Timing of cyclin D1 expression within G1 phase is controlled by Rho

The expression of cyclin D1 in mid-G1 phase is associated with sustained ER K activity, and we show here that Rho is required for the sustained ERK sig nal. However, we also re port that Rho inhibits an alternative Rac/Cdc42-de pendent pathway, which results in a strikingly early GI-phase expression of cyclin D1. Thus, cyclin D1 is induced in mid-G1 phase because a Rho switch couples its expression to sustained ERK activity rather than Rac and Cdc42 . Our results show that Rho is crucial for maintaining the correct timing o f cyclin D1 expression in G1 phase and describe a new role for cytoskeletal integrity in the regulation of cell cycle progression.