E2-C, A CYCLIN-SELECTIVE UBIQUITIN CARRIER PROTEIN REQUIRED FOR THE DESTRUCTION OF MITOTIC CYCLINS

Ubiquitin-dependent proteolysis of the mitotic cyclins A and B is requ ired for the completion of mitosis and entry into the next cell cycle. This process is catalyzed by the cyclosome, an approximate to 22S par ticle that contains a cyclin-selective ubiquitin ligase activity, E3-C , that requires a cyclin-selective ubiquitin carrier protein (UBC) E2- C. Here we report the purification and cloning of E2-C from clam oocyt es. The deduced amino acid sequence of E2-C indicates that it is a new UBC family member. Bacterially expressed recombinant E2-C is active i n in vitro cyclin ubiquitination assays, where it exhibits the same su bstrate specificities seen with native E2-C. These results demonstrate that E2-C is not a homolog of UBC4 or UBC9, proteins previously sugge sted to be involved in cyclin ubiquitination, but is a new UBC family member with unique properties.