ACTIVATION OF THE HUMAN ANAPHASE-PROMOTING COMPLEX BY PROTEINS OF THECDC20 FIZZY FAMILY/

The initiation of anaphase and exit from mitosis depend on the activat ion of the cyclosome/anaphase-promoting complex (APC) that ubiquitinat es regulatory proteins such as anaphase inhibitors and mitotic cyclins [1-4]. Genetic experiments have demonstrated that two related WD40-re peat proteins - called Cdc20p and Hct1p/Cdh1p in budding yeast and Fiz zy and Fizzy-related in Drosophila - are essential for APC-dependent p roteolysis [5-11]. Human orthologs of these proteins - hCDC20/p55(CDC) [12] and hCDH1 - have recently been found to associate with APC in a cell-cycle-dependent manner [13,14]. Here, we show that the amount of hCDC20 and hCDH1 bound to APC correlates with a high ubiquitination ac tivity of APC and that binding of recombinant hCDC20 and hCDH1 can act ivate APC in vitro. Our results suggest that the association between h CDH1 and APC is regulated by post-translational mechanisms, whereas th e amount of hCDC20 bound to APC may in addition be controlled by hCDC2 0 synthesis and destruction [15]. The temporally distinct association of hCDC20 and hCDH1 with APC suggests that these proteins are, respect ively, mitosis-specific and G1-specific activating subunits of APC.