BubR1 and APC/EB1 cooperate to maintain metaphase chromosome alignment

The accurate segregation of chromosomes in mitosis requires the stable attachment of microtubules to kinetockores. The details of this complex and dynamic process are poorly understood. In this study, we report the interaction of a kinetochore-associated mitotic checkpoint kinase, l with two microtubule plus end-associated proteins, adenomatous polyposis coli (APC) and EB1, providing a potential link in stable kinetochore microtubule attachment. Using immunodepletion from and antibody addition to Xenopus laevis egg extracts, we show that l and its kinase activity are essential for positioning chromosomes at the metaphase plate. BubR1 associates with APC and l in egg extracts, and the complex formation is necessary for metaphase chromosome alignment. Using purified components, l directly phosphorylates APC and forms a ternary complex with APC and microtubules. These findings support a model in which l kinase may directly regulate APC function involved in stable kinetochore microtubule attachment.