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 a nd the F-box protein Skp2, the substrate-recognition subunit. Levels of Skp 2 decrease as cells exit the cell cycle and increase as cells re-enter the cycle, Ectopic expression of Skp2 in quiescent fibroblasts causes mitogen-i ndependent S-phase entry. Hence, mechanisms must exist for limiting Skp2 pr otein expression during the G(0)/G(1) phases. Here we show that Skp2 is deg raded by the proteasome in G(0)/G(1) and is stabilized when cells reenter t he 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 ubiquitylatio n 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.