Activator protein that regulates the ubiquitin ligase activity and substrate specificity of the anaphase promoting complex/cyclosome (APC/C). At the metaphase-to-anaphase transition, recognizes and binds proteins containing a D-box including the B-type cyclins CLB2 and CLB5, HSL1 and securin PDS1, and recruits them in a C-box-dependent manner to the APC/C for ubiquitination and subsequent proteolysis. Required for sister chromatid separation and disassembly of the mitotic spindle. Target of the spindle checkpoint pathway through participation in the mitotic checkpoint complex (MCC) and the MAD2-CDC20 subcomplex. MCC and presumably the MAD2-CDC20 subcomplex inhibit the ubiquitin ligase activity of the anaphase promoting complex/cyclosome (APC/C) by preventing its activation by CDC20 until proper attachment of all chromosomes to the spindle. Component of the mitotic checkpoint complex (MCC) which consists of MAD2, MAD3, BUB3 and CDC20, and of the MAD2-CDC20 subcomplex, both of which appear to be assembled during mitoisis independently of the kinetochore. In mitose-arrested cells, MAD2-CDC20 occurs in a larger amount than MCC. MCC associates with the with the APC/C complex. Interacts with MAD2, MAD3, BUB3, HSL1 and PDS1. Expressed in late S and M phase of the cell cycle. Degraded in G1 by the APC/C in its CDH1-bound form. The KEN box is required for the association with the APC/C complex. The C-box is required for the association with the APC/C complex. The N-terminal destruction box (D-box) acts as a recognition signal for degradation via the ubiquitin-proteasome pathway. Ubiquitinated by the APC/C complex. Phosphorylated by CDC28. Belongs to the WD repeat CDC20/Fizzy family.