First characterization of the gene RGD1 in the yeast Saccharomyces cerevisiae

Putative RhoGAP proteins orchestrate vegetative growth, conidiogenesis and pathogenicity of the rice blast fungus Magnaporthe oryzae

Rho GTPases, acting as molecular switches, are involved in the regulation of diverse cellular functions. Rho GTPase activating proteins (Rho GAPs) function as negative regulators of Rho GTPases and are required for a variety of signaling processes in cell development. But the mechanisms underlying Rho GAPs in Rho-mediated signaling pathways in fungi are still elusive. There are eight RhoGAP domain-containing genes annotated in the Magnaporthe oryzae genome. To understand the function of these RhoGAP genes, we generated knockout mutants of each of the RhoGAP genes through a homologous recombination-based method. Phenotypic analysis showed that growth rate of aerial hyphae of the Molrg1 deletion mutant decreased dramatically. The ΔMolrg1 mutant showed significantly reduced conidiation and appressorium formation by germ tubes. Moreover, it lost pathogenicity completely. Deletion of another Rho GAP (MoRga1) resulted in high percentage of larger or gherkin-shaped conidia and slight decrease in conidiation. Appressorial formation of the ΔMoRga1 mutant was delayed significantly on hydrophobic surface, while the development of mycelial growth and pathogenicity in plants was not affected. Confocal fluorescence microscopy imaging showed that MoRga1-GFP localizes to septal pore of the conidium, and this localization pattern requires both LIM and RhoGAP domains. Furthermore, either deleting the LIM or RhoGAP domain or introducing an inactivating R1032A mutation in the RhoGAP domain of MoRga1 caused similar defects as the Morga1 deletion mutant in terms of conidial morphology and appressorial formation, suggesting that MoRga1 is a stage-specific regulator of conidial differentiation by regulating some specific Rho GTPases. In this regard, MoRga1 and MoLrg1 physically interacted with both MoRac1-CA and MoCdc42-CA in the yeast two-hybrid and pull-down assays, suggesting that the actions of these two GAPs are involved in MoRac1 and MoCdc42 pathways. On the other hand, six other putative Rho GAPs (MoRga2 to MoRga7) were dispensable for conidiation, vegetative growth, appressorial formation and pathogenicity, suggesting that these Rho GAPs function redundantly during fungal development. Taking together, Rho GAP genes play important roles in M. oryzae development and infectious processes through coordination and modulation of Rho GTPases.

Mechanisms of membrane deformation by lipid-binding domains

Among an increasing number of lipid-binding domains, a group that not only binds to membrane lipids but also changes the shape of the membrane has been found. These domains are characterized by their strong ability to transform globular liposomes as well as flat plasma membranes into elongated membrane tubules both in vitro and in vivo. Biochemical studies on the structures of these proteins have revealed the importance of the amphipathic helix, which potentially intercalates into the lipid bilayer to induce and/or sense membrane curvature. Among such membrane-deforming domains, BAR and F-BAR/EFC domains form crescent-shaped dimers, suggesting a preference for a curved membrane, which is important for curvature sensing. Bioinformatics in combination with structural analyses has been identifying an increasing number of novel families of lipid-binding domains. This review attempts to summarize the evidence obtained by recent studies in order to gain general insights into the roles of membrane-deforming domains in a variety of biological events.

Rga2 is a Rho2 GAP that regulates morphogenesis and cell integrity in S. pombe

Schizosaccharomyces pombe Rho2 GTPase regulates alpha-D-glucan synthesis and acts upstream of Pck2 to activate the MAP kinase pathway for cell integrity. However, little is known about its regulation. Here we describe Rga2 as a Rho2 GTPase-activating protein (GAP) that regulates cell morphology. rga2+ gene is not essential for growth but its deletion causes longer and thinner cells whereas rga2+ overexpression causes shorter and broader cells. rga2+ overexpression also causes abnormal accumulation of Calcofluor-stained material and cell lysis, suggesting that it also participates in cell wall integrity. Rga2 localizes to growth tips and septum region. The N-terminal region of the protein is required for its correct localization whereas the PH domain is necessary exclusively for Rga2 localization to the division area. Also, Rga2 localization depends on polarity markers and on actin polymerization. Rga2 interacts with Rho2 and possesses in vitro and in vivo GAP activity for this GTPase. Accordingly, rga2Delta cells contain more alpha-D-glucan and therefore partially suppress the thermosensitivity of mok1-664 cells, which have a defective alpha-D-glucan synthase. Additionally, genetic interactions and biochemical analysis suggest that Rga2 regulates Rho2-Pck2 interaction and might participate in the regulation of the MAPK cell integrity pathway.

Dictyostelium MEGAPs: F-BAR domain proteins that regulate motility and membrane tubulation in contractile vacuoles

PCH family proteins are fundamentally important proteins, linking membrane curvature events with cytoskeletal reorganisation. One group, the MEGAPs (also called srGAPs and WRPs) contain RhoGAP domains in addition to the F-BAR domain. We disrupted MEGAP1 and MEGAP2 in Dictyostelium both singly and in combination. We found a strong cytoskeletal phenotype in MEGAP1– cells and a subtle phototaxis defect in MEGAP2– slugs. MEGAP1–/2– cells have an overabundance of filopodia and slug motility and function are affected. The most dramatic changes, however, are on contractile vacuoles. MEGAP1–/2– cells empty their contractile vacuoles less efficiently than normal and consequently have three times the usual number. GFP-tagged MEGAP1 localises to tubules of the contractile vacuole network and when vacuoles start to empty they recruit cytosolic GFP-MEGAP1. Mutants in the Saccharomyces homologues RGD1 and RGD2 also show abnormal vacuoles, implying that this role is conserved. Thus, MEGAP is an important regulator of the contractile vacuole network, and we propose that tubulation of the contractile vacuole by MEGAP1 represents a novel mechanism for driving vacuole emptying.

Roles of F-BAR/PCH proteins in the regulation of membrane dynamics and actin reorganization

The Pombe Cdc15 Homology (PCH) proteins have emerged in many species as important coordinators of signaling pathways that regulate actomyosin assembly and membrane dynamics. The hallmark of the PCH proteins is the presence of a Fes/CIP4 homology-Bin/Amphiphysin/Rvsp (F-BAR) domain; therefore they are commonly referred to as F-BAR proteins. The prototype F-BAR protein, Cdc15p of Schizosaccharomyces pombe, has a role in the formation of the contractile actomyosin ring during cytokinesis. Vertebrate F-BAR proteins have an established role in binding phospholipids and they participate in membrane deformations, for instance, during the internalization of transmembrane receptors. This way the F-BAR proteins will function as linkers between the actin polymerization apparatus and the machinery regulating membrane dynamics. Interestingly, some members of the F-BAR proteins are implicated in inflammatory or neurodegenerative disorders and the observations can be expected to have clinical implications for the treatment of the diseases.

The Rho3 and Rho4 small GTPases interact functionally with Wsc1p, a cell surface sensor of the protein kinase C cell-integrity pathway in Saccharomyces cerevisiae

Rgd1, a GTPase-activating protein, is the only known negative regulator of the Rho3 and Rho4 small GTPases in the yeastSaccharomyces cerevisiae. Rho3p and Rho4p are involved in regulating cell polarity by controlling polarized exocytosis. Co-inactivation ofRGD1andWSC1, which is a cell wall sensor-encoding gene, is lethal. Another plasma membrane sensor, Mid2p, is known to rescue thergd1Δwsc1Δ synthetic lethality. It has been proposed that Wsc1p and Mid2p act upstream of the protein kinase C (PKC) pathway to function as mechanosensors of cell wall stress. Analysis of the synthetic lethal phenomenon revealed that production of activated Rho3p and Rho4p leads to lethality inwsc1Δ cells. Inactivation ofRHO3orRHO4was able to rescue thergd1Δwsc1Δ synthetic lethality, supporting the idea that the accumulation of GTP-bound Rho proteins, following loss of Rgd1p, is detrimental if the Wsc1 sensor is absent. In contrast, the genetic interaction betweenRGD1andMID2was not due to an accumulation of GTP-bound Rho proteins. It was proposed that simultaneous inactivation ofRGD1andWSC1constitutively activates the PKC–mitogen-activated protein kinase (MAP kinase) pathway. Moreover, it was shown that the activity of this pathway was not involved in the synthetic lethal interaction, which suggests the existence of another mechanism. Consistent with this idea, it was found that perturbations in Rho3-mediated polarized exocytosis specifically impair the abundance and processing of Wsc1 and Mid2 proteins. Hence, it is proposed that Wsc1p participates in the regulation of a Rho3/4-dependent cellular mechanism, and that this is distinct from the role of Wsc1p in the PKC–MAP kinase pathway.

RGD1, encoding a RhoGAP involved in low-pH survival, is an Msn2p/Msn4p regulated gene in Saccharomyces cerevisiae

The RhoGAP Rgd1p is involved in different signal transduction pathways in Saccharomyces cerevisiae through its regulatory activity upon the Rho3 and Rho4 GTPases. The rgd1Delta mutant, which presents a mortality at the entry into the stationary phase in minimal medium, is sensitive to medium acidification caused by biomass augmentation. We showed that low-pH shock leads to abnormal intracellular acidification of the rgd1Delta mutant. Transcriptional regulation of RGD1 was studied in several stress conditions and we observed an activation of RGD1 transcription at low pH and after heat and oxidative shocks. The transcription level at low pH and after heat shock was demonstrated to depend on the STRE box located in the RGD1 promoter. The general stress-activated transcription factors Msn2p and Msn4p as well as the HOG pathway were shown to mainly act on the basal RGD1 transcriptional level in normal and stress conditions.

Glucose and trehalose PTS permeases of Spiroplasma citri probably share a single IIA domain, enabling the spiroplasma to adapt quickly to carbohydrate changes in its environment

Spiroplasma citri is a plant-pathogenic mollicute phylogenetically related to Gram-positive bacteria. Spiroplasma cells are restricted to the phloem sieve tubes and are transmitted from plant to plant by the leafhopper vector Circulifer haematoceps. In the plant sieve tubes, S. citri grows on glucose and fructose, whereas in the leafhopper haemolymph the spiroplasma must grow on trehalose, the major sugar in insects. Previous studies in this laboratory have shown that fructose utilization was a key factor of spiroplasmal pathogenicity. To further study the implication of sugar metabolism in the interactions of S. citri with its plant host and its leafhopper vector, genes encoding permease enzymes II (EII(Glc) and EII(Tre)) of the S. citri phosphoenolpyruvate : glucose and phosphoenolpyruvate : trehalose phosphotransferase systems (PTS) were characterized. Mapping studies revealed that the EII(Glc) complex was split into two distinct polypeptides, IIA(Glc) and IICB(Glc), encoded by two separate genes, crr and ptsG, respectively. As expected, S. citri polypeptides IIA(Glc) and IICB(Glc) were more phylogenetically related to their counterparts from Gram-positive than to those from Gram-negative bacteria. The trehalose operon consisted of three genes treR, treP and treA, encoding a transcriptional regulator, the PTS permease (EII(Tre)) and the amylase, respectively. However, in contrast to the fructose-PTS permease, which is encoded as a single polypeptide (IIABC(Fru)) containing the three domains A, B and C, the trehalose-PTS permease (IIBC(Tre)) lacks its own IIA domain. No trehalose-specific IIA could be identified in the spiroplasmal genome, suggesting that the IIBC(Tre) permease probably functions with the IIA(Glc) domain. In agreement with this statement, yeast two-hybrid system experiments revealed that the IIA(Glc) domain interacted not only with IIB(Glc) but also with the IIB(Tre) domain. The results are discussed with respect to the ability of the spiroplasma to adapt from the phloem sap of the host plant to the haemolymph and salivary gland cells of the insect vector.

GTPase-activating proteins for Cdc42

The Rho-type GTPase, Cdc42, has been implicated in a variety of functions in the yeast life cycle, including septin organization for cytokinesis, pheromone response, and haploid invasive growth. A group of proteins called GTPase-activating proteins (GAPs) catalyze the hydrolysis of GTP to GDP, thereby inactivating Cdc42. At the time this study began, there was one known GAP, Bem3, and one putative GAP, Rga1, for Cdc42. We identified another putative GAP for Cdc42 and named it Rga2 (Rho GTPase-activating protein 2). We confirmed by genetic and biochemical criteria that Rga1, Rga2, and Bem3 act as GAPs for Cdc42. A detailed characterization of Rga1, Rga2, and Bem3 suggested that they regulate different subsets of Cdc42 function. In particular, deletion of the individual GAPs conferred different phenotypes. For example, deletion of RGA1, but not RGA2 or BEM3, caused hyperinvasive growth. Furthermore, overproduction or loss of Rga1 and Rga2, but not Bem3, affected the two-hybrid interaction of Cdc42 with Ste20, a p21-activated kinase (PAK) kinase required for haploid invasive growth. These results suggest Rga1, and possibly Rga2, facilitate the interaction of Cdc42 with Ste20 to mediate signaling in the haploid invasive growth pathway. Deletion of BEM3 resulted in cells with severe morphological defects not observed in rga1delta or rga2delta strains. These data suggest that Bem3 and, to a lesser extent, Rga1 and Rga2 facilitate the role of Cdc42 in septin organization. Thus, it appears that the GAPs play a role in modulating specific aspects of Cdc42 function. Alternatively, the different phenotypes could reflect quantitative rather than qualitative differences in GAP activity in the mutant strains.

Overactivation of the protein kinase C-signaling pathway suppresses the defects of cells lacking the Rho3/Rho4-GAP Rgd1p in Saccharomyces cerevisiae

The nonessential RGD1 gene encodes a Rho-GTPase activating protein for the Rho3 and Rho4 proteins in Saccharomyces cerevisiae. Previous studies have revealed genetic interactions between RGD1 and the SLG1 and MID2 genes, encoding two putative sensors for cell integrity signaling, and VRP1 encoding an actin and myosin interacting protein involved in polarized growth. To better understand the role of Rgd1p, we isolated multicopy suppressor genes of the cell lethality of the double mutant rgd1Delta mid2Delta. RHO1 and RHO2 encoding two small GTPases, MKK1 encoding one of the MAP-kinase kinases in the protein kinase C (PKC) pathway, and MTL1, a MID2-homolog, were shown to suppress the rgd1Delta defects strengthening the functional links between RGD1 and the cell integrity pathway. Study of the transcriptional activity of Rlm1p, which is under the control of Mpk1p, the last kinase of the PKC pathway, and follow-up of the PST1 transcription, which is positively regulated by Rlm1p, indicate that the lack of RGD1 function diminishes the PKC pathway activity. We hypothesize that the rgd1Delta inactivation, at least through the hyperactivation of the small GTPases Rho3p and Rho4p, alters the secretory pathway and/or the actin cytoskeleton and decreases activity of the PKC pathway.

Functional characterization of the Bag7, Lrg1 and Rgd2 RhoGAP proteins from Saccharomyces cerevisiae

Rho proteins are down-regulated in vivo by specific GTPase activating proteins (RhoGAP). We have functionally studied three Saccharomyces cerevisiae putative RhoGAP. By first identifying Rho partners with a systematic two-hybrid approach and then using an in vitro assay, we have demonstrated that the Bag7 protein stimulated the GTPase activity of the Rho1 protein, Lrg1p acted on the Cdc42 and Rho2 GTPases and we showed that Rgd2p has a GAP activity on both Cdc42p and Rho5p. In addition, we brought the first evidence for the existence of a sixth functional Rho in yeast, the Cdc42/Rac-like GTPase Rho5.

Evidence for the genetic interaction between the actin-binding protein Vrp1 and the RhoGAP Rgd1 mediated through Rho3p and Rho4p in Saccharomyces cerevisiae

The non-essential RGD1 gene from Saccharomyces cerevisiae encodes a protein that has been characterized in vitro as a Rho GTPase activating protein (RhoGAP) for the Rho3 and Rho4 proteins. Rgd1p, which displays a conserved FCH-coiled coil-Rho-GAP domain organization, showed a patch-like distribution in the cell, including a localization in growing buds. Using a genetic screen, we found that rgd1delta and vrp1alpha mutations exhibited a synthetic lethality, thus revealing an interaction between these genes. The VRP1 product is an actin and myosin interacting protein involved in polarized growth. Using mutant forms of both Rho3 and Rho4 proteins, we provide evidence for the involvement of these two GTPases in RGD1-VRP1 co-lethality. In addition, these results strongly argue in favour of Rho3p and Rho4p being the targets of Rgd1p RhoGAP activity in vivo. Genetic relationships between either VRP1 or RGD1 and actin cytoskeleton-linked genes were also studied. These and other well-established data support the idea that Vrp1, Las17, Rvs167 proteins belong to the same complex. This protein structure might act with myosins in various actin cytoskeleton-based activities, in co-operation with a Rho3p/Rho4p signalling pathway that is negatively regulated by Rgd1p GAP activity.

RGD1 genetically interacts with MID2 and SLG1, encoding two putative sensors for cell integrity signalling in Saccharomyces cerevisiae

The RGD1 gene was identified during systematic genome sequencing of Saccharomyces cerevisiae. To further understand Rgd1p function, we set up a synthetic lethal screen for genes interacting with RGD1. Study of one lethal mutant made it possible to identify the SLG1 and MID2 genes. The gene SLG1/HCS77/WSC1 was mutated in the original synthetic lethal strain, whereas MID2/SMS1 acted as a monocopy suppressor. The SLG1 gene has been described to be an upstream component in the yeast PKC pathway and encodes a putative cell surface sensor for the activation of cell integrity signalling. First identified by viability loss of shmoos after pheromone exposure, and since found in different genetic screens, MID2 was recently reported as also encoding an upstream activator of the PKC pathway. The RGD1 gene showed genetic interactions with both sensors of cell integrity pathway. The rgd1 slg1 synthetic lethality was rescued by osmotic stabilization, as expected for mutants altered in cell wall integrity. The slight viability defect of rgd1 in minimal medium, which was exacerbated by mid2, was not osmoremediated. As for mutants altered in PKC pathway, the accumulation of small-budded dead cells in slg1, rgd1 and mid2 after heat shock was prevented by 1 M sorbitol. In addition, the rgd1 strain also displayed dead shmoos after pheromone treatment, like mid2. Taken together, the present results indicate close functional links between RGD1, MID2 and SLG1 and suggest that RGD1 and MID2 interact in a cell integrity signalling functionally linked to the PKC pathway.

The yeast Rgd1p is a GTPase activating protein of the Rho3 and rho4 proteins

The RGD1 gene, identified during sequencing of the Saccharomyces cerevisiae genome, encodes a protein with a Rho-GTPase activating protein (GAP) domain at the carboxy-terminal end. The Rgd1 protein showed two-hybrid interactions with the activated forms of Rho2p, Rho3p and Rho4p. Using in vitro assays, we demonstrated that Rgd1p stimulated the GTPase activity of both Rho3p and Rho4p; no stimulation was observed on Rho2p. In addition, the rho3Deltargd1Delta double mutant exhibited a dramatic growth defect compared to the single mutants, suggesting that Rgd1p has a GAP activity in vivo. The present study allowed the identification of the first GAP of Rho3p and Rho4p.