Verprolin Cytokinesis Function Mediated by the Hof One Trap Domain

Yeast as a Model to Understand Actin-Mediated Cellular Functions in Mammals-Illustrated with Four Actin Cytoskeleton Proteins

The budding yeast Saccharomyces cerevisiae has an actin cytoskeleton that comprises a set of protein components analogous to those found in the actin cytoskeletons of higher eukaryotes. Furthermore, the actin cytoskeletons of S. cerevisiae and of higher eukaryotes have some similar physiological roles. The genetic tractability of budding yeast and the availability of a stable haploid cell type facilitates the application of molecular genetic approaches to assign functions to the various actin cytoskeleton components. This has provided information that is in general complementary to that provided by studies of the equivalent proteins of higher eukaryotes and hence has enabled a more complete view of the role of these proteins. Several human functional homologues of yeast actin effectors are implicated in diseases. A better understanding of the molecular mechanisms underpinning the functions of these proteins is critical to develop improved therapeutic strategies. In this article we chose as examples four evolutionarily conserved proteins that associate with the actin cytoskeleton: (1) yeast Hof1p/mammalian PSTPIP1, (2) yeast Rvs167p/mammalian BIN1, (3) yeast eEF1A/eEF1A1 and eEF1A2 and (4) yeast Yih1p/mammalian IMPACT. We compare the knowledge on the functions of these actin cytoskeleton-associated proteins that has arisen from studies of their homologues in yeast with information that has been obtained from in vivo studies using live animals or in vitro studies using cultured animal cell lines.

Hof1 and Chs4 Interact via F-BAR Domain and Sel1-like Repeats to Control Extracellular Matrix Deposition during Cytokinesis

Localized extracellular matrix (ECM) remodeling is thought to stabilize the cleavage furrow and maintain cell shape during cytokinesis [1-14]. This remodeling is spatiotemporally coordinated with a cytoskeletal structure pertaining to a kingdom of life, for example the FtsZ ring in bacteria [15], the phragmoplast in plants [16], and the actomyosin ring in fungi and animals [17, 18]. Although the cytoskeletal structures have been analyzed extensively, the mechanisms of ECM remodeling remain poorly understood. In the budding yeast Saccharomyces cerevisiae, ECM remodeling refers to sequential formations of the primary and secondary septa that are catalyzed by chitin synthase-II (Chs2) and chitin synthase-III (the catalytic subunit Chs3 and its activator Chs4), respectively [18, 19]. Surprisingly, both Chs2 and Chs3 are delivered to the division site at the onset of cytokinesis [6, 20]. What keeps Chs3 inactive until secondary septum formation remains unknown. Here, we show that Hof1 binds to the Sel1-like repeats (SLRs) of Chs4 via its F-BAR domain and inhibits Chs3-mediated chitin synthesis during cytokinesis. In addition, Hof1 is required for rapid accumulation as well as efficient removal of Chs4 at the division site. This study uncovers a mechanism by which Hof1 controls timely activation of Chs3 during cytokinesis and defines a novel interaction and function for the conserved F-BAR domain and SLR that are otherwise known for their abilities to bind membrane lipids [21, 22] and scaffold protein complex formation [23].

Blocking endocytotic mechanisms to improve heterologous protein titers in Saccharomyces cerevisiae

Saccharomyces cerevisiae is a useful platform for protein production of biopharmaceuticals and industrial enzymes. To date, substantial effort has focused on alleviating several bottlenecks in expression and the secretory pathway. Recently, it has been shown that highly active endocytosis could decrease the overall protein titer in the supernatant. In this study, we block endocytosis and trafficking to the vacuole using a modified TEV Protease-Mediated Induction of Protein Instability (mTIPI) system to disrupt the endocytotic and vacuolar complexes. We report that conditional knock-down of endocytosis gene Rvs161 improved the concentration of α-amylase in supernatant of S. cerevisiae cultures by 63.7% compared to controls. By adaptive evolution, we obtained knock-down mutants in Rvs161 and End3 genes with 2-fold and 3-fold α-amylase concentrations compared to controls that were not evolved. Our study demonstrates that genetic blocking of endocytotic mechanisms can improve heterologous protein production in S. cerevisiae. This result is likely generalizable to other eukaryotic secretion hosts.

Yeast studies reveal moonlighting functions of the ancient actin cytoskeleton

Classic functions of the actin cytoskeleton include control of cell size and shape and the internal organization of cells. These functions are manifest in cellular processes of fundamental importance throughout biology such as the generation of cell polarity, cell migration, cell adhesion, and cell division. However, studies in the unicellular model eukaryote Saccharomyces cerevisiae (Baker's yeast) are giving insights into other functions in which the actin cytoskeleton plays a critical role. These include endocytosis, control of protein translation, and determination of protein 3-dimensional shape (especially conversion of normal cellular proteins into prions). Here, we present a concise overview of these new "moonlighting" roles for the actin cytoskeleton and how some of these roles might lie at the heart of important molecular switches. This is an exciting time for researchers interested in the actin cytoskeleton. We show here how studies of actin are leading us into many new and exciting realms at the interface of genetics, biochemistry, and cell biology. While many of the pioneering studies have been conducted using yeast, the conservation of the actin cytoskeleton and its component proteins throughout eukaryotes suggests that these new roles for the actin cytoskeleton may not be restricted to yeast cells but rather may reflect new roles for the actin cytoskeleton of all eukaryotes.

Regulation of cytokinesis in the milk yeast Kluyveromyces lactis

Cytokinesis in yeast and mammalian cells is a highly coordinated process mediated by the constriction of an actomyosin ring. In yeasts, it is accompanied by the formation of a chitinous primary septum. Although much is known about the regulation of cytokinesis in budding yeast, overlapping functions of redundant genes complicates genetic analyses. Here, we investigated the effects of various deletion mutants on cytokinesis in the milk yeast Kluyveromyces lactis. To determine the spatiotemporal parameters of cytokinesis components, live-cell imaging of fluorophor-tagged KlMyo1 and a new Lifeact probe for KlAct1 was employed. In contrast to Saccharomyces cerevisiae, where deletion of ScMYO1 is lethal, Klmyo1 deletion was temperature-sensitive. Transmission and scanning electron microscopy demonstrated that the Klmyo1 deletion cells had a defect in the formation of the primary septum and in cell separation; this result was confirmed by FACS analyses. Deletion of KlCYK3 was lethal, whereas in S. cerevisiae a cyk3 deletion is synthetically lethal with hof1 deletion. Growth of Klhof1 mutants was osmoremedial at 25°C, as it is in S. cerevisiae. CYK3 and HOF1 genes cross-complemented in both species, suggesting that they are functional homologs. Inn1, a common interactor for these two regulators, was essential in both yeasts and the encoding genes did not cross-complement. The C2 domain of the Inn1 homologs conferred species specificity. Thus, our work establishes K. lactis as a model yeast to study cytokinesis with less genetic redundancy than S. cerevisiae. The viability of Klmyo1 deletions provides an advantage over budding yeast to study actomyosin-independent cytokinesis. Moreover, the lethality of Klcyk3 null mutants suggests that there are fewer functional redundancies with KlHof1 in K. lactis.

Targeting and functional mechanisms of the cytokinesis-related F-BAR protein Hof1 during the cell cycle

Hof1 targets to the division site by interacting with septins and myosin II sequentially during the cell cycle. It plays a role in cytokinesis by coupling actomyosin ring constriction to primary septum formation through interactions with Myo1 and Chs2.

F-BAR proteins are membrane‑associated proteins believed to link the plasma membrane to the actin cytoskeleton in cellular processes such as cytokinesis and endocytosis. In the budding yeast Saccharomyces cerevisiae, the F‑BAR protein Hof1 localizes to the division site in a complex pattern during the cell cycle and plays an important role in cytokinesis. However, the mechanisms underlying its localization and function are poorly understood. Here we show that Hof1 contains three distinct targeting domains that contribute to cytokinesis differentially. The N‑terminal half of Hof1 localizes to the bud neck and the sites of polarized growth during the cell cycle. The neck localization is mediated mainly by an interaction between the second coiled‑coil region in the N‑terminus and the septin Cdc10, whereas the localization to the sites of polarized growth is mediated entirely by the F‑BAR domain. In contrast, the C‑terminal half of Hof1 interacts with Myo1, the sole myosin‑II heavy chain in budding yeast, and localizes to the bud neck in a Myo1‑dependent manner from the onset to the completion of cytokinesis. We also show that the SH3 domain in the C‑terminus plays an important role in maintaining the symmetry of Myo1 ring constriction during cytokinesis and that Hof1 interacts with Chs2, a chitin synthase that is required for primary septum formation. Together these data define a mechanism that accounts for the localization of Hof1 during the cell cycle and suggest that Hof1 may function in cytokinesis by coupling actomyosin ring constriction to primary septum formation through interactions with Myo1 and Chs2.

Dual function of the NDR-kinase Dbf2 in the regulation of the F-BAR protein Hof1 during cytokinesis

The conserved NDR-kinase Dbf2 plays a critical role in cytokinesis in budding yeast. Among its cytokinesis-related substrates is the F-BAR protein Hof1. Hof1 colocalizes at the cell division site with the septin complex and, as mitotic exit progresses, moves to the actomyosin ring (AMR). Neither the function of Hof1 at the septin complex nor the mechanism by which Hof1 supports AMR constriction is understood. Here we establish that Dbf2 has a dual function in Hof1 regulation. First, we show that the coiled-coil region, which is adjacent to the conserved F-BAR domain, is required for the binding of Hof1 to septins. The Dbf2-dependent phosphorylation of Hof1 at a single serine residue (serine 313) in this region diminishes the recruitment of Hof1 to septins both in vitro and in vivo. Genetic and functional analysis indicates that the binding of Hof1 to septins is important for septin rearrangement and integrity during cytokinesis. Furthermore, Dbf2 phosphorylation of Hof1 at serines 533 and 563 promotes AMR constriction most likely by inhibiting the SH3-domain-dependent interactions of Hof1. Thus our data show that Dbf2 coordinates septin and AMR functions during cytokinesis through the regulation/control of Hof1.

Mutation of Hof1 PEST motif phosphorylation sites leads to retention of Hof1 at the bud neck and a decrease in the rate of myosin contraction

Regulation of actomyosin ring contraction is important for the coordination of cytokinesis with mitosis. Hof1, a member of the Pombe Cdc15 homology (PCH) family of proteins, is required for efficient cytokinesis in budding yeast. Phosphorylation of Hof1 depends on the mitotic exit network (MEN), and its degradation at the end of mitosis depends on its PEST motif and interaction with the E3 ligase Grr1. To test the hypothesis that targeted destruction of Hof1 temporally couples mitotic exit with contraction of the actomyosin ring, we mutated the Hof1 PEST motif to prevent phosphorylation and subsequent degradation. These mutations increased the amount of Hof1 at the bud neck during cytokinesis, resulted in smaller bud neck diameter, and slowed the rate of myosin contraction. However, Hof1 PEST motif phosphorylation site mutants did not have cytokinesis defects, indicating that regulation of Hof1 levels does not control the onset of actomyosin ring contraction as predicted.

Candida albicans Vrp1 is required for polarized morphogenesis and interacts with Wal1 and Myo5

Recently, a link between endocytosis and hyphal morphogenesis has been identified in Candida albicans via the Wiskott–Aldrich syndrome gene homologue WAL1. To get a more detailed mechanistic understanding of this link we have investigated a potentially conserved interaction between Wal1 and the C. albicans WASP-interacting protein (WIP) homologue encoded by VRP1. Deletion of both alleles of VRP1 results in strong hyphal growth defects under serum inducing conditions but filamentation can be observed on Spider medium. Mutant vrp1 cells show a delay in endocytosis – measured as the uptake and delivery of the lipophilic dye FM4-64 into small endocytic vesicles – compared to the wild-type. Vacuolar morphology was found to be fragmented in a subset of cells and the cortical actin cytoskeleton was depolarized in vrp1 daughter cells. The morphology of the vrp1 null mutant could be complemented by reintegration of the wild-type VRP1 gene at the BUD3 locus. Using the yeast two-hybrid system we could demonstrate an interaction between the C-terminal part of Vrp1 and the N-terminal part of Wal1, which contains the WH1 domain. Furthermore, we found that Myo5 has several potential interaction sites on Vrp1. This suggests that a Wal1–Vrp1–Myo5 complex plays an important role in endocytosis and the polarized localization of the cortical actin cytoskeleton to promote polarized hyphal growth in C. albicans.

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.

Systems biology of the cell cycle of Saccharomyces cerevisiae: From network mining to system-level properties

Following a brief description of the operational procedures of systems biology (SB), the cell cycle of budding yeast is discussed as a successful example of a top-down SB analysis. After the reconstruction of the steps that have led to the identification of a sizer plus timer network in the G1 to S transition, it is shown that basic functions of the cell cycle (the setting of the critical cell size and the accuracy of DNA replication) are system-level properties, detected only by integrating molecular analysis with modelling and simulation of their underlying networks. A detailed network structure of a second relevant regulatory step of the cell cycle, the exit from mitosis, derived from extensive data mining, is constructed and discussed. To reach a quantitative understanding of how nutrients control, through signalling, metabolism and transcription, cell growth and cycle is a very relevant aim of SB. Since we know that about 900 gene products are required for cell cycle execution and control in budding yeast, it is quite clear that a purely systematic approach would require too much time. Therefore lines for a modular SB approach, which prioritises molecular and computational investigations for faster cell cycle understanding, are proposed. The relevance of the insight coming from the cell cycle SB studies in developing a new framework for tackling very complex biological processes, such as cancer and aging, is discussed.

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.

Pombe Cdc15 homology (PCH) proteins: coordinators of membrane-cytoskeletal interactions

Cellular adhesion, motility, endocytosis, exocytosis and cytokinesis involve the coordinated reorganization of the cytoskeleton and of the plasma membrane. The 'Pombe Cdc15 homology' (PCH) family of adaptor proteins has recently been shown to coordinate the membrane and cytoskeletal dynamics involved in these processes by curving membranes, recruiting dynamin and controlling the architecture of the actin cytoskeleton. Mutations in PCH family members or proteins that interact with them are associated with autoinflammatory, neurological or neoplastic diseases. Here, we review the nature, actions and disease associations of the vertebrate PCH family members, highlighting their fundamental roles in the regulation of processes involving membrane-cytoskeletal interactions.

IQGAP and mitotic exit network (MEN) proteins are required for cytokinesis and re-polarization of the actin cytoskeleton in the budding yeast, Saccharomyces cerevisiae

In budding yeast the final stages of the cell division cycle, cytokinesis and cell separation, are distinct events that require to be coupled, both together and with mitotic exit. Here we demonstrate that mutations in genes of the mitotic exit network (MEN) prevent cell separation and are synthetically lethal in combination with both cytokinesis and septation defective mutations. Analysis of the synthetic lethal phenotypes reveals that Iqg1p functions in combination with the MEN components, Tem1p, Cdc15p Dbf20p and Dbf2p to govern the re-polarization of the actin cytoskeleton to either side of the bud neck. In addition phosphorylation of the conserved PCH protein, Hof1p, is dependent upon these activities and requires actin ring assembly. Recruitment of Dbf2p to the bud neck is dependent upon actin ring assembly and correlates with Hof1p phosphorylation. Failure to phosphorylate Hof1p results in the increased stability of the protein and its persistence at the bud neck. These data establish a mechanistic dependency of cell separation upon an intermediate step requiring actomyosin ring assembly.

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.

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.