LOCALIZATION OF THE KAR3 KINESIN HEAVY CHAIN-RELATED PROTEIN REQUIRESTHE CIK1 INTERACTING PROTEIN

The Kar3 protein (Kar3p), a protein related to kinesin heavy chain, an d the Cik1 protein (Cik1p) appear to participate in the same cellular processes in S. cerevisiae. Phenotypic analysis of mutants indicates t hat both CIK1 and KAR3 participate in spindle formation and karyogamy. In addition, the expression of both genes is induced by pheromone tre atment. In vegetatively growing cells, both Cik1::beta-gal and Kar3::b eta-gal fusions localize to the spindle pole body (SPB), and after phe romone treatment both fusion proteins localize to the spindle pole bod y and cytoplasmic microtubules. The dependence of Cik1p and Kar3p loca lization upon one another was investigated by indirect immunofluoresce nce of fusion proteins in pheromone-treated cells. The Cik1p::beta-gal fusion does not localize to the SPB or microtubules in a kar3 Delta s train, and the Kar3p::beta-gal fusion protein does not localize to mic rotubule-associated structures in a cik1 Delta strain. Thus, these pro teins appear to be interdependent for localization to the SPB and micr otubules. Analysis by both the two-hybrid system and coimmunoprecipita tion experiments indicates that Cik1p and Kar3p interact, suggesting t hat they are part of the same protein complex. These data indicate tha t interaction between a putative kinesin heavy chain-related protein a nd another protein can determine the localization of motor activity an d thereby affect the functional specificity of the motor complex.