| We speculate that the F box protein Skp 2 , in addition to utilizing Cul 1 Skp1 , utilizes Cul4A DDB 1 to induce proteolysis of p27Kip1 . . ^^^ Cul4A and DDB 1 associate with Skp 2 to target p27Kip1 for proteolysis involving the COP 9 signalosome . ^^^ Surprisingly , the DDB 1 induced proteolysis of p27Kip1 also involves Skp 2 , an F box protein that allows targeting of p27Kip1 for ubiquitination by the Skp 1 Cul1 F box complex . ^^^ Moreover , we provide evidence for a physical association between Cul4A , DDB 1 , and Skp 2 . ^^^ |