Abstract
Formation of a bipolar spindle is the first step in the accurate segregation of chromosomes during cell division. Kinesin-5 is an highly conserved motor protein required to form the bipolar spindle, and can both cross-link and slide anti-parallel microtubules. Yet how these two modalities combine to form a bipolar spindle remains unclear. In this study, we report that Kinesin-5 cross-linking and sliding of microtubules have distinct roles in the fast, irreversible formation of a stable bipolar spindle in budding yeast. We report that mutations in Kinesin-5 orthologs that impair microtubule sliding do not reduce the velocity of pole separation or delay the onset of spindle formation. Using electron tomography models, we show that microtubule pairs in the initial monopolar state have short overlaps and intersect at a high angle, and are unsuited to ensemble kinesin-5 sliding. However, this highly coupled system can support a maximal extension of cross-linked microtubules consistent with the length of nascent bipolar spindles. Finally, we find microtubule sliding by kinesin-5 determines both the equilibrium length of the nascent bipolar spindle and reduces fluctuations in spindle length. We propose that kinesin-5 crosslinks anti-parallel microtubules to first form the bipolar spindle and then slides these microtubules apart to maintain a stable bipolar state at a stereotyped length.