A splice variant of Skp2 is retained in the cytoplasm and fails to direct cyclin D1 ubiquitination in the uterine cancer cell line SK-UT

Cyclin D1 is an important regulator of the transition from G1 into S phase of the cell cycle. The level to which cyclin D1 accumulates is tightly regu lated. One mechanism contributing to the control of cyclin D1 levels is the regulation of its ubiquitination, SK-UT-1B cells are deficient in the degr adation of D-type cyclins, We show here that p27, a substrate of the SCFSkp 2 ubiquitin Ligase complex, is coordinately stabilized in SK-UT-IB cells. F urther, we show that expression of Skp2 in SK-UT-IB cells rescues the cycli n D1 and p27 degradation defect observed in this cell line. These results t herefore indicate that the SCFSkp2 ubiquitin ligase complex affects the ubi quitination of cyclin D1, In addition, we show that SK-UT-LB cells express a novel splice variant of Skp2 that localizes to the cytoplasm and that cyc lin D1 ubiquitination takes place in the nucleus. We propose that the trans location of Skp2 into the nucleus is required for the ubiquitination of cyc lin DI and that the absence of the SCFSkp2 complex in the nucleus of SK-UT- IB cells is the mechanism underlying the ubiquitination defect observed in this cell line, Finally, our data indicates that differential splicing of F -box proteins may represent an additional level of regulation of the F-box mediated ubiquitination pathway.