DIRECT BINDING OF CDC20 PROTEIN FAMILY MEMBERS ACTIVATES THE ANAPHASE-PROMOTING COMPLEX IN MITOSIS AND G1

Activation of the anaphase-promoting complex (APC) is required for ana phase initiation and for exit from mitosis. We show that APC is activa ted during mitosis and G1 by two regulatory factors, hCDC20 and hCDH1. These proteins directly bind to APC and activate its cyclin ubiquitin ation activity. hCDC20 confers a strict destruction-box (D-box) depend ence on APC, while hCDH1 shows a much more relaxed specificity for the D-box. In HeLa cells, the protein levels of hCDC20 as well as its bin ding to APC peak in mitosis and decrease drastically at early G1. Thus , hCDC20 is the mitotic activator of APC and directs the degradation o f substrates containing the D-box. The hCDH1 protein level remains con stant during the cell cycle and may target specific substrates lacking the D-box in G1, such as polo-like kinase, for ubiquitination.