Stimulation of actin polymerization by vacuoles via Cdc42p-dependent signaling

The interdependent transport of yeast vacuole Ca2+ and H+ and the role of phosphatidylinositol 3,5-bisphosphate

Yeast vacuoles are acidified by the v-type H+-ATPase (V-ATPase) that is comprised of the membrane embedded VO complex and the soluble cytoplasmic V1 complex. The assembly of the V1-VO holoenzyme on the vacuole is stabilized in part through interactions between the VO a-subunit ortholog Vph1 and the lipid phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2). PI(3,5)P2 also affects vacuolar Ca2+ release through the channel Yvc1 and uptake through the Ca2+ pump Pmc1. Here, we asked if H+ and Ca2+ transport activities were connected through PI(3,5)P2. We found that overproduction of PI(3,5)P2 by the hyperactive fab1T2250A mutant augmented vacuole acidification, whereas the kinase-inactive fab1EEE mutant attenuated the formation of a H+ gradient. Separately, we tested the effects of excess Ca2+ on vacuole acidification. Adding micromolar Ca2+ blocked vacuole acidification, whereas chelating Ca2+ accelerated acidification. The effect of adding Ca2+ on acidification was eliminated when the Ca2+/H+ antiporter Vcx1 was absent, indicating that the vacuolar H+ gradient can collapse during Ca2+ stress through Vcx1 activity. This, however, was independent of PI(3,5)P2, suggesting that PI(3,5)P2 plays a role in submicromolar Ca2+ flux but not under Ca2+ shock. To see if the link between Ca2+ and H+ transport was bidirectional, we examined Ca2+ transport when vacuole acidification was inhibited. We found that Ca2+ transport was inhibited by halting V-ATPase activity with Bafilomycin or neutralizing vacuolar pH with chloroquine. Together, these data show that Ca2+ transport and V-ATPase efficacy are connected but not necessarily through PI(3,5)P2.

High throughput analysis of vacuolar acidification

Eukaryotic cells are compartmentalized into membrane-bound organelles, allowing each organelle to maintain the specialized conditions needed for their specific functions. One of the features that change between organelles is lumenal pH. In the endocytic and secretory pathways, lumenal pH is controlled by isoforms and concentration of the vacuolar-type H⁺-ATPase (V-ATPase). In the endolysosomal pathway, copies of complete V-ATPase complexes accumulate as membranes mature from early endosomes to late endosomes and lysosomes. Thus, each compartment becomes more acidic as maturation proceeds. Lysosome acidification is essential for the breakdown of macromolecules delivered from endosomes as well as cargo from different autophagic pathways, and dysregulation of this process is linked to various diseases. Thus, it is important to understand the regulation of the V-ATPase. Here we describe a high-throughput method for screening inhibitors/activators of V-ATPase activity using Acridine Orange (AO) as a fluorescent reporter for acidified yeast vacuolar lysosomes. Through this method, the acidification of purified vacuoles can be measured in real-time in half-volume 96-well plates or a larger 384-well format. This not only reduces the cost of expensive low abundance reagents, but it drastically reduces the time needed to measure individual conditions in large volume cuvettes.

The R-SNARE Ykt6 is required for multiple events during oogenesis in Drosophila

Ykt6 has emerged as a key protein involved in a wide array of trafficking events, and has also been implicated in a number of human pathologies, including the progression of several cancers. It is a complex protein that simultaneously exhibits a high degree of structural and functional homology, and yet adopts differing roles in different cellular contexts. Because Ykt6 has been implicated in a variety of vesicle fusion events, we characterized the role of Ykt6 in oogenesis by observing the phenotype of Ykt6 germline clones. Immunofluorescence was used to visualize the expression of membrane proteins, organelles, and vesicular trafficking markers in mutant egg chambers. We find that Ykt6 germline clones have morphological and actin defects affecting both the nurse cells and oocyte, consistent with a role in regulating membrane growth during mid-oogenesis. Additionally, these egg chambers exhibit defects in bicoid and oskar RNA localization, and in the trafficking of Gurken during mid-to-late oogenesis. Finally, we show that Ykt6 mutations result in defects in late endosomal pathways, including endo- and exocytosis. These findings suggest a role for Ykt6 in endosome maturation and in the movement of membranes to and from the cell surface.

Actin Cytoskeleton Regulation by the Yeast NADPH Oxidase Yno1p Impacts Processes Controlled by MAPK Pathways

Reactive oxygen species (ROS) that exceed the antioxidative capacity of the cell can be harmful and are termed oxidative stress. Increasing evidence suggests that ROS are not exclusively detrimental, but can fulfill important signaling functions. Recently, we have been able to demonstrate that a NADPH oxidase-like enzyme (termed Yno1p) exists in the single-celled organism Saccharomyces cerevisiae. This enzyme resides in the peripheral and perinuclear endoplasmic reticulum and functions in close proximity to the plasma membrane. Its product, hydrogen peroxide, which is also produced by the action of the superoxide dismutase, Sod1p, influences signaling of key regulatory proteins Ras2p and Yck1p/2p. In the present work, we demonstrate that Yno1p-derived H₂O₂ regulates outputs controlled by three MAP kinase pathways that can share components: the filamentous growth (filamentous growth MAPK (fMAPK)), pheromone response, and osmotic stress response (hyperosmolarity glycerol response, HOG) pathways. A key structural component and regulator in this process is the actin cytoskeleton. The nucleation and stabilization of actin are regulated by Yno1p. Cells lacking YNO1 showed reduced invasive growth, which could be reversed by stimulation of actin nucleation. Additionally, under osmotic stress, the vacuoles of a ∆yno1 strain show an enhanced fragmentation. During pheromone response induced by the addition of alpha-factor, Yno1p is responsible for a burst of ROS. Collectively, these results broaden the roles of ROS to encompass microbial differentiation responses and stress responses controlled by MAPK pathways.

SNAREs, tethers and SM proteins: how to overcome the final barriers to membrane fusion?

Physiological membrane vesicles are built to separate reaction spaces in a stable manner, even when they accidentally collide or are kept in apposition by spatial constraints in the cell. This requires a natural resistance to fusion and mixing of their content, which originates from substantial energetic barriers to membrane fusion [1]. To facilitate intracellular membrane fusion reactions in a controlled manner, proteinaceous fusion machineries have evolved. An important open question is whether protein fusion machineries actively pull the fusion reaction over the present free energy barriers, or whether they rather catalyze fusion by lowering those barriers. At first sight, fusion proteins such as SNARE complexes and viral fusion proteins appear to act as nano-machines, which mechanically transduce force to the membranes and thereby overcome the free energy barriers [2,3]. Whether fusion proteins additionally alter the free energy landscape of the fusion reaction via catalytic roles is less obvious. This is a question that we shall discuss in this review, with particular focus on the influence of the eukaryotic SNARE-dependent fusion machinery on the final step of the reaction, the formation and expansion of the fusion pore.

OsVPE3 Mediates GA-induced Programmed Cell Death in Rice Aleurone Layers via Interacting with Actin Microfilaments

BACKGROUND

Vacuolar processing enzymes (VPEs) have been identified as the enzymes that regulate vacuole-mediated programmed cell death (PCD) in plants. The mechanism that VPE regulates the PCD in rice aleurone layers remains unknown.

RESULTS

The aleurone layers treated with distilled water exerted caspase-1 and VPE activity, both of which were inhibited by the caspase-1 specific inhibitor Ac-YVAD-CMK but not by the caspase-3 specific inhibitor Ac-DEVD-CHO. However, the caspase-1 and caspase-3 inhibitors weakened the activity of caspase-3. Combined with the effects of endogenous gibberellin (GA) on the induction of OsVPEs, we suggest that the OsVPE3 in the aleurone layers, which exhibits caspase-1-like activity, is a key molecule in GA-induced PCD via regulating the protease with caspase-3-like activity. Many studies have confirmed that vacuolar fusion is an important feature of vacuole-mediated PCD in plants. In this experiment, the process of vacuole fusion was accompanied by changes in the structure of actin filaments (AFs), specifically, their depolymerization and polymerization. The process of vacuolar fusion was accelerated or delayed by the promotion or inhibition of the depolymerization of AFs, respectively. Here, the inhibition of OsVPE3 blocked the depolymerization of AFs and delayed the fusion of vacuoles, indicating that OsVPE3 can regulate the fusion of vacuoles in rice aleurone layers via mediating AFs. Furthermore, the depolymerization of AFs contributed to the up-regulation of OsVPE3 gene expression and VPE activity, resulting in accelerated PCD in rice aleurone layers. However, the inhibitor of VPE reversed the effects of AF depolymerization on the activity of VPE, then postponing the process of PCD, implying that AF can involve in GA-induced PCD of rice aleurone layers by mediating OsVPE3.

CONCLUSIONS

Together, activation of OsVPE3 and depolymerization of AFs shortened the process of vacuolation and PCD in rice aleurone layers, and OsVPE3 interacted with AFs during regulation.

Cdc42 activation state affects its localization and protein levels in fission yeast

Rho GTPases control polarized cell growth and are well-known regulators of exocytic and endocytic processes. Cdc42 is an essential GTPase, conserved from yeast to humans, that is critical for cell polarization. Cdc42 is negatively regulated by the GTPase-activating proteins (GAPs) and the GDP dissociation inhibitors (GDIs), and positively regulated by guanine nucleotide exchange factors (GEFs). Cdc42 GTPase can be found in a GTP- or GDP-bound state, which determines the ability to bind downstream effector proteins and activate signalling pathways. Only GTP-bound Cdc42 is active. In this study we have analysed the localization of the different nucleotide-bound states of Cdc42 in Schizosaccharomyces pombe: the wild-type Cdc42 protein that cycles between an active and inactive form, the Cdc42G12V form that is permanently bound to GTP and the Cdc42T17N form that is constitutively inactive. Our results indicate that Cdc42 localizes to several membrane compartments in the cell and this localization is mediated by its C-terminal prenylation. Constitutively active Cdc42 localizes mainly to the plasma membrane and concentrates at the growing tips where it is considerably less dynamic than wild-type or GDP-bound Cdc42. Additionally we show that the activation state of Cdc42 also participates in the regulation of its protein levels mediated by endocytosis and by the exocyst complex.

Yeast translation elongation factor-1A binds vacuole-localized Rho1p to facilitate membrane integrity through F-actin remodeling

Rho GTPases are molecular switches that modulate a variety of cellular processes, most notably those involving actin dynamics. We have previously shown that yeast vacuolar membrane fusion requires re-organization of actin filaments mediated by two Rho GTPases, Rho1p and Cdc42p. Cdc42p initiates actin polymerization to facilitate membrane tethering; Rho1p has a role in the late stages of vacuolar fusion, but its mode of action is unknown. Here, we identified eEF1A as a vacuolar Rho1p-interacting protein. eEF1A (encoded by the TEF1 and TEF2 genes in yeast) is an aminoacyl-tRNA transferase needed during protein translation. eEF1A also has a second function that is independent of translation; it binds and organizes actin filaments into ordered cable structures. Here, we report that eEF1A interacts with Rho1p via a C-terminal subdomain. This interaction occurs predominantly when both proteins are in the GDP-bound state. Therefore, eEF1A is an atypical downstream effector of Rho1p. eEF1A does not promote vacuolar fusion; however, overexpression of the Rho1p-interacting subdomain affects vacuolar morphology. Vacuoles were destabilized and prone to leakage when treated with the eEF1A inhibitor narciclasine. We propose a model whereby eEF1A binds to Rho1p-GDP on the vacuolar membrane; it is released upon Rho1p activation and then bundles actin filaments to stabilize fused vacuoles. Therefore, the Rho1p-eEF1A complex acts to spatially localize a pool of eEF1A to vacuoles where it can readily organize F-actin.

Plant vacuole morphology and vacuolar trafficking

Plant vacuoles are essential organelles for plant growth and development, and have multiple functions. Vacuoles are highly dynamic and pleiomorphic, and their size varies depending on the cell type and growth conditions. Vacuoles compartmentalize different cellular components such as proteins, sugars, ions and other secondary metabolites and play critical roles in plants response to different biotic/abiotic signaling pathways. In this review, we will summarize the patterns of changes in vacuole morphology in certain cell types, our understanding of the mechanisms of plant vacuole biogenesis, and the role of SNAREs and Rab GTPases in vacuolar trafficking.

The MAP kinase Slt2 is involved in vacuolar function and actin remodeling in Saccharomyces cerevisiae mutants affected by endogenous oxidative stress

Oxidative stress causes transient actin cytoskeleton depolarization and also provokes vacuole fragmentation in wild-type cells. Under conditions of oxidative stress induced by hydrogen peroxide, the Slt2 protein is required to repolarize the actin cytoskeleton and to promote vacuole fusion. In this study, we show that grx3 grx4 and grx5 mutants are cellular models of endogenous oxidative stress. This stress is the result of alterations in iron homeostasis that lead to impairment of vacuolar function and also to disorganization of the actin cytoskeleton. Slt2 overexpression suppresses defects in vacuolar function and actin cytoskeleton organization in the grx3 grx4 mutant. Slt2 exerts this effect independently of the intracellular levels of reactive oxygen species (ROS) and of iron homeostasis. The deletion of SLT2 in the grx3 grx4 mutant results in synthetic lethality related to vacuolar function with substantial vacuole fragmentation. The observation that both Vps4 and Vps73 (two proteins related to vacuole sorting) suppress vacuole fragmentation and actin depolarization in the grx3 grx4 slt2 triple mutant strengthens the hypothesis that Slt2 plays a role in vacuole homeostasis related to actin dynamics. Here we show that in sod1, grx5, and grx3 grx4 slt2 mutants, all of which are affected by chronic oxidative stress, the overexpression of Slt2 favors vacuole fusion through a mechanism dependent on an active actin cytoskeleton.

Identification of Drosophila gene products required for phagocytosis of Leishmania donovani

The identity and function of host factors required for efficient phagocytosis and intracellular maintenance of the protozoan parasite Leishmania donovani are poorly understood. Utilising the phagocytic capability of Drosophila S2 cells, together with available tools for modulating gene expression by RNAi, we have developed an experimental system in which to identify host proteins of this type on a genome-wide scale. We have shown that L. donovani amastigotes can be phagocytosed by S2 cells, in which they replicate and are maintained in a compartment with features characteristic of mammalian phagolysosomes. Screening with dsRNAs from 1920 conserved metazoan genes has identified transcripts that, when reduced in expression, cause either increased or decreased phagocytosis. Focussing on genes in the latter class, RNAi-mediated knockdown of the small GTPase Rab5, the prenylated SNARE protein YKT6, one sub-unit of serine palmitoyltransferase (spt2/lace), the Rac1-associated protein Sra1 and the actin cytoskeleton regulatory protein, SCAR, all lead to a significant reduction in parasite phagocytosis. A role for the lace mammalian homologue in amastigote uptake by mammalian macrophages has been verified using the serine palmitoyltransferase inhibitor, myriocin. These observations suggest that this experimental approach has the potential to identify a large number of host effectors required for efficient parasite uptake and maintenance.

The 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) pathway regulates developmental cerebral-vascular stability via prenylation-dependent signalling pathway

Spontaneous intracranial hemorrhage is a debilitating form of stroke, often leading to death or permanent cognitive impairment. Many of the causative genes and the underlying mechanisms implicated in developmental cerebral-vascular malformations are unknown. Recent in vitro and in vivo studies in mice have shown inhibition of the 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) pathway to be effective in stabilizing cranial vessels. Using a combination of pharmacological and genetic approaches to specifically inhibit the HMGCR pathway in zebrafish (Danio rerio), we demonstrate a requirement for this metabolic pathway in developmental vascular stability. Here we report that inhibition of HMGCR function perturbs cerebral-vascular stability, resulting in progressive dilation of blood vessels, followed by vessel rupture, mimicking cerebral cavernous malformation (CCM)-like lesions in humans and murine models. The hemorrhages in the brain are rescued by prior exogenous supplementation with geranylgeranyl pyrophosphate (GGPP), a 20-carbon metabolite of the HMGCR pathway, required for the membrane localization and activation of Rho GTPases. Consistent with this observation, morpholino-induced depletion of the β-subunit of geranylgeranyltransferase I (GGTase I), an enzyme that facilitates the post-translational transfer of the GGPP moiety to the C-terminus of Rho family of GTPases, mimics the cerebral hemorrhaging induced by the pharmacological and genetic ablation of HMGCR. In embryos with cerebral hemorrhage, the endothelial-specific expression of cdc42, a Rho GTPase involved in the regulation of vascular permeability, was significantly reduced. Taken together, our data reveal a metabolic contribution to the stabilization of nascent cranial vessels, requiring protein geranylgeranylation acting downstream of the HMGCR pathway.

Cdc42 regulation of polarized traffic in fission yeast

Cdc42 is a key factor in the control of cell polarity and morphogenesis. Fission yeast Cdc42 regulates formin activation and actin cable assembly. Cdc42 is also required for exocyst function, contributing to polarized secretion. Additionally, Cdc42 participates in membrane trafficking, endosome recycling, and vacuole formation. We show here how Cdc42 is required for the correct transport/recycling to the plasma membrane of the glucan synthases Bgs1 and Bgs4, responsible of cell wall biosynthesis and polarized growth at the cell tips.

Termination of isoform-selective Vps21/Rab5 signaling at endolysosomal organelles by Msb3/Gyp3

Traffic through endosomes and lysosomes is controlled by small G-proteins of the Rab5 and Rab7 families. Like humans, Saccharomyces cerevisiae has three Rab5s (Vps21, Ypt52 and Ypt53) and one Rab7 (Ypt7). Here, we elucidate the functional roles and regulation of the yeast Rab5s. Using GFP-tagged cargoes, a novel quantitative multivesicular body (MVB) sorting assay, and electron microscopy, we show that MVB biogenesis and thus MVB cargo sorting is severely impaired in vps21Δ ypt52Δ double mutants. Ypt53, the third Rab5 paralog, is hardly expressed during normal growth but its transcription is strongly induced by cellular stress through the calcineurin-Crz1 pathway. The requirement for Rab5 activity in stress tolerance facilitated identification of Msb3/Gyp3 as the principal Rab5 GAP (GTPase accelerating protein). In vitro GAP assays verified that Vps21 is a preferred Gyp3 target. Moreover, we demonstrate that Gyp3 spatially restricts active Vps21 to intermediate endosomal compartments by preventing Vps21 accumulation on lysosomal vacuoles. Gyp3, therefore, operates as a compartmental insulator that helps to define the spatial domain of Vps21 signaling in the endolysosomal pathway.

Cdk1-dependent control of membrane-trafficking dynamics

Cyclin-dependent kinase 1 (Cdk1) is required for initiation and maintenance of polarized cell growth in budding yeast. Cdk1 activates Rho-family GTPases, which trigger polarization of the actin cytoskeleton for delivery of membrane to growth sites. It is found that Cdk1's function in polarized growth extends beyond that of actin organization.

Cyclin-dependent kinase 1 (Cdk1) is required for initiation and maintenance of polarized cell growth in budding yeast. Cdk1 activates Rho-family GTPases, which polarize the actin cytoskeleton for delivery of membrane to growth sites via the secretory pathway. Here we investigate whether Cdk1 plays additional roles in the initiation and maintenance of polarized cell growth. We find that inhibition of Cdk1 causes a cell surface growth defect that is as severe as that caused by actin depolymerization. However, unlike actin depolymerization, Cdk1 inhibition does not result in a massive accumulation of intracellular secretory vesicles or their cargoes. Analysis of post-Golgi vesicle dynamics after Cdk1 inhibition demonstrates that exocytic vesicles are rapidly mistargeted away from the growing bud, possibly to the endomembrane/vacuolar system. Inhibition of Cdk1 also causes defects in the organization of endocytic and exocytic zones at the site of growth. Cdk1 thus modulates membrane-trafficking dynamics, which is likely to play an important role in coordinating cell surface growth with cell cycle progression.

GTPase networks in membrane traffic

Members of the Rab or ARF/Sar branches of the Ras GTPase superfamily regulate almost every step of intracellular membrane traffic. A rapidly growing body of evidence indicates that these GTPases do not act as lone agents but are networked to one another through a variety of mechanisms to coordinate the individual events of one stage of transport and to link together the different stages of an entire transport pathway. These mechanisms include guanine nucleotide exchange factor (GEF) cascades, GTPase-activating protein (GAP) cascades, effectors that bind to multiple GTPases, and positive-feedback loops generated by exchange factor-effector interactions. Together these mechanisms can lead to an ordered series of transitions from one GTPase to the next. As each GTPase recruits a unique set of effectors, these transitions help to define changes in the functionality of the membrane compartments with which they are associated.

SNAREs, HOPS and regulatory lipids control the dynamics of vacuolar actin during homotypic fusion in S. cerevisiae

Homotypic vacuole fusion requires SNAREs, the Rab Ypt7p, the tethering complex HOPS, regulatory lipids and actin. In Saccharomyces cerevisiae, actin functions at two stages of vacuole fusion. Pre-existing actin filaments are depolymerized to allow docking and assembly of the vertex ring (a microdomain enriched in proteins and lipids that mediate fusion). Actin is then polymerized late in the pathway to aid fusion. Here, we report that the fusion machinery regulates the accumulation of actin at the vertex ring. Using Cy3-labeled yeast actin to track its dynamics, we found that its vertex enrichment was abolished when actin monomers were stabilized by latrunculin-B, independent of the extent of incorporation. By contrast, stabilization of filamentous actin with jasplakinolide markedly augmented actin vertex enrichment. Importantly, agents that inhibit SNAREs, Ypt7p and HOPS inhibited the vertex enrichment of actin, demonstrating that the cytoskeleton and the fusion machinery are interdependently regulated. Actin mobilization was also inhibited by ligating ergosterol and PtdIns(3)P, whereas the ligation or modification of PtdIns(4,5)P(2) augmented the vertex enrichment of actin. The proteins and lipids that regulated actin mobilization to the vertex did not affect the total incorporation of Cy3-actin, indicating that actin mobilization and polymerization activities can be dissociated during membrane fusion.

Functional analysis of RhoGDI inhibitory activity on vacuole membrane fusion

RhoGDIs (Rho GDP-dissociation inhibitors) are the natural inhibitors of Rho GTPases. They interfere with Rho protein function by either blocking upstream activation or association with downstream signalling molecules. RhoGDIs can also extract membrane-bound Rho GTPases to form soluble cytosolic complexes. We have shown previously that purified yeast RhoGDI Rdi1p, can inhibit vacuole membrane fusion in vitro. In the present paper we functionally dissect Rdi1p to discover its mode of regulating membrane fusion. Overexpression of Rdi1p in vivo profoundly affected cell morphology including increased actin patches in mother cells indicative of polarity defects, delayed ALP (alkaline phosphatase) sorting and the presence of highly fragmented vacuoles indicative of membrane fusion defects. These defects were not caused by the loss of typical transport and fusion proteins, but rather were linked to the reduction of membrane localization and activation of Cdc42p and Rho1p. Subcellular fractionation showed that Rdi1p is predominantly a cytosolic monomer, free of bound Rho GTPases. Overexpression of endogenous Rdi1p, or the addition of exogenous Rdi1p, generated stable cytosolic complexes. Rdi1p structure-function analysis showed that membrane association via the C-terminal β-sheet domain was required for the functional inhibition of membrane fusion. Furthermore, Rdi1p inhibited membrane fusion through the binding of Rho GTPases independent from its extraction activity.

Ezrin promotes actin assembly at the phagosome membrane and regulates phago-lysosomal fusion

Phagosome maturation is defined as the process by which phagosomes fuse sequentially with endosomes and lysosomes to acquire an acidic pH and hydrolases that degrade ingested particles. While the essential role of actin cytoskeleton remodeling during particle internalization is well established, its role during the later stages of phagosome maturation remains largely unknown. We have previously shown that purified mature phagosomes assemble F-actin at their membrane, and that the ezrin-radixin-moesin (ERM) proteins ezrin and moesin participate in this process. Moreover, we provided evidence that actin assembly on purified phagosomes stimulates their fusion with late endocytic compartments in vitro. In this study, we further investigated the role of ezrin in phagosome maturation. We engineered a structurally open form of ezrin and demonstrated that ezrin binds directly to the actin assembly promoting factor N-WASP (Neural Wiskott-Aldrich Syndrome Protein) by its FERM domain. Using a cell-free system, we found that ezrin stimulates F-actin assembly on purified phagosomes by recruiting the N-WASP-Arp2/3 machinery. Accordingly, we showed that the down-regulation of ezrin activity in macrophages by a dominant-negative approach caused reduced F-actin accumulation on maturing phagosomes. Furthermore, using fluorescence and electron microscopy, we found that ezrin is required for the efficient fusion between phagosomes and lysosomes. Live-cell imaging analysis supported the notion that ezrin is necessary for the fusogenic process itself, promoting the transfer of the lysosome content into the phagosomal lumen.

Rho GTPases and exocytosis: what are the molecular links?

Delivery of proteins or lipids to the plasma membrane or into the extracellular space occurs through exocytosis, a process that requires tethering, docking, priming and fusion of vesicles, as well as F-actin rearrangements in response to specific extracellular cues. GTPases of the Rho family have been implicated as important regulators of exocytosis, but how Rho proteins control this process is an open question. In this review, we focus on molecular connections that drive Rho-dependent exocytosis in polarized and regulated exocytosis. Specifically, we present data showing that Rho proteins interaction with the exocyst complex and IQGAP mediates polarized exocytosis, whereas interaction with actin-binding proteins like N-WASP mediates regulated exocytosis.

The Na+/H+ exchanger Nhx1p regulates the initiation of Saccharomyces cerevisiae vacuole fusion

Nhx1p is a Na+(K+)/H+ antiporter localized at the vacuolar membrane of the yeast Saccharomyces cerevisiae. Nhx1p regulates the acidification of cytosol and vacuole lumen, and is involved in membrane traffic from late endosomes to the vacuole. Deletion of the gene leads to aberrant vacuolar morphology and defective vacuolar protein sorting. These phenotypes are hallmarks of malfunctioning vacuole homeostasis and indicate that membrane fusion is probably altered. Here, we investigated the role of Nhx1p in the regulation of homotypic vacuole fusion. Vacuoles isolated from nhx1Δ yeast showed attenuated fusion. Assays configured to differentiate between the first round of fusion and ongoing rounds showed that nhx1Δ vacuoles were only defective in the first round of fusion, suggesting that Nhx1p regulates an early step in the pathway. Although fusion was impaired on nhx1Δ vacuoles, SNARE complex formation was indistinguishable from wild-type vacuoles. Fusion could be rescued by adding the soluble SNARE Vam7p. However, Vam7p only activated the first round of nhx1Δ vacuole fusion. Once fusion was initiated, nhx1Δ vacuoles appeared behave in a wild-type manner. Complementation studies showed that ion transport function was required for Nhx1p-mediated support of fusion. In addition, the weak base chloroquine restored nhx1Δ fusion to wild-type levels. Together, these data indicate that Nhx1p regulates the initiation of fusion by controlling vacuole lumen pH.

The platelet actin cytoskeleton associates with SNAREs and participates in alpha-granule secretion

Following platelet activation, platelets undergo a dramatic shape change mediated by the actin cytoskeleton and accompanied by secretion of granule contents. While the actin cytoskeleton is thought to influence platelet granule secretion, the mechanism for this putative regulation is not known. We found that disruption of the actin cytoskeleton by latrunculin A inhibited alpha-granule secretion induced by several different platelet agonists without significantly affecting activation-induced platelet aggregation. In a cell-free secretory system, platelet cytosol was required for alpha-granule secretion. Inhibition of actin polymerization prevented alpha-granule secretion in this system, and purified platelet actin could substitute for platelet cytosol to support alpha-granule secretion. To determine whether SNAREs physically associate with the actin cytoskeleton, we isolated the Triton X-100 insoluble actin cytoskeleton from platelets. VAMP-8 and syntaxin-2 associated only with actin cytoskeletons of activated platelets. Syntaxin-4 and SNAP-23 associated with cytoskeletons isolated from either resting or activated platelets. When syntaxin-4 and SNAP-23 were tested for actin binding in a purified protein system, only syntaxin-4 associated directly with polymerized platelet actin. These data show that the platelet cytoskeleton interacts with select SNAREs and that actin polymerization facilitates alpha-granule release.

Natamycin inhibits vacuole fusion at the priming phase via a specific interaction with ergosterol

The antifungal antibiotic natamycin belongs to the family of polyene antibiotics. Its antifungal activity arises via a specific interaction with ergosterol in the plasma membrane (te Welscher et al., J. Biol. Chem. 283:6393-6401, 2008). However, this activity does not involve disruption of the membrane barrier function, a well-known property of other members of the polyene antibiotic family, such as filipin and nystatin. Here we tested the effect of natamycin on vacuole membrane fusion, which is known to be ergosterol dependent. Natamycin blocked the fusion of isolated vacuoles without compromising the barrier function of the vacuolar membrane. Sublethal doses of natamycin perturbed the cellular vacuole morphology, causing the formation of many more small vacuolar structures in yeast cells. Using vacuoles isolated from yeast strains deficient in the ergosterol biosynthesis pathway, we showed that the inhibitory activity of natamycin was dependent on the presence of specific chemical features in the structure of ergosterol that allow the binding of natamycin. We found that natamycin inhibited the priming stage of vacuole fusion. Similar results were obtained with nystatin. These results suggest a novel mode of action of natamycin and perhaps all polyene antibiotics, which involves the impairment of membrane fusion via perturbation of ergosterol-dependent priming reactions that precede membrane fusion, and they may point to an effect of natamycin on ergosterol-dependent protein function in general.

Cdc42p is activated during vacuole membrane fusion in a sterol-dependent subreaction of priming

Cdc42p is a Rho GTPase that initiates signaling cascades at spatially defined intracellular sites for many cellular functions. We have previously shown that Cdc42p is localized to the yeast vacuole where it initiates actin polymerization during membrane fusion. Here we examine the activation cycle of Cdc42p during vacuole membrane fusion. Expression of either GTP- or GDP-locked Cdc42p mutants caused several morphological defects including enlarged cells and fragmented vacuoles. Stimulation of multiple rounds of fusion enhanced vacuole fragmentation, suggesting that cycles of Cdc42p activation, involving rounds of GTP binding and hydrolysis, are required to propagate Cdc42p signaling. We developed an assay to directly examine Cdc42p activation based on affinity to a probe derived from the p21-activated kinase effector, Ste20p. Cdc42p was rapidly activated during vacuole membrane fusion, which kinetically coincided with priming subreaction. During priming, Sec18p ATPase activity dissociates SNARE complexes and releases Sec17p, however, priming inhibitors such as Sec17p and Sec18p ligands did not block Cdc42p activation. Therefore, Cdc42p activation seems to be a parallel subreaction of priming, distinct from Sec18p activity. Specific mutants in the ergosterol synthesis pathway block both Sec17p release and Cdc42p activation. Taken together, our results define a novel sterol-dependent subreaction of vacuole priming that activates cycles of Cdc42p activity to facilitate membrane fusion.

Unraveling a novel Rac1-mediated signaling pathway that regulates cofilin dephosphorylation and secretion in thrombin-stimulated platelets

In platelets stimulated by thrombin to secrete and aggregate, cofilin is rapidly dephosphorylated leading to its activation. Cofilin by severing existing actin filaments and stimulating F-actin polymerization on newly created barbed ends dynamizes the actin cytoskeleton. We previously found that cofilin dephosphorylation is Ca2+-dependent and occurs upstream of degranulation in stimulated platelets. We report now in thrombin-stimulated platelets that Rac1 and class II PAKs (PAK4/5/6) were rapidly (within 5 seconds) activated, whereas PAK1/2 (class I PAKs) phosphorylation was slower. The Rac1-specific inhibitor NSC23766 blocked phosphorylation of class II PAKs, but not PAK1/2. Moreover, inhibition of the Ca2+/calmodulin-dependent phosphatase calcineurin inhibited Rac1 activation and class II PAKs phosphorylation. Prevention of Rac1 activation by calcineurin inhibition or NSC23766 also blocked cofilin dephosphorylation and platelet granule secretion indicating that a calcineurin/Rac1/class II PAKs pathway regulates cofilin dephosphorylation leading to secretion. We further found that PI3-kinases were activated downstream of Rac1, but were not involved in regulating cofilin dephosphorylation and secretion in thrombin-stimulated platelets. Our study unravels a Ca2+-dependent pathway of secretion in stimulated platelets as a signaling pathway linking Rac1 activation to actin dynamics: calcineurin→Rac1→class II PAKs→cofilin activation. We further demonstrate that this pathway is separate and independent of the protein kinase C (PKC) pathway mediating secretion.

Primary granule exocytosis in human neutrophils is regulated by Rac-dependent actin remodeling

The actin cytoskeleton regulates exocytosis in all secretory cells. In neutrophils, Rac2 GTPase has been shown to control primary (azurophilic) granule exocytosis. In this report, we propose that Rac2 is required for actin cytoskeletal remodeling to promote primary granule exocytosis. Treatment of neutrophils with low doses (< or = 10 microM) of the actin-depolymerizing drugs latrunculin B (Lat B) or cytochalasin B (CB) enhanced both formyl peptide receptor- and Ca(2+) ionophore-stimulated exocytosis. Higher concentrations of CB or Lat B, or stabilization of F-actin with jasplakinolide (JP), inhibited primary granule exocytosis measured as myeloperoxidase release but did not affect secondary granule exocytosis determined by lactoferrin release. These results suggest an obligatory role for F-actin disassembly before primary granule exocytosis. However, lysates from secretagogue-stimulated neutrophils showed enhanced actin polymerization activity in vitro. Microscopic analysis showed that resting neutrophils contain significant cortical F-actin, which was redistributed to sites of primary granule translocation when stimulated. Exocytosis and actin remodeling was highly polarized when cells were primed with CB; however, polarization was reduced by Lat B preincubation, and both polarization and exocytosis were blocked when F-actin was stabilized with JP. Treatment of cells with the small molecule Rac inhibitor NSC23766 also inhibited actin remodeling and primary granule exocytosis induced by Lat B/fMLF or CB/fMLF, but not by Ca(2+) ionophore. Therefore, we propose a role for F-actin depolymerization at the cell cortex coupled with Rac-dependent F-actin polymerization in the cell cytoplasm to promote primary granule exocytosis.

Swf1p, a member of the DHHC-CRD family of palmitoyltransferases, regulates the actin cytoskeleton and polarized secretion independently of its DHHC motif

Rho and Rab family GTPases play a key role in cytoskeletal organization and vesicular trafficking, but the exact mechanisms by which these GTPases regulate polarized cell growth are incompletely understood. A previous screen for genes that interact with CDC42, which encodes a Rho GTPase, found SWF1/PSL10. Here, we show Swf1p, a member of the DHHC-CRD family of palmitoyltransferases, localizes to actin cables and cortical actin patches in Saccharomyces cerevisiae. Deletion of SWF1 results in misorganization of the actin cytoskeleton and decreased stability of actin filaments in vivo. Cdc42p localization depends upon Swf1p primarily after bud emergence. Importantly, we revealed that the actin regulating activity of Swf1p is independent of its DHHC motif. A swf1 mutant, in which alanine substituted for the cysteine required for the palmitoylation activity of DHHC-CRD proteins, displayed wild-type actin organization and Cdc42p localization. Bgl2p-marked exocytosis was found wild type in this mutant, although invertase secretion was impaired. These data indicate Swf1p has at least two distinct functions, one of which regulates actin organization and Bgl2p-marked secretion. This report is the first to link the function of a DHHC-CRD protein to Cdc42p and the regulation of the actin cytoskeleton.

Actin in the endocytic pathway: from yeast to mammals

Genetic analysis of endocytosis in yeast early pointed to the essential role of actin in the uptake step. Efforts to identify the machinery involved demonstrated the important contribution of Arp2/3 and the myosins-I. Analysis of the process using live-cell fluorescence microscopy and electron microscopy have recently contributed to refine molecular models explaining clathrin and actin-dependent endocytic uptake. Increasing evidence now also indicates that actin plays important roles in post-internalization events along the endocytic pathway in yeast, including transport of vesicles, motility of endosomes and vacuole fusion. This review describes the present knowledge state on the roles of actin in endocytosis in yeast and points to similarities and differences with analogous processes in mammals.

Filamentous actin regulates insulin exocytosis through direct interaction with Syntaxin 4

Glucose-induced insulin exocytosis is coupled to associations between F-actin and SNARE proteins, although the nature and function of these interactions remains unknown. Toward this end we show here that both Syntaxin 1A and Syntaxin 4 associated with F-actin in MIN6 cells and that each interaction was rapidly and transiently diminished by stimulation of cells with d-glucose. Of the two isoforms, only Syntaxin 4 was capable of interacting directly with F-actin in an in vitro sedimentation assay, conferred by the N-terminal 39-112 residues of Syntaxin 4. The 39-112 fragment was capable of selective competitive inhibitory action, disrupting endogenous F-actin-Syntaxin 4 binding in MIN6 cells. Disruption of F-actin-Syntaxin 4 binding correlated with enhanced glucose-stimulated insulin secretion, mediated by increased granule accumulation at the plasma membrane and increased Syntaxin 4 accessibility under basal conditions. However, no increase in basal level Syntaxin 4-VAMP2 association occurred with either latrunculin treatment or expression of the 39-112 fragment. Taken together, these data disclose a new underlying mechanism by which F-actin negatively regulates exocytosis via binding and blocking Syntaxin 4 accessibility, but they also reveal the existence of additional signals and/or steps required to trigger the subsequent docking and fusion steps of exocytosis.