Essential component of the mitotic checkpoint. Required for normal mitosis progression. The mitotic checkpoint delays anaphase until all chromosomes are properly attached to the mitotic spindle. One of its checkpoint functions may be to inhibit the activity of the anaphase-promoting complex/cyclosome (APC/C) by blocking the binding of CDC20 to APC/C (By similarity). Interacts with anaphase-promoting complex/cyclosome (APC/C) (By similarity). Part of the mitotic checkpoint complex (MCC); interacts with CDC20-1 and CDC20-2. Interacts with MAD2 at chromocenters and with BUB3.1. Cytoplasmic in interphase cells. Accumulates onto both kinetochores and the spindle microtubules in cell arrested in metaphase. Associates with unattached kinetochores in early prophase upon spindle assembly checkpoint (SAC) activation. Present in chromocenters. Expressed in actively dividing tissues, early in organ development, in young leaves, lateral root primordia and root meristems. Cell cycle regulated expression with a distinct expression peak at the G2/M boundary. The KEN box is required for its ubiquitination and degradation. BUB1 N-terminal domain directs kinetochore localization and binding to BUB3.