β-tubulin phosphorylation by Chk1 is required for normal spindle formation during cell division

The mitotic spindle is a microtubule-based apparatus that is responsible for accurate segregation of chromosomes into two daughter cells. In this study, we show that the DNA damage kinase Chk1 is required for optimal density and efficient nucleation of spindle microtubules during unperturbed mitosis in vertebrate cells. Chk1 phosphorylates β-tubulin at the identified conserved site threonine-285 (T285) in vitro, and at mitotic centrosomes in prometaphase and metaphase. Impaired β-tubulin-T285 phosphorylation correlates with improper spindles, delayed anaphase onset, erroneous chromosome alignment and segregation, unequal daughter cell-size and reduced cell proliferation. The ATR-interacting protein ATRIP promotes localization of ATR kinase and the mediator protein TopBP1 to mitotic centrosomes; furthermore, interaction of ATRIP with ATR and TopBP1 is required for Chk1 activation and β-tubulin-T285 phosphorylation. These results identify a signaling pathway that promotes spindle maturation and function in human cells, through Chk1-mediated β-tubulin-T285 phosphorylation.