FISSION YEAST PKL1 IS A KINESIN-RELATED PROTEIN INVOLVED IN MITOTIC SPINDLE FUNCTION

We have used anti-peptide antibodies raised against highly conserved r egions of the kinesin motor domain to identify kinesin-related protein s in the fission yeast Schizosaccharomyces pombe. Here we report the i dentification of a new kinesin-related protein, which we have named pk l1. Sequence homology and domain organization place pkl1 in the Kar3/n cd subfamily of kinesin-related proteins. Bacterially expressed pkl1 f usion proteins display microtubule-stimulated ATPase activity, nucleot ide-sensitive binding, and bundling of microtubules. Immunofluorescenc e studies with affinity-purified antibodies indicate that the pkl1 pro tein localizes to the nucleus and the mitotic spindle. Pkl1 null mutan ts are viable but have increased sensitivity to microtubule-disrupting drugs. Disruption of pkl1(+) suppresses mutations in another kinesin- related protein, cut7, which is known to act in the spindle. Overexpre ssion of pkl1 to very high levels causes a similar phenotype to that s een in cut7 mutants: V-shaped and star-shaped microtubule structures a re observed, which we interpret to be spindles with unseparated spindl e poles. These observations suggest that pkl1 and cut7 provide opposin g forces in the spindle. We propose that pkl1 functions as a microtubu le-dependent motor that is involved in microtubule organization in the mitotic spindle.