Loss of Rhb1, a Rheb-related GTPase in fission yeast, causes growth arrestwith 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 ce ll types. A likely fission yeast homologue of mammalian Rheb, which we desi gnated Rhb1, was identified by genome sequencing. Our investigation of rhb1 showed that rhb1(-) cells arrested cell growth and division with a termina l phenotype similar to that of nitrogen-starved cells. In particular, cells depleted of Rhb1 arrested as small, round cells with 1N DNA content, arres ted more quickly in low-nitrogen medium, and induced expression of fnx1 and mei2 mRNA, two mRNAs that were normally induced by nitrogen starvation. Si nce mammalian Rheb binds and may regulate Raf-1, a Ras effector, rye tested for functional overlap between Ras1 and Rhb1 in fission yeast. This analys is showed that Ras1 overexpression did not suppress rhb1(-) mutant phenotyp es, Rhb1 overexpression did not suppress ras1(-) mutant phenotypes, and ras 1(-) rhb1(-) double mutants had phenotypes equal to the sum of the correspo nding single-mutant phenotypes. Hence, there is no evidence for overlapping functions between Ras1 and Rhb1. On the basis of this study, we hypothesiz e that Rhb1 negatively regulates entry into stationary phase when extracell ular nitrogen levels are adequate for growth. If this hypothesis is correct , then Rhb1 and Ras1 regulate alternative responses to limiting nutrients.