Required for cell cycle arrest in response to loss of microtubule function. Component of the spindle assembly checkpoint which is a feedback control that prevents cells with incompletely assembled spindles from leaving mitosis. Component of the mitotic checkpoint complex (MCC) which inhibits the ubiquitin ligase activity of the anaphase promoting complex/cyclosome (APC/C) by preventing its activation by CDC20. The formation of a MAD1-BUB1-BUB3 complex seems to be required for the spindle checkpoint mechanism. Component of the mitotic checkpoint complex (MCC) which consists of MAD2, MAD3, BUB3 and CDC20. Part of complex consisting of MAD1, BUB1 and BUB3 after activation of spindle checkpoint. Interacts with MAD1, MAD2 and CDC20. Phosphorylated by BUB1. Present with 1430 molecules/cell in log phase SD medium. Belongs to the WD repeat BUB3 family.