The cyclin-dependent kinase inhibitor p27Kip1 is degraded at the G0-G1 transition of the cell cycle by the ubiquitin-proteasome pathway. Although the nuclear ubiquitin ligase (E3) SCFSkp2 is implicated in p27Kip1 degradation, proteolysis of p27Kip1 at the G0-G1 transition proceeds normally in Skp2-/- cells. Moreover, p27Kip1 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 p27Kip1 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 ubiquitin-like 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 p27Kip1, whereas a dominant-negative mutant of KPC1 delayed p27Kip1 degradation. The nuclear export of p27Kip1 by CRM1 seems to be necessary for KPC-mediated proteolysis. Depletion of KPC1 by RNA interference also inhibited p27Kip1 degradation. KPC thus probably controls degradation of p27Kip1 in G1 phase after export of the latter from the nucleus.