Bub1 is required for kinetochore localization of BubR1, Cenp-E, Cenp-F and Mad2, and chromosome congression

During mitosis, the recruitment of spindle-checkpoint-associated proteins to the kinetochore occurs in a defined order. The protein kinase Bub1 localizes to the kinetochore very early during mitosis, followed by Cenp-F, BubR1, Cenp-E and finally Mad2. Using RNA interference, we have investigated whether this order of binding reflects a level of dependency in human somatic cells. Specifically, we show that Bub1 plays a key role in the assembly of checkpoint proteins at the kinetochore, being required for the subsequent localization of Cenp-F, BubR1, Cenp-E and Mad2. In contrast to studies in Xenopus, we also show that BubR1 is not required for kinetochore localization of Bub1. Repression of Bub1 increases the number of cells with lagging chromosomes at metaphase, suggesting that Bub1 plays a role in chromosome congression. However, repression of Bub1 does not appear to compromise spindle checkpoint function either during normal mitosis or in response to spindle damage. This raises the possibility that, in the absence of Bub1, other mechanisms contribute to spindle checkpoint function.