The F-box protein Skp2 is a ubiquitylation target of a Cul1-based core ubiquitin ligase complex:: evidence for a role of Cul1 in the suppression of Skp2 expression in quiescent fibroblasts

The ubiquitin protein ligase SCFSkp2 is composed of Skp1, Cul1, Roc1/Rbx1 and the F-box protein Skp2, the substrate-recognition subunit. Levels of Skp2 decrease as cells exit the cell cycle and increase as cells re-enter the cycle, Ectopic expression of Skp2 in quiescent fibroblasts causes mitogen-independent S-phase entry. Hence, mechanisms must exist for limiting Skp2 protein expression during the G(0)/G(1) phases. Here we show that Skp2 is degraded by the proteasome in G(0)/G(1) and is stabilized when cells reenter the cell cycle. Rapid degradation of Skp2 in quiescent cells depends on Skp2 sequences that contribute to Cull binding and interference with endogenous Cull function in serum-deprived cells induces Skp2 expression. Furthermore, recombinant Cul1-Roc1/Rbx1-Skp1 complexes can catalyse Skp2 ubiquitylation in vitro. These results suggest that degradation of Skp2 in G(0)/G(1) is mediated, at least in part, by an autocatalytic mechanism involving a Skp2-bound Cul1-based core ubiquitin ligase and imply a role for this mechanism in the suppression of SCFSkp2 ubiquitin protein ligase function during the G(0)/G(1) phases of the cell cycle.