Involved in cell cycle checkpoint enforcement. Acts to stabilize the spindle during mitosis. Required for the correct localization of bub3 and mad3 to the kinetochore. Appears to have a role in chromosome segregation. Catalyzes the phosphorylation of bub3. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with bub3 and mad3. Associates with kinetochores and centromeres during the early stages of mitosis. Autophosphorylated. Simultaneous disruption of nucleoporin alm1 results in a growth defect. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. BUB1 subfamily.