APC15 mediates CDC20 autoubiquitylation by [APC/C.sup.MCC] and disassembly of the mitotic checkpoint complex

The anaphase-promoting complex/cyclosome (APC/C) bound to CDC20 ([APC/C.sup.CDC20]) initiates anaphase by ubiquitylating B-type cyclins and securin. During chromosome bi-orientation, CDC20 assembles with MAD2, BLJBR1 and BuB3 into a mitotic checkpoint complex (MCC) that inhibits substrate recruitment to the APC/C. APC/C activation depends on MCC disassembly, which was proposed to require CDC20 autoubiquitylation. Here we characterize APC15, a human APC/C subunit related to yeast Mnd2. APC15 is located near APC/C's MCC binding site; it is required for APC/C-bound MCC ([APC/C.sup.MCC])- dependent CDC20 autoubiquitylation and degradation and for timely anaphase initiation but is dispensable for substrate ubiquitylation by [APC/C.sup.CDC20] and APC/CCDH1. Our results support the model wherein MCC is continuously assembled and disassembled to enable rapid activation of [APC/C.sup.CDC20] and CDC20 autoubiquitylation promotes MCC disassembly. We propose that APC15 and Mnd2 negatively regulate APC/C coactivators and report generation of recombinant human APC/C.