Loss of Rhb1, a Rheb-related GTPase in fission yeast, causes growth arrest with a terminal phenotype similar to that caused by nitrogen starvation.

The Rheb GTPase is most similar in primary sequence to the Ras, Rap, R-Ras, and Ral GTPases, which regulate cell growth and differentiation in many cell types. A likely fission yeast homologue of mammalian Rheb, which we designated Rhb1, was identified by genome sequencing. Our investigation of rhb1 showed that rhb1(-) cells arrested cell growth and division with a terminal phenotype similar to that of nitrogen-starved cells. In particular, cells depleted of Rhb1 arrested as small, round cells with 1N DNA content, arrested more quickly in low-nitrogen medium, and induced expression of fnx1 and mei2 mRNA, two mRNAs that were normally induced by nitrogen starvation. Since mammalian Rheb binds and may regulate Raf-1, a Ras effector, we tested for functional overlap between Ras1 and Rhb1 in fission yeast. This analysis showed that Ras1 overexpression did not suppress rhb1(-) mutant phenotypes, Rhb1 overexpression did not suppress ras1(-) mutant phenotypes, and ras1(-) rhb1(-) double mutants had phenotypes equal to the sum of the corresponding single-mutant phenotypes. Hence, there is no evidence for overlapping functions between Ras1 and Rhb1. On the basis of this study, we hypothesize that Rhb1 negatively regulates entry into stationary phase when extracellular nitrogen levels are adequate for growth. If this hypothesis is correct, then Rhb1 and Ras1 regulate alternative responses to limiting nutrients.