Has a dual function in spindle-assembly checkpoint signaling and in promoting the establishment of correct kinetochore-microtubule (K-MT) attachments. Promotes the formation of stable end-on bipolar attachments. Necessary for kinetochore localization of BUB1. The BUB1/BUB3 complex plays a role in the inhibition of anaphase-promoting complex or cyclosome (APC/C) when spindle-assembly checkpoint is activated and inhibits the ubiquitin ligase activity of APC/C by phosphorylating its activator CDC20 (By similarity). Part of the mitotic checkpoint complex (MCC). Accumulates onto both kinetochores and the spindle microtubules in cell arrested in metaphase. Starts to localize at kinetochores in prometaphase I (Pro-MI) stage and maintains the localization until the metaphase I-anaphase I (MI-AI) transition. Associates with unattached kinetochores upon spindle assembly checkpoint (SAC) activation. Present in the phragmoplast midline during the final step of cell division (By similarity). Expressed in actively dividing tissues such as roots, flower buds, flowers and siliques. No visible phenotype. Belongs to the WD repeat BUB3 family.