Mutations in the PMA1 gene, encoding plasma membrane H + -ATPase, were isolated that are able to suppress the temperature sensitivity (ts) phenotype of mdp1 mutations located in RSP5 , the ubiquitin-protein ligase gene. The mdp1 mutants were previously found to change the mitochondrial/cytosolic distribution of Mod5p-I, the tRNA modifying enzyme, and to affect fluid phase endocytosis. The data presented reveal that mdp1 mutants are also pH sensitive, and hypersensitive to hygromycin B and paromomycin. The ts phenotype, hygromycin B and paromomycin sensitivity are suppressed by pma1-t , but the pH sensitivity, the effect of mdp1 on Mod5p-I cytoplasmic/mitochondrial localization and endocytosis are not. Characterization of pma1-t revealed the substitution of amino acid G(653)V in the ATP-binding domain of the H + -ATPase. Our results indicate that Rsp5 ubiquitin-protein ligase may also influence, in addition to protein distribution, the functioning of plasma membrane H + -ATPase and the response of cells to stress.
