Failure to farnesylate Rheb protein contributes to the enrichment of G0/G1phase cells in the Schizosaccharomyces pombe farnesyltransferase mutant

Protein farnesylation is important for a number of physiological processes, including proliferation and cell morphology. The Schizosaccharomyces pombe mutant, cpp1(-), defective in farnesylation, exhibits distinct phenotypes, including morphological changes and sensitivity to the arginine analogue, canavanine. In this work, we report a novel phenotype of this mutant, enric hment of G0/G1 phase cells. This phenotype results mainly from the inabilit y to farnesylate the Rheb G-protein, as normal cell cycle progression can b e restored to the mutant by expressing a mutant form of SpRheb (SpRheb-CVIL ) that can bypass farnesylation. In contrast, a farnesylation-defective mut ant of SpRheb (SpRheb-SVIA) is incapable of restoring the normal cell cycle profile to the cpp1(-) mutant. Inhibition of SpRheb expression leads to th e accumulation of cells at the G0/G1 phase of the cell cycle. This growth a rrest phenotype of the sprheb(-) disruption can be complemented by the intr oduction of wild-type sprheb(+). The complementation is dependent on farnes ylation, as the farnesylation-defective SpRheb-SVIA mutant is incapable of complementing the sprheb(-) disruption. Other mutants of SpRheb, E40K and S 20N, are also incapable of complementing the sprheb(-) disruption. Furtherm ore, efficient complementation can be obtained by the expression of human R heb but not Saccharomyces cerevisiae Rheb. Our findings suggest that protei n farnesylation is important for cell cycle progression of S. pombe cells a nd that farnesylated SpRheb is critical in this process.