Cytoplasmic ubiquitin ligase KPC regulates proteolysis of p27Kip1 at G1 phase

The cyclin-dependent kinase inhibitor p27(Kip1) is degraded at the G0-G1 transition of the cell cycle by the ubiquitin proteasome pathway(1,2). Although the nuclear ubiquitin ligase (E3) SCFSkp2 is implicated in p27(Kip1) degradation(3-6), proteolysis of p27(Kip1) at the G0 - G1 transition proceeds normally in Skp2(-/-) cells(7,8). Moreover, p27(Kip1) is exported from the nucleus to the cytoplasm at G0 - G1 ( refs 9 - 11). These data suggest the existence of a Skp2- independent pathway for the degradation of p27(Kip1) at G1 phase. We now describe a previously unidentified E3 complex: KPC ( Kip1 ubiquitination- promoting complex), consisting of KPC1 and KPC2. KPC1 contains a RING- finger domain, and KPC2 contains a ubiquitinlike domain and two ubiquitin- associated domains. KPC interacts with and ubiquitinates p27Kip1 and is localized to the cytoplasm. Overexpression of KPC promoted the degradation of p27(Kip1), whereas a dominant- negative mutant of KPC1 delayed p27(Kip1) degradation. The nuclear export of p27(Kip1) by CRM1 seems to be necessary for KPC- mediated proteolysis. Depletion of KPC1 by RNA interference also inhibited p27(Kip1) degradation. KPC thus probably controls degradation of p27(Kip1) in G1 phase after export of the latter from the nucleus.