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The Rho GTPase-activating protein Rgd1p positively regulates the GTPase activity of Rho3p and Rho4p, which are involved in bud growth and cytokinesis, respectively, in the budding yeast Saccharomyces cerevisiae. Two-hybrid screening identified Tos2p as a candidate Rgd1p-binding protein. Further analyses confirmed that Tos2p binds to the RhoGAP Rgd1p through its C-terminal region. Both Tos2p and Rgd1p are localized to polarized growth sites during the cell cycle and associated with detergent-resistant membranes. We observed that TOS2 overexpression suppressed rgd1Δ sensitivity to a low pH. In the tos2Δ strain, the amount of GTP-bound Rho3p was increased, suggesting an influence of Tos2p on Rgd1p activity in vivo. We also showed a functional interaction between the TOS2 and the RHO3 genes: TOS2 overexpression partially suppressed the growth defect of rho3-V51 cells at a restrictive temperature. We propose that Tos2p, a protein involved in polarized growth and most probably associated with the plasma membrane, modulates the action of Rgd1p and Rho3p in S. cerevisiae. [ABSTRACT FROM AUTHOR]

Abstract: Rho proteins are essential regulators of polarized growth in eukaryotic cells. These proteins are down-regulated in vivo by specific Rho GTPase Activating Proteins (RhoGAP). We investigated the role of Rgd1 RhoGAP, encoded by the Candida albicans RGD1 gene. We demonstrated that CaCdc42, CaRho3 and CaRho4 proteins had an intrinsic GTPase activity and that CaRgd1 stimulates in vitro GTP hydrolysis of these GTPases. Deletion of RGD1 in C. albicans results in sensitivity to low pH as already described for rgd1Δ in Saccharomyces cerevisiae. The role of Rgd1 in survival at low pH is conserved in the two yeast species as the CaRGD1 gene complements the Scrgd1Δ sensitivity. By tagging the RhoGAP with GFP, we found that CaRgd1 is localized at the tip and cortex of growing cells and during cytokinesis at the septation sites in yeast and filamentous forms. We investigated the effect of CaRgd1 on the control of the polarized growth. Removing CaRGD1 alleles increased filamentous growth and cells lacking CaRgd1 presented longer germ tubes. Conversely, RGD1 overexpression restricted hyphae growth. Our results demonstrate that Rgd1 is critical for filamentous formation in C. albicans especially for filamentous elongation. [Copyright &y& Elsevier]