MUTATIONS IN THE BIMC BOX OF CUT7 INDICATE DIVERGENCE OF REGULATION WITHIN THE BIMC FAMILY OF KINESIN-RELATED PROTEINS

Members of the bimC family of kinesin related proteins (KRPs) play vit al roles in the formation and function of the mitotic spindle. Althoug h they share little amino acid homology outside the highly conserved m icrotubule motor domain, several family members do contain a 'bimC box ', a sequence motif around a p34(cdc2) Consensus phosphorylation site in their carboxy-terminal 'tail' region. One family member, Eg5, requi res phosphorylation at this site for association with the mitotic spin dle. We show that mutations in the Schizosaccharomyces pombe cut7(+) g ene that change the bimC box p34(cdc2) Consensus phosphorylation site at position 1,011 and a neighbouring MAP kinase consensus phosphorylat ion site at position 1,020 to non-phosphorylatable residues did not af fect the ability of S. pombe cut7 genes to complement temperature sens itive cut7 mutants. Phosphorylation site mutants expressed as fusions to green fluorescent protein associated with the mitotic spindle with a localisation indistinguishable from similarly expressed wild-type Cu t7, Cells in which cut7.T1011A replaced the genomic copy of cut7(+) we re viable and formed normal spindles. Deletion of the entire carboxy-t erminal tail region did not affect the ability of Cut7 to associate wi th the mitotic spindle but did inhibit normal spindle formation. Thus, unlike Eg5, neither the p34(cdc2) consensus phosphorylation site in t he bimC box nor the entire tail region of Cut7 are required for associ ation with the mitotic spindle.