Serine/threonine-protein kinase that performs 2 crucial functions during mitosis: it is essential for spindle-assembly checkpoint signaling and for correct chromosome alignment. Has a key role in the assembly of checkpoint proteins at the kinetochore. Acts as a substrate for anaphase-promoting complex or cyclosome (APC/C). Necessary for ensuring proper chromosome segregation. Can regulate chromosome segregation in a kinetochore-independent manner. The BUB1-BUB3 complex plays a role in the inhibition of APC/C when spindle-assembly checkpoint is activated and inhibits the ubiquitin ligase activity of APC/C by phosphorylating its activator CDC20. Kinase activity is essential for inhibition of APC/CCDC20 and for chromosome alignment but does not play a major role in the spindle-assembly checkpoint activity (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Autophosphorylated when the cells enters mitosis. Part of the mitotic checkpoint complex (MCC); interacts with CDC20-1, CDC20-2 and CDC20-5. Nuclear in interphase cells. Accumulates gradually during G1 and S phase of the cell cycle, peaks at G2/M, and drops dramatically after mitosis. Localizes to the outer kinetochore. Kinetochore localization is required for normal mitotic timing and checkpoint response to spindle damage and occurs very early in prophase (By similarity). BUB1 N-terminal domain directs kinetochore localization and binding to BUB3. Ubiquitinated and degraded during mitotic exit. Upon spindle-assembly checkpoint activation it is hyperphosphorylated and its kinase activity toward CDC20 is stimulated. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. BUB1 subfamily.