Bub1 Multitasking in Mitosis

Accurate partition of duplicated genetic material to the daughter cells during mitosis relies on the maintenance of the physical linkage (cohesion) between sister chromatids until their bipolar attachment to the mitotic spindle. In response to a single straying chromatid within a cell, a surveillance mechanism called the spindle checkpoint blocks the ubiquitin ligase activity of the anaphase-promoting complex or cyclosome (APC/C), stabilizes securin (an APC/C substrate and an inhibitor of separase), and delays the activation of separase. This in turn prevents cleavage of cohesin by separase, preserves sister chromatid cohesion, and delays the onset of anaphase. The protein kinase, Bub1, is a key component of the spindle checkpoint. Bub1 has an upstream function in regulating the kinetochore localization of Mad2 and other downstream checkpoint components. In addition, recent biochemical studies have shown that Bub1 directly phosphorylates the APC/C activator, Cdc20, and inhibits APC/C. Finally, Bub1 has a noncheckpoint function at the kinetochores and preserves centromeric cohesion through the MEI-S332/shugoshin family of proteins. Therefore, Bub1 performs multiple tasks in mitosis that ensure the proper inheritance of chromosomes.