Polar Ejection Forces Promote the Conversion from Lateral to End-on Kinetochore-Microtubule Attachments on Mono-oriented Chromosomes.

Chromosome bi-orientation occurs after conversion of initial lateral attachments between kinetochores and spindle microtubules into stable end-on attachments near the cell equator. After bi-orientation, chromosomes experience tension from spindle forces, which plays a key role in the stabilization of correct kinetochore-microtubule attachments. However, how end-on kinetochore-microtubule attachments are first stabilized in the absence of tension remains a key unanswered question. To address this, we generated Drosophila S2 cells undergoing mitosis with unreplicated genomes (SMUGs). SMUGs retained single condensed chromatids that attached laterally to spindle microtubules. Over time, laterally attached kinetochores converted into end-on attachments and experienced intra-kinetochore stretch/structural deformation, and SMUGs eventually exited a delayed mitosis with mono-oriented chromosomes after satisfying the spindle-assembly checkpoint (SAC). Polar ejection forces (PEFs) generated by Chromokinesins promoted the conversion from lateral to end-on kinetochore-microtubule attachments that satisfied the SAC in SMUGs. Thus, PEFs convert lateral to stable end-on kinetochore-microtubule attachments, independently of chromosome bi-orientation.

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