REGULATION OF RAS-RELATED RHOB PROTEIN EXPRESSION DURING THE CELL-CYCLE

The immediate-early gene rhoB codes for a small GTP-binding protein hi ghly homologous to the RhoA protein, While RhoA is known to regulate t he assembly of focal adhesions and stress fibers in response to growth factors, the function of RhoB remains unknown. In a first attempt to elucidate its function, we examined the variation of the RhoB protein expression in response to induction of its mRNA. We report here that R hoB is an unstable protein rapidly and transiently induced by growth f actors in PC12 and HeLa cells. Moreover, RhoB protein accumulation is periodic through the cell cycle. First detected at the G1/S phase tran sition, the level of the RhoB protein is maximal during the S phase an d declines at the S/G2-M transition. This timing suggests that RhoB pl ays a role in the G1/S phase transition and/or in the S phase of the c ell cycle. We also confirm here a vesicular and perinuclear localizati on of the endogenous RhoB protein induced by growth factors.