Successive Kinesin-5 Microtubule Crosslinking and Sliding Promote Fast, Irreversible Formation of a Stereotyped Bipolar Spindle.

Separation of duplicated spindle poles is the first step in forming the mitotic spindle. Kinesin-5 crosslinks and slides anti-parallel microtubules (MTs), but it is unclear how these two activities contribute to the first steps in spindle formation. In this study, we report that in monopolar spindles, the duplicated spindle poles snap apart in a fast and irreversible step that produces a nascent bipolar spindle. Using mutations in Kinesin-5 that inhibit microtubule sliding, we show that the fast, irreversible pole separation is primarily driven by microtubule crosslinking. Electron tomography revealed microtubule pairs in monopolar spindles have short overlaps that intersect at high angles and are unsuited for ensemble Kinesin-5 sliding. However, maximal extension of a subset of anti-parallel microtubule pairs approaches the length of nascent bipolar spindles and is consistent with a Kinesin-5 crosslinking-driven transition. Nonetheless, microtubule sliding by Kinesin-5 contributes to stabilizing the nascent spindle and setting its stereotyped equilibrium length. • The monopolar-to-bipolar spindle transition is fast and irreversible • The fast transition is driven by Cin8 (Kinesin-5) microtubule crosslinking • Nascent bipolar spindles need Kinesin-5 sliding for steady-state lengths >1 μm • Spindle formation sequentially integrates Kinesin-5 MT crosslinking and sliding Leary et al. show that bipolar spindle formation is a fast and irreversible process driven by Kinesin-5 microtubule crosslinking consistent with electron tomography revealing an initial microtubule architecture unsuited to Kinesin-5 sliding. However, Kinesin-5 sliding plays a role in ensuring subsequent bipolar spindle elongation and stability. [ABSTRACT FROM AUTHOR]