Rom1p and Rom2p are GDP/GTP exchange proteins (GEPs) for the Rho1p small GTP binding protein in Saccharomyces cerevisiae

The RNO1 gene encodes a homolog of the mammalian RhoA small GTP binding protein in the yeast Saccharomyces cerevisiae. Rho1p is localized at the growth site and is required for bud formation. Multicopy suppressors of a temperature-sensitive, dominant negative mutant allele of RHO1, RHO1 (G22S, D125N), were isolated and named ROM (RHO1 multicopy suppressor). Rom1p and Rom2p were found to contain a DH (Dbl homologous) domain and a PH (pleckstrin homologous) domain, both of which are conserved among the GDP/GTP exchange proteins (GEPs) for the Rho family small GTP binding proteins, Disruption of ROM2 resulted in a temperature-sensitiv:e growth phenotype, whereas disruption of both ROM1 and ROM2 resulted in lethality. The phenotypes of Delta rom1 Delta rom2 cells were similar to those of Delta rho1 cells, including growth arrest with a small bud and cell lysis, Moreover, the temperature-sensitive growth phenotype of Delta rom2 was suppressed by overexpression of RHO1 or RHO2, but not of CDC42. The glutathione-S-transferase (GST) fusion protein containing the DH domain of Rom2p showed the lipid-modified Rho1p-specific GDP/GTP exchange activity which was sensitive to Rho GDP dissociation inhibitor, These results indicate that Rom1p and Rom2p are GEPs that activate Rho1p in S.cerevisiae.