Skp2 connects Rb, cyclin A, and p27kip1 in oncogenesis
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Skp2 was first identified in multi-protein complexes with cyclin A and Cdk2 in transformed cells. Subsequent studies revealed that Skp2 is a member of the F-box protein family and recruits the kinase inhibitor p27 for ubiquitination in the SCF-Skp2 complex. Here, we report our recent studies that shed light on these two aspects of Skp2.;We revealed that the tumor suppressor retinoblastoma protein (Rb) interacts with the N-terminus of Skp2 to interfere with Skp2-p27 interaction and inhibit ubiquitination of p27, which leads to acute G1 cell cycle arrest. Disruption of p27 function or expression of the Skp2 N-terminus prevents Rb from causing G1 arrest. This function of Rb is fully retained in a partial penetrance Rb mutant, suggesting that the Rb-Skp2-p27 pathway may suppress progression of tumors initiated when the Rb-E2F pathway is inactivated.;We determined that the Skp2 N-terminus is responsible for the stable Skp2-cyclin A interaction. We demonstrated that the Skp2-cyclin A interaction is mediated by novel interaction sequences on both Skp2 and cyclin A, distinguishing it from the well-known RxL-HP interaction between cyclins and cyclin-binding proteins. We show that Skp2cyclin A interaction is separable from Skp2's ability to mediate p27 ubiquitination, but can directly protect cyclin A/Cdk2 from inhibition by p27 through competitive binding. Mutations in the cyclin A binding domain of Skp2 significantly compromise the proliferation-stimulating activity of Skp2 without affecting its ability to cause degradation of p27 and p21. We also identified an eighteen-residue peptide from cyclin A binding sequences in Skp2 that can block Skp2-cyclin A/Cdk2 interaction but not p27-cyclin A/Cdk2 interaction and can therefore abolish Skp2's protective effects on cyclin A/Cdk2 activity. When delivered into cells, this peptide can cause cell death selectively in cancer cells with IC50 positively correlated with Skp2 abundance. These results identify a new functional mechanism of Skp2 that may play a survival role during tumorigenesis and can be targeted in designing mechanism-based cancer cell-specific therapeutics.
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