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. {ECO:0000269|PubMed:12152084; ECO:0000269|PubMed:15198982; ECO:0000269|PubMed:15879521}.