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). Essential for gametophyte development. Part of the mitotic checkpoint complex (MCC); interacts with CDC20-1 and CDC20-2. Interacts with MAD2 and BUBR1. 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. Expressed in actively dividing tissues, early in organ development, in young leaves, lateral root primordia and root meristems, flower buds, flowers and siliques. Cell cycle regulated expression with a distinct expression peak at the G2/M boundary. Survives only in heterozygous state. Displays defects in development of male and female gametophytes. Belongs to the WD repeat BUB3 family.