The Saccharomyces cerevisiae Rheb G-protein is involved in regulating canavanine resistance and arginine uptake

The new member of the Ras superfamily of G-proteins, Rheb, has been identif ied in rat and human, but its function has not been defined. We report here the identification of Rheb homologues in the budding yeast Saccharomyces c erevisiae (ScRheb) as well as in Schizosaccharomyces pombe, Drosophila mela nogaster, zebrafish, and Ciona intestinalis, These proteins define a new cl ass of G-proteins based on 1) their overall sequence similarity, 2) high co nservation of their effector domain sequence, 3) presence of a unique argin ine in their G1 box, and 4) presence of a conserved CAAX farnesylation moti f. Characterization of an S. cerevisiae strain deficient in ScRheb showed t hat it is hypersensitive to growth inhibitory effects of canavanine and thi alysine, which are analogues of arginine and lysine, respectively. Accordin gly, the uptake of arginine and lysine was increased in the ScRheb-deficien t strain. This increased arginine uptake requires the arginine-specific per mease Can1p. The function of ScRheb is dependent on having an intact effect or domain since mutations in the effector domain of ScRheb are incapable of complementing canavanine hypersensitivity of scrheb disruptant cells. Furt hermore, the conserved arginine in the G1 box plays a role in the activity of ScRheb, as a mutation of this arginine to glycine significantly reduced the ability of ScRheb to complement canavanine hypersensitivity of ScRheb-d eficient yeast. Finally, a mutation in the C-terminal CAAX farnesylation mo tif resulted in a loss of ScRheb function. This result, in combination with our finding that ScRheb is farnesylated, suggests that farnesylation plays a key role in ScRheb function. Our findings assign the regulation of argin ine and lysine uptake as the first physiological function for this new farn esylated Ras superfamily G-protein.