Multi-functional Dynamic Control of Bipolarity, Chromosome segregation and Spindle Elongation by a Novel Essential Chromatin Binding Klp in Fission Yeast

Abstract

A subset of all known kinesin-like microtubule motor proteins (Klps) regulates spindle function for fidelity in chromosome segregation. While some families are ubiquitous, such as Kinesin-5 and Kinesin-14 Klps, others families seem restricted to multi-cellular eukaryotes, such as chromokinesins and MKlp1-like passenger proteins. We report a novel essential chromatin-binding Klp in fission yeast Schizosaccharomyces pombe Ckk1 (Cho1 and Kid-like kinesin). S. pombe Ckk1p has an N-terminal motor domain and carboxy-terminal tandem basic-Zip DNA binding domains. It localizes to chromosome arms in prometaphase and kinetochores in metaphase. In prometaphase to anaphase it also associates with overlapping anti-parallel microtubules of the spindle midzone. Passenger proteins show dynamic relocalization from chromosome arms to kinetochores to spindle midzone and regulate cytokinesis. Although Ckk1p has the conserved Cho-domain, it does not have actin binding domains and exhibits no cytokinesis defects. Ckk1p microtubule localization regulates microtubule sliding but not spindle midzone integrity, a role attributed to Ase1p in fission yeast. A single microtubule-binding site within the motor domain suggests that like MKlp1, protein association with the stalk is needed to mediate other microtubule interactions. Premature overexpression of Ckk1 can stabilize bipolar spindles in a compromised Kinesin-5 cut7-22ts strain, overriding the opposing effect of Kinesin-14 Pkl1. However in wild type cells overexpressed Ckk1p results in spindle collapse. Upon removal of pkl1, the ability of Ckk1p overexpression to cause spindle collapse is reduced, however pre-prophase spindle elongation occurs. The result is non-central, off-side anaphase A segregation of chromosomes. Ckk1p represents a novel chromatin-binding Klp in fission yeast, absent in budding yeast, that has multiple roles in stabilizing spindle assembly, equatorial chromosome alignment and spindle elongation and does not fall cleanly into either Kinesin-6 or Kinesin-10 families of Klps.