Guo et al. describe how the mitotic kinase BubR1 phosphorylates itself in order to promote accurate chromosome segregation.
BubR1 localizes to kinetochores and has two important functions in mitosis: it helps chromosomes to align correctly on the mitotic spindle, and, by maintaining the mitotic checkpoint, it prevents cells from entering anaphase until every chromosome is properly attached to spindle microtubules. Whether BubR1’s kinase activity is required for these processes is unclear, however. No substrates have been identified in cells, and purified BubR1 shows little activity in vitro, though the enzyme will phosphorylate itself in the presence of its binding partner, the microtubule-based motor protein CENP-E.
Guo et al. found that CENP-E also stimulates BubR1’s autophosphorylation in vivo. Cells expressing a nonphosphorylatable version of BubR1 frequently missegregated their chromosomes in mitosis. The cells’ mitotic checkpoint was weakened because, unlike in wild-type cells, the checkpoint proteins Mad1 and Mad2 failed to accumulate strongly on kinetochores that weren't attached to the spindle. In addition, many kinetochores formed incorrect spindle attachments when BubR1 couldn't phosphorylate itself, preventing chromosomes from aligning properly in metaphase. Incorrect attachments are normally destabilized by the kinase Aurora B, which phosphorylates and inhibits microtubule-binding kinetochore proteins like Hec1. Guo et al. found that Aurora B's ability to phosphorylate Hec1 was diminished in the presence of nonphosphorylatable BubR1.
Hec1 phosphorylation was also decreased in CENP-E–deficient cells, but chromosome alignment was restored by a phosphomimetic BubR1 mutant. Senior author Yinghui Mao now wants to investigate how CENP-E–stimulated BubR1 autophosphorylation regulates the mitotic checkpoint and the ability of Aurora B to destabilize incorrect spindle attachments.
