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. {ECO:0000269|PubMed:15509783; ECO:0000269|PubMed:15525673; ECO:0000269|PubMed:9864354}.