Figure 2.

Biochemical analysis of cell cycle regulators in papillomas of Skp2-null and WT mice. A: Immunoblot analysis of WT (Skp2^+/+) and Skp2^−/− papilloma lysates developed with antibodies against Skp2, p27^Kip1, p21^Cip1, cyclin A, cyclin E, CDK2, CDK4, and actin (a loading control). B: CDK4 and CDK2 kinase activities of papillomas from Skp2^−/− and Skp2^+/+ mice. Fresh tumor lysates were immunoprecipitated with specific antibodies against CDKs, and in vitro kinase assays were performed with pRb or histone H1 peptides as substrates. Con and Skp2^+/+ control lysates were immunoprecipitated with normal rabbit IgG.