and stimulate the Arp2/3 complex which in turn nucleates assembly of actin monomers into filaments at polarized sites at the cortex. WASP-WIP interaction in yeast and humans are important for Arp2/3 complex stimulation in vitro. It has been proposed that these interactions are also important for polarized actin assembly in vivo. However, the redundancy of actin-associated proteins has made it difficult to test this hypothesis. We have identified two point mutations (L80T and H94L) in yeast WASP that in combination abolish WASP-WIP interaction in yeast. We also identify an N-terminal fragment of Las17p (N-Las17p1-368) able to interact with Vrp1p but not Arp2/3. Using these mutant and truncated forms of yeast WASP we provide novel evidence that WASP interaction with WIP is more important than interaction with Arp2/3 for polarized actin assembly and endocytosis in yeast.

Actin and endocytosis: mechanisms and phylogeny

The regulated assembly of actin filament networks is a crucial part of endocytosis, with crucial temporal and spatial relationships between proteins of the endocytic and actin assembly machinery. Of particular importance has been a wealth of studies in budding and fission yeast. Cell biology approaches, combined with molecular genetics, have begun to uncover the complexity of the regulation of actin dynamics during the endocytic process. In a wide range of organisms, clathrin-mediated endocytosis appears to be linked to Arp2/3-mediated actin assembly. The conservation of the components, across a wide range eukaryotic species, suggests that the partnership between endocytosis and actin may be evolutionarily ancient.

Live-cell imaging of the cytoskeleton and mitochondrial-cytoskeletal interactions in budding yeast

This chapter describes labeling methods and optical approaches for live-cell imaging of the cytoskeleton and of a specific organelle-cytoskeleton interaction in budding yeast.

The role of actin, fimbrin and endocytosis in growth of hyphae in Aspergillus nidulans

Filamentous fungi are ideal systems to study the process of polarized growth, as their life cycle is dominated by hyphal growth exclusively at the cell apex. The actin cytoskeleton plays an important role in this growth. Until now, there have been no tools to visualize actin or the actin-binding protein fimbrin in live cells of a filamentous fungus. We investigated the roles of actin (ActA) and fimbrin (FimA) in hyphal growth in Aspergillus nidulans. We examined the localization of ActA::GFP and FimA::GFP in live cells, and each displayed a similar localization pattern. In actively growing hyphae, cortical ActA::GFP and FimA::GFP patches were highly mobile throughout the hypha and were concentrated near hyphal apices. A patch-depleted zone occupied the apical 0.5 microm of growing hypha. Both FimA::GFP and Act::GFP also localize transiently to septa. Movement and later localization of both was compromised after cytochalasin treatment. Disruption of fimA resulted in delayed polarity establishment during conidium germination, abnormal hyphal growth and endocytosis defects in apolar cells. Endocytosis was severely impaired in apolar fimA disruption cells. Our data support a novel apical recycling model which indicates a critical role for actin patch-mediated endocytosis to maintain polarized growth at the apex.

Wiskott-Aldrich syndrome protein is a key regulator of the phagocytic cup formation in macrophages

Phagocytosis is a vital first-line host defense mechanism against infection involving the ingestion and digestion of foreign materials such as bacteria by specialized cells, phagocytes. For phagocytes to ingest the foreign materials, they form an actin-based membrane structure called phagocytic cup at the plasma membranes. Formation of the phagocytic cup is impaired in phagocytes from patients with a genetic immunodeficiency disorder, Wiskott-Aldrich syndrome (WAS). The gene defective in WAS encodes Wiskott-Aldrich syndrome protein (WASP). Mutation or deletion of WASP causes impaired formation of the phagocytic cup, suggesting that WASP plays an important role in the phagocytic cup formation. However, the molecular details of its formation remain unknown. We have shown that the WASP C-terminal activity is critical for the phagocytic cup formation in macrophages. We demonstrated that WASP is phosphorylated on tyrosine 291 in macrophages, and the WASP phosphorylation is important for the phagocytic cup formation. In addition, we showed that WASP and WASP-interacting protein (WIP) form a complex at the phagocytic cup and that the WASP.WIP complex plays a critical role in the phagocytic cup formation. Our results indicate that the phosphorylation of WASP and the complex formation of WASP with WIP are the essential molecular steps for the efficient formation of the phagocytic cup in macrophages, suggesting a possible disease mechanism underlying phagocytic defects and recurrent infections in WAS patients.

Verprolin function in endocytosis and actin organization

Vrp1p (verprolin, End5p) is the yeast ortholog of human Wiskott-Aldrich syndrome protein (WASP)-interacting protein (WIP). Vrp1p localizes to the cortical actin cytoskeleton, is necessary for its polarization to sites of growth and is also essential for endocytosis. At elevated temperature, Vrp1p becomes essential for growth. A C-terminal Vrp1p fragment (C-Vrp1p) retains the ability to localize to the cortical actin cytoskeleton and function in actin-cytoskeleton polarization, endocytosis and growth. Here, we demonstrate that two submodules in C-Vrp1p are required for actin-cytoskeleton polarization: a novel C-terminal actin-binding submodule (CABS) that contains a novel G-actin-binding domain, which we call a verprolin homology 2 C-terminal (VH2-C) domain; and a second submodule comprising the Las17p-binding domain (LBD) that binds Las17p (yeast WASP). The LBD localizes C-Vrp1p to membranes and the cortical actin cytoskeleton. Intriguingly, the LBD is sufficient to restore endocytosis and growth at elevated temperature to Vrp1p-deficient cells. The CABS also restores these functions, but only if modified by a lipid anchor to provide membrane association. Our findings highlight the role of Las17p binding for Vrp1p membrane association, suggest general membrane association may be more important than specific targeting to the cortical actin cytoskeleton for Vrp1p function in endocytosis and cell growth, and suggest that Vrp1p binding to individual effectors may alter their physiological activity.

Localization of Gts1p in cortical actin patches of yeast and its possible role in endocytosis

Herein we report that Gts1p fused with green-fluorescent protein (GFP) is localized in the cortical actin patch besides nuclei in yeast and the cortical Gts1p changed its position together with the patch depending on the cell-cycle phase, while nuclear Gts1p accumulated predominantly in the budding phase. Whereas Gts1p does not directly bind to actin, it associated mainly with the actin-associated protein Pan1p. In the GTS1-deleted transformant gts1Delta, the number of cells containing either a fragmented vacuole or an enlarged single central vacuole increased and the uptake of the hydrophilic dye Lucifer yellow (LY) in the vacuole decreased. Further, gts1Delta transformed with a mutant Gts1p having two cysteine-to-alanine substitutions in a zinc finger resembling that of GTPase-activating proteins of ADP-ribosylation factors (ARF-GAP) neither recovered the LY uptake unlike gts1Delta transformed with the wild-type GTS1, nor reduced the average size of central vacuoles as much as the latter did. These results suggested that Gts1p in the actin patch is involved in the fluid-phase endocytosis and membrane trafficking for vacuole formation and that the putative ARF-GAP domain in Gts1p plays an important role in these functions.

The Saccharomyces cerevisiae Arf3 protein is involved in actin cable and cortical patch formation

We show that Arf3p, a member of the ADP ribosylation family, is involved in the organization of actin cables and cortical patches in Saccharomyces cerevisiae. Profilin-deficient cells (pfy1Delta) have severe growth defects and lack actin cables. Overexpression of ARF3 restores actin cables and corrects growth defects in these cells. Cells deficient for the cortical patch proteins Las17p and Vrp1p have growth defects and a random cortical patch distribution. Overexpression of ARF3 in las17Delta and in vrp1Delta cells partially corrects growth defects and restores the polarized distribution of cortical patches. The N-terminal glycine, a myristoylation site in Arf3p, is necessary for its suppressor activity. arf3Delta cells show a random budding pattern. Overexpression of BNI1, GEA2 or SYP1, three genes involved in actin cytoskeleton formation, restores the normal axial budding pattern of arf3Delta cells. BUD6 is a polarity gene and GEA2 is involved in retrograde transport and the organization of the actin cytoskeleton. We have identified genetic interactions between ARF3 and BUD6, and between ARF3 and GEA2. Both double mutant strains have actin cytoskeleton defects. Our results support a role for ARF3 in cell polarity and the organization of the actin cytoskeleton.

Actin binding and proline rich motifs of CR16 play redundant role in growth of vrp1Delta cells

CR16, (Glucocorticoid-regulated) belongs to the verprolin family of proteins which are characterized by the presence of a V domain (verprolin) at the N-terminal. Expression of CR16 suppressed the growth and endocytosis defect of vrp1Delta strain without correcting the actin patch polarization defect. The V domain of CR16 is critical for suppression of the growth defect of vrp1Delta strain but not for localisation to cortical actin patches. Mutations in the actin binding motif alone did not abolish the activity of CR16 but the mutations in combination with deletion of N-terminal proline rich motif abolished the ability of CR16 to suppress the growth defect. This suggests that the V domain of CR16 has two functionally redundant motifs and either one of these motifs is sufficient for suppressing the growth defect of vrp1Delta strain. This is in contrast to the observation that both WIP and WIRE require the actin binding motif for their activity.

Central roles of small GTPases in the development of cell polarity in yeast and beyond

SUMMARY

The establishment of cell polarity is critical for the development of many organisms and for the function of many cell types. A large number of studies of diverse organisms from yeast to humans indicate that the conserved, small-molecular-weight GTPases function as key signaling proteins involved in cell polarization. The budding yeast Saccharomyces cerevisiae is a particularly attractive model because it displays pronounced cell polarity in response to intracellular and extracellular cues. Cells of S. cerevisiae undergo polarized growth during various phases of their life cycle, such as during vegetative growth, mating between haploid cells of opposite mating types, and filamentous growth upon deprivation of nutrition such as nitrogen. Substantial progress has been made in deciphering the molecular basis of cell polarity in budding yeast. In particular, it becomes increasingly clear how small GTPases regulate polarized cytoskeletal organization, cell wall assembly, and exocytosis at the molecular level and how these GTPases are regulated. In this review, we discuss the key signaling pathways that regulate cell polarization during the mitotic cell cycle and during mating.

Requirement for a complex of Wiskott-Aldrich syndrome protein (WASP) with WASP interacting protein in podosome formation in macrophages

Chemotactic migration of macrophages is critical for the recruitment of leukocytes to inflamed tissues. Macrophages use a specialized adhesive structure called a podosome to migrate. Podosome formation requires the Wiskott-Aldrich syndrome protein (WASP), which is a product of the gene defective in an X-linked inherited immunodeficiency disorder, the Wiskott-Aldrich syndrome. Macrophages from WASP-deficient Wiskott-Aldrich syndrome patients lack podosomes, resulting in defective chemotactic migration. However, the molecular basis for podosome formation is not fully understood. I have shown that the WASP interacting protein (WIP), a binding partner of WASP, plays an important role in podosome formation in macrophages. I showed that WASP bound WIP to form a complex at podosomes and that the knockdown of WIP impairs podosome formation. When WASP binding to WIP was blocked, podosome formation was also impaired. When WASP expression was reduced by small interfering RNA transfection, the amount of the complex of WASP with WIP decreased, resulting in reduced podosome formation. Podosomes were restored by reconstitution of the WASP-WIP complex in WASP knockdown cells. These results indicate that the WASP-WIP complex is required for podosome formation in macrophages. When podosome formation was reduced by blocking WASP binding to WIP, transendothelial migration of macrophages, the most crucial process in macrophage trafficking, was impaired. These results suggest that a complex of WASP with WIP plays a critical role in podosome formation, thereby mediating efficient transendothelial migration of macrophages.

The yeast actin cytoskeleton: from cellular function to biochemical mechanism

All cells undergo rapid remodeling of their actin networks to regulate such critical processes as endocytosis, cytokinesis, cell polarity, and cell morphogenesis. These events are driven by the coordinated activities of a set of 20 to 30 highly conserved actin-associated proteins, in addition to many cell-specific actin-associated proteins and numerous upstream signaling molecules. The combined activities of these factors control with exquisite precision the spatial and temporal assembly of actin structures and ensure dynamic turnover of actin structures such that cells can rapidly alter their cytoskeletons in response to internal and external cues. One of the most exciting principles to emerge from the last decade of research on actin is that the assembly of architecturally diverse actin structures is governed by highly conserved machinery and mechanisms. With this realization, it has become apparent that pioneering efforts in budding yeast have contributed substantially to defining the universal mechanisms regulating actin dynamics in eukaryotes. In this review, we first describe the filamentous actin structures found in Saccharomyces cerevisiae (patches, cables, and rings) and their physiological functions, and then we discuss in detail the specific roles of actin-associated proteins and their biochemical mechanisms of action.

WASP-interacting protein is important for actin filament elongation and prompt pseudopod formation in response to a dynamic chemoattractant gradient

The role of WASP-interacting protein (WIP) in the process of F-actin assembly during chemotaxis of Dictyostelium was examined. Mutations of the WH1 domain of WASP led to a reduction in binding to WIPa, a newly identified homolog of mammalian WIP, a reduction of F-actin polymerization at the leading edge, and a reduction in chemotactic efficiency. WIPa localizes to sites of new pseudopod protrusion and colocalizes with WASP at the leading edge. WIPa increases F-actin elongation in vivo and in vitro in a WASP-dependent manner. WIPa translocates to the cortical membrane upon uniform cAMP stimulation in a time course that parallels F-actin polymerization. WIPa-overexpressing cells exhibit multiple microspike formation and defects in chemotactic efficiency due to frequent changes of direction. Reduced expression of WIPa by expressing a hairpin WIPa (hp WIPa) construct resulted in more polarized cells that exhibit a delayed response to a new chemoattractant source due to delayed extension of pseudopod toward the new gradient. These results suggest that WIPa is required for new pseudopod protrusion and prompt reorientation of cells toward a new gradient by initiating localized bursts of actin polymerization and/or elongation.

A complex of Wiskott-Aldrich syndrome protein with mammalian verprolins plays an important role in monocyte chemotaxis

The Wiskott-Aldrich syndrome protein (WASP) is a product of the gene defective in an Xid disorder, Wiskott-Aldrich syndrome. WASP expression is limited to hemopoietic cells, and WASP regulates the actin cytoskeleton. It has been reported that monocytes/macrophages from WASP-deficient Wiskott-Aldrich syndrome patients are severely defective in chemotaxis, resulting in recurrent infection. However, the molecular basis of such chemotactic defects is not understood. Recently, the WASP N-terminal region was found to bind to the three mammalian verprolin homologs: WASP interacting protein (WIP); WIP and CR16 homologous protein (WICH)/WIP-related protein (WIRE); and CR16. Verprolin was originally found to play an important role in the regulation of actin cytoskeleton in yeast. We have shown that WASP, WIP, and WICH/WIRE are expressed predominantly in the human monocyte cell line THP-1 and that WIP and WICH/WIRE are involved in monocyte chemotaxis. When WASP binding to verprolins was blocked, chemotactic migration of monocytes was impaired in both THP-1 cells and primary human monocytes. Increased expression of WASP and WIP enhanced monocyte chemotaxis. Blocking WASP binding to verprolins impaired cell polarization but not actin polymerization. These results indicate that a complex of WASP with mammalian verprolins plays an important role in chemotaxis of monocytes. Our results suggest that WASP and mammalian verprolins function as a unit in monocyte chemotaxis and that the activity of this unit is critical to establish cell polarization. In addition, our results also indicate that the WASP-verprolin complex is involved in other functions such as podosome formation and phagocytosis.

Proteomic analysis of the binding partners to enteropathogenic Escherichia coli virulence proteins expressed in Saccharomyces cerevisiae

Enteropathogenic Escherichia coli (EPEC) is an enteric human pathogen responsible for much worldwide morbidity and mortality. EPEC uses a type III secretion system to inject bacterial proteins into the cytosol of intestinal epithelial cells to cause diarrheal disease. We are interested in determining the host proteins to which EPEC translocator and effector proteins bind during infection. To facilitate protein enrichment, we created fusions between GST and EPEC virulence proteins, and expressed these fusions individually in Saccharomyces cerevisiae. The biology of S. cerevisiae is well understood and often employed as a model eukaryote to study the function of bacterial virulence factors. We isolated the yeast proteins that interact with individual EPEC proteins by affinity purifying against the GST tag. These complexes were subjected to ICAT combined with ESI-MS/MS. Database searching of sequenced peptides provided a list of proteins that bound specifically to each EPEC virulence protein. The dataset suggests several potential mammalian targets of these proteins that may guide future experimentation.

Identification of novel mutations in ACT1 and SLA2 that suppress the actin-cable-overproducing phenotype caused by overexpression of a dominant active form of Bni1p in Saccharomyces cerevisiae

A formin Bni1p nucleates actin to assemble actin cables, which guide the polarized transport of secretory vesicles in budding yeast. We identified mutations that suppressed both the lethality and the excessive actin cable formation caused by overexpression of a truncated Bni1p (BNI1DeltaN). Two recessive mutations, act1-301 in the actin gene and sla2-82 in a gene involved in cortical actin patch assembly, were identified. The isolation of sla2-82 was unexpected, because cortical actin patches are required for the internalization step of endocytosis. Both act1-301 and sla2-82 exhibited synthetic growth defects with bni1Delta. act1-301, which resulted in an E117K substitution, interacted genetically with mutations in profilin (PFY1) and BUD6, suggesting that Act1-301p was not fully functional in formin-mediated polymerization. sla2-82 also interacted genetically with genes involved in actin cable assembly. Some experiments, however, suggested that the effects of sla2-82 were caused by depletion of actin monomers, because the temperature-sensitive growth phenotype of the bni1Delta sla2-82 mutant was suppressed by increased expression of ACT1. The isolation of suppressors of the BNI1DeltaN phenotype may provide a useful system for identification of actin amino-acid residues that are important for formin-mediated actin polymerization and mutations that affect the availability of actin monomers.

The BAR domain proteins: molding membranes in fission, fusion, and phagy

The Bin1/amphiphysin/Rvs167 (BAR) domain proteins are a ubiquitous protein family. Genes encoding members of this family have not yet been found in the genomes of prokaryotes, but within eukaryotes, BAR domain proteins are found universally from unicellular eukaryotes such as yeast through to plants, insects, and vertebrates. BAR domain proteins share an N-terminal BAR domain with a high propensity to adopt alpha-helical structure and engage in coiled-coil interactions with other proteins. BAR domain proteins are implicated in processes as fundamental and diverse as fission of synaptic vesicles, cell polarity, endocytosis, regulation of the actin cytoskeleton, transcriptional repression, cell-cell fusion, signal transduction, apoptosis, secretory vesicle fusion, excitation-contraction coupling, learning and memory, tissue differentiation, ion flux across membranes, and tumor suppression. What has been lacking is a molecular understanding of the role of the BAR domain protein in each process. The three-dimensional structure of the BAR domain has now been determined and valuable insight has been gained in understanding the interactions of BAR domains with membranes. The cellular roles of BAR domain proteins, characterized over the past decade in cells as distinct as yeasts, neurons, and myocytes, can now be understood in terms of a fundamental molecular function of all BAR domain proteins: to sense membrane curvature, to bind GTPases, and to mold a diversity of cellular membranes.

Identification of multiple actin-binding sites in cofilin-phosphatase Slingshot-1L

Slingshot-1L (SSH1L) is a phosphatase that specifically dephosphorylates and activates cofilin, an actin-severing and -depolymerizing protein. SSH1L binds to and is activated by F-actin in vitro, and co-localizes with F-actin in cultured cells. We examined the F-actin-binding activity, F-actin-mediated phosphatase activation, and subcellular distribution of various mutants of SSH1L. We identified three sites involved in F-actin binding of SSH1L: Trp-458 close to the C-terminus of the phosphatase domain, an LHK motif in the N-terminal region, and an LKR motif in the C-terminal region. These sites play unique roles in the control of subcellular localization and F-actin-mediated activation of SSH1L.

The verprolin family of proteins: Regulators of cell morphogenesis and endocytosis

The verprolin family of proteins, WIP, CR16 and WIRE/WICH, has emerged as critical regulators of cytoskeletal organisation in vertebrate cells. The founding father of the family, verprolin, was originally identified in budding yeast and later shown to be needed for actin polymerisation during polarised growth and during endocytosis. The vertebrate verprolins regulate actin dynamics either by binding directly to actin, by binding the WASP family of proteins or by binding to other actin regulating proteins. Interestingly, also the vertebrate verprolins have been implicated in endocytosis, demonstrating that most of the functional modules in this fascinating group of proteins have been conserved from yeast to man.

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

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

Verprolin Cytokinesis Function Mediated by the Hof One Trap Domain

In budding yeast, partitioning of the cytoplasm during cytokinesis can proceed via a pathway dependent on the contractile actomyosin ring, as in other eukaryotes, or alternatively via a septum deposition pathway dependent on an SH3 domain protein, Hof1/Cyk2 (the yeast PSTPIP1 ortholog). In dividing yeast cells, Hof1 forms a ring at the bud neck distinct from the actomyosin ring, and this zone is active in septum deposition. We previously showed the yeast Wiskott-Aldrich syndrome protein (WASP)-interacting protein (WIP) ortholog, verprolin/Vrp1/End5, interacts with Hof1 and facilitates Hof1 recruitment to the bud neck. A Vrp1 fragment unable to interact with yeast WASP (Las17/Bee1), localize to the actin cytoskeleton or function in polarization of the cortical actin cytoskeleton nevertheless retains function in Hof1 recruitment and cytokinesis. Here, we show the ability of this Vrp1 fragment to bind the Hof1 SH3 domain via its Hof one trap (HOT) domain is critical for cytokinesis. The Vrp1 HOT domain consists of three tandem proline-rich motifs flanked by serines. Unexpectedly, the Hof1 SH3 domain itself is not required for cytokinesis and indeed appears to negatively regulate cytokinesis. The Vrp1 HOT domain promotes cytokinesis by binding to the Hof1 SH3 domain and counteracting its inhibitory effect.

WICH, a member of WASP-interacting protein family, cross-links actin filaments

In yeast, Verprolin plays an important role in rearrangement of the actin cytoskeleton. There are three mammalian homologues of Verprolin, WIP, CR16, and WICH, and all of them bind actin and Wiskott-Aldrich syndrome protein (WASP) and/or neural-WASP. Here, we describe a novel function of WICH. In vitro co-sedimentation analysis revealed that WICH not only binds to actin filaments but also cross-links them. Fluorescence and electron microscopy detected that this cross-linking results in straight bundled actin filaments. Overexpression of WICH alone in cultured fibroblast caused the formation of thick actin fibers. This ability of WICH depended on its own actin cross-linking activity. Importantly, the actin cross-linking activity of WICH was modified through a direct association with N-WASP. Taken together, these data suggest that WICH induces a bundled form of actin filament with actin cross-linking activity and the association with N-WASP suppresses that activity. WICH thus appears to be a novel actin bundling protein.

The mammalian verprolin homologue WIRE participates in receptor-mediated endocytosis and regulation of the actin filament system by distinct mechanisms

The mammalian verprolin family consists of three family members: WIP, WIRE and CR16. WIRE was recently found to bind to WASP and N-WASP and to have roles in regulating actin dynamics downstream of the platelet-derived growth factor beta-receptor. In the current study, the WASP-binding domain of WIRE was identified, with the core of the binding motif encompassing amino acid residues 408-412. A stretch of aromatic amino acid residues close to the core motif also participates in WASP binding. Amino acid substitutions in each of these motifs abrogated WASP binding, suggesting that both motifs are involved in the binding of WIRE to WASP. Interestingly, WIRE mutants unable to bind WASP were still able to induce a reorganisation of the actin filament system, indicating that WASP did not participate in the signalling pathway that link WIRE to actin dynamics. In cells ectopically expressing WIRE, the endocytosis of the platelet-derived growth factor beta-receptor was drastically reduced. However, in contrast to the effect on the actin filament system, the WIRE-induced ablation of the receptor endocytosis required an intact WASP-binding domain. Moreover, WIRE was more efficient than WIP in inhibiting the receptor endocytosis, implicating that these two mammalian verprolins have distinct roles in mammalian cells.

Structural basis of actin sequestration by thymosin-beta4: implications for WH2 proteins

The WH2 (Wiscott-Aldridge syndrome protein homology domain 2) repeat is an actin interacting motif found in monomer sequestering and filament assembly proteins. We have stabilized the prototypical WH2 family member, thymosin-beta4 (Tbeta4), with respect to actin, by creating a hybrid between gelsolin domain 1 and the C-terminal half of Tbeta4 (G1-Tbeta4). This hybrid protein sequesters actin monomers, severs actin filaments and acts as a leaky barbed end cap. Here, we present the structure of the G1-Tbeta4:actin complex at 2 A resolution. The structure reveals that Tbeta4 sequesters by capping both ends of the actin monomer, and that exchange of actin between Tbeta4 and profilin is mediated by a minor overlap in binding sites. The structure implies that multiple WH2 motif-containing proteins will associate longitudinally with actin filaments. Finally, we discuss the role of the WH2 motif in arp2/3 activation.

Regulation of cytoskeletal dynamics by actin-monomer-binding proteins

The actin cytoskeleton is a vital component of several key cellular and developmental processes in eukaryotes. Many proteins that interact with filamentous and/or monomeric actin regulate the structure and dynamics of the actin cytoskeleton. Actin-filament-binding proteins control the nucleation, assembly, disassembly and crosslinking of actin filaments, whereas actin-monomer-binding proteins regulate the size, localization and dynamics of the large pool of unpolymerized actin in cells. In this article, we focus on recent advances in understanding how the six evolutionarily conserved actin-monomer-binding proteins - profilin, ADF/cofilin, twinfilin, Srv2/CAP, WASP/WAVE and verprolin/WIP - interact with actin monomers and regulate their incorporation into filament ends. We also present a model of how, together, these ubiquitous actin-monomer-binding proteins contribute to cytoskeletal dynamics and actin-dependent cellular processes.

Actin assembly and endocytosis: from yeast to mammals

Internalization of receptors, lipids, pathogens, and other cargo at the plasma membrane involves several different pathways and requires coordinated interactions between a variety of protein and lipid molecules. The actin cytoskeleton is an integral part of the cell cortex, and there is growing evidence that F-actin plays a direct role in these endocytic events. Genetic studies in yeast have firmly established a functional connection between actin and endocytosis. Identification of several proteins that may function at the interface between actin and the endocytic machinery has provided further evidence for this association in both yeast and mammalian cells. Several of these proteins are directly involved in regulating actin assembly and could thus harness forces produced during actin polymerization to facilitate specific steps in the endocytic process. Recent microscopy studies in mammalian cells provide powerful evidence that localized recruitment and polymerization of actin occurs at endocytic sites. In this review, we focus on progress made in elucidating the functions of the actin cytoskeleton in endocytosis.

Negative regulation of yeast WASp by two SH3 domain-containing proteins

BACKGROUND

WASp family proteins promote actin filament assembly by activating Arp2/3 complex and are regulated spatially and temporally to assemble specialized actin structures used in diverse cellular processes. Some WASp family members are autoinhibited until bound by activating ligands; however, regulation of the budding yeast WASp homolog (Las17/Bee1) has not yet been explored.

RESULTS

We isolated full-length Las17 and characterized its biochemical activities on yeast Arp2/3 complex. Purified Las17 was not autoinhibited; in this respect, it is more similar to SCAR/WAVE than to WASp proteins. Las17 was a much stronger activator of Arp2/3 complex than its carboxyl-terminal (WA) fragment. In addition, actin polymerization stimulated by Las17-Arp2/3 was much less sensitive to the inhibitory effects of profilin compared to polymerization stimulated by WA-Arp2/3. Two SH3 domain-containing binding partners of Las17, Sla1 and Bbc1, were purified and were shown to cooperate in inhibiting Las17 activity. The two SLA1 SH3 domains required for this inhibitory activity in vitro were also required in vivo, in combination with BBC1, for cell viability and normal actin organization.

CONCLUSIONS

Full-length Las17 is not autoinhibited and activates Arp2/3 complex more strongly than its WA domain alone, revealing an important role for the Las17 amino terminus in Arp2/3 complex activation. Two of the SH3 domain-containing ligands of Las17, Sla1 and Bbc1, cooperate to inhibit Las17 activity in vitro and are required for a shared function in actin organization in vivo. Our results show that, like SCAR/WAVE, WASp proteins can be controlled by negative regulation through the combined actions of multiple ligands.

Unconventional myosins, actin dynamics and endocytosis: a ménage à trois?

Ever since the discovery of class I myosins, the first nonmuscle myosins, about 30 years ago, the history of unconventional myosins has been linked to the organization and working of actin filaments. It slowly emerged from studies of class I myosins in lower eukaryotes that they are involved in mechanisms of endocytosis. Most interestingly, a flurry of recent findings assign a more active role to class I myosins in regulating the spatial and temporal organization of actin filament nucleation and elongation. The results highlight the multiple links between class I myosins and the major actin nucleator, the Arp2/3 complex, and its newly described activators. Two additional types of unconventional myosins, myosinIX, and Dictyostelium discoideum MyoM, have recently been tied to the signaling pathways controlling actin cytoskeleton remodeling. The present review surveys the links between these three classes of molecular motors and the complex cellular processes of endocytosis and actin dynamics, and concentrates on a working model accounting for the function of class I myosins via recruitment of the machinery responsible for actin nucleation and elongation.

MIM-B, a putative metastasis suppressor protein, binds to actin and to protein tyrosine phosphatase delta

We have found that MIM-B, a putative metastasis suppressor protein, is implicated in actin cytoskeletal control and interaction with a protein tyrosine phosphatase (PTP). MIM was originally described as a protein whose mRNA was Missing in Metastasis, as it was found not to be present in metastatic bladder carcinoma cell lines [Lee, Y. G., Macoska, J. A., Korenchuk, S. and Pienta, K. J. (2002) Neoplasia 4, 291-294]. We further characterized a variant of MIM, which we call MIM-B, and which we believe may be a link between tyrosine kinase signalling and the actin cytoskeleton. We have shown, using purified proteins and cell extracts, that MIM-B is an actin-binding protein, probably via a WASP (Wiskott-Aldrich syndrome protein)-homology 2 domain at its C-terminus. We have also found that MIM-B binds to the cytoplasmic domain of receptor PTPdelta. Expression of full-length MIM-B induces actin-rich protrusions resembling microspikes and lamellipodia at the plasma membrane and promotes disassembly of actin stress fibres. The C-terminal portion of MIM-B is localized in the cytoplasm and does not affect the actin cytoskeleton when expressed, while the N-terminal portion localizes to internal vesicles and probably targets the protein to membranes. We postulate that MIM-B may be a regulator of actin assembly downstream of tyrosine kinase signalling and that this activity may explain the involvement of MIM in the metastasis of cancer cells.

Cortactin interacts with WIP in regulating Arp2/3 activation and membrane protrusion

BACKGROUND

Modulation of actin cytoskeleton assembly is an integral step in many cellular events. A key regulator of actin polymerization is Arp2/3 complex. Cortactin, an F-actin binding protein that localizes to membrane ruffles, is an activator of Arp2/3 complex.

RESULTS

A yeast two-hybrid screen revealed the interaction of the cortactin Src homology 3 (SH3) domain with a peptide fragment derived from a cDNA encoding a region of WASp-Interacting Protein (WIP). GST-cortactin interacted with WIP in an SH3-dependent manner. The subcellular localization of cortactin and WIP coincided at the cell periphery. WIP increased the efficiency of cortactin-mediated Arp2/3 complex activation of actin polymerization in a concentration-dependent manner. Lastly, coexpression of cortactin and WIP stimulated membrane protrusions.

CONCLUSIONS

WIP, a protein involved in filopodia formation, binds to both actin monomers and cortactin. Thus, recruitment of actin monomers to a cortactin-activated Arp2/3 complex likely leads to the observed increase in cortactin activation of Arp2/3 complex by WIP. These data suggest that a cortactin-WIP complex functions in regulating actin-based structures at the cell periphery.

SYD-1, a presynaptic protein with PDZ, C2 and rhoGAP-like domains, specifies axon identity in C. elegans

Axons are defined by the presence of presynaptic specializations at specific locations. We show here that loss-of-function mutations in the C. elegans gene syd-1 cause presynaptic specializations to form in the dendritic processes of GABA-expressing motor neurons during initial differentiation. At a later developmental stage, however, syd-1 is not required for the polarity respecification of a subset of these neurons. The SYD-1 protein contains PDZ, C2 and rho-GTPase activating protein (GAP)-like domains, and is localized to presynaptic terminals in mature neurons. A truncated SYD-1 that lacks the rhoGAP domain interferes with neurite outgrowth and guidance. Our data indicate that syd-1 may be involved in specifying axon identity during initial polarity acquisition.

Saccharomyces cerevisiae Bzz1p is implicated with type I myosins in actin patch polarization and is able to recruit actin-polymerizing machinery in vitro

In Saccharomyces cerevisiae, the WASP (Wiskott-Aldrich syndrome protein) homologue Las17p (also called Bee1p) is an important component of cortical actin patches. Las17p is part of a high-molecular-weight protein complex that regulates Arp2/3 complex-dependent actin polymerization at the cell cortex and that includes the type I myosins Myo3p and Myo5p and verprolin (Vrp1p). To identify other factors implicated with this complex in actin regulation, we isolated proteins that bind to Las17p by two-hybrid screening and affinity chromatography. Here, we report the characterization of Lsb7/Bzz1p (for Las seventeen binding protein 7), an Src homology 3 (SH3) domain protein that interacts directly with Las17p via a polyproline-SH3 interaction. Bzz1p coimmunoprecipitates in a complex with Las17p, Vrp1p, Myo3/5p, Bbc1p, Hsp70p, and actin. It colocalizes with cortical actin patches and with Las17p. This localization is dependent on Las17p, but not on F-actin. Bzz1p interacts physically and genetically with type I myosins. While deletion of BZZ1 shows no obvious phenotype, simultaneous deletion of the BZZ1, MYO3, and MYO5 genes is lethal. Overexpression of Bzz1p inhibits cell growth, and a bzz1Delta myo5Delta double mutant is unable to restore actin polarity after NaCl stress. Finally, Bzz1p in vitro is able to recruit a functional actin polymerization machinery through its SH3 domains. Its interactions with Las17p, Vrp1p, and the type I myosins are essential for this process. This suggests that Bzz1p could be implicated in the regulation of actin polymerization.

Rsp5p, a new link between the actin cytoskeleton and endocytosis in the yeast Saccharomyces cerevisiae

Rsp5p is an ubiquitin-protein ligase of Saccharomyces cerevisiae that has been implicated in numerous processes including transcription, mitochondrial inheritance, and endocytosis. Rsp5p functions at multiple steps of endocytosis, including ubiquitination of substrates and other undefined steps. We propose that one of the roles of Rsp5p in endocytosis involves maintenance and remodeling of the actin cytoskeleton. We report the following. (i) There are genetic interactions between rsp5 and several mutant genes encoding actin cytoskeletal proteins. rsp5 arp2, rsp5 end3, and rsp5 sla2 double mutants all show synthetic growth defects. Overexpressed wild-type RSP5 or mutant rsp5 genes with lesions of some WW domains suppress growth defects of arp2 and end3 cells. The defects in endocytosis, actin cytoskeleton, and morphology of arp2 are also suppressed. (ii) Rsp5p and Sla2p colocalize in abnormal F-actin-containing clumps in arp2 and pan1 mutants. Immunoprecipitation experiments confirmed that Rsp5p and Act1p colocalize in pan1 mutants. (iii) Rsp5p and Sla2p coimmunoprecipitate and partially colocalize to punctate structures in wild-type cells. These studies provide the first evidence for an interaction of an actin cytoskeleton protein with Rsp5p. (iv) rsp5-w1 mutants are resistant to latrunculin A, a drug that sequesters actin monomers and depolymerizes actin filaments, consistent with the fact that Rsp5p is involved in actin cytoskeleton dynamics.

Cyclase-associated proteins: CAPacity for linking signal transduction and actin polymerization

Many extracellular signals elicit changes in the actin cytoskeleton, which are mediated through an array of signaling proteins and pathways. One family of proteins that plays a role in regulating actin remodeling in response to cellular signals are the cyclase-associated proteins (CAPs). CAPs are highly conserved monomeric actin binding proteins present in a wide range of organisms including yeast, fly, plants, and mammals. The original CAP was isolated as a component of the Saccharomyces cerevisiae adenylyl cyclase complex that serves as an effector of Ras during nutritional signaling. CAPs are multifunctional molecules that contain domains involved in actin binding, adenylyl cyclase association in yeast, SH3 binding, and oligomerization. Genetic studies in yeast have implicated CAPs in vesicle trafficking and endocytosis. CAPs play a developmental role in multicellular organisms, and studies of Drosophila have illuminated the importance of the actin cytoskeleton during eye development and in establishing oocyte polarity. This review will highlight the critical structural and functional domains of CAPs, describe recent studies that have implied important roles for these proteins in linking cell signaling with actin polymerization, and highlight their roles in vesicle trafficking and development.