The kinesin-related proteins, Kip2p and Kip3p, function differently in nuclear migration in yeast

The roles of two kinesin-related proteins, Kip2p and Kip3p, in microtubule function and nuclear migration were investigated. Deletion of either gene resulted in nuclear migration defects similar to those described for dynein and kar9 mutants. By indirect immunofluorescence, the cytoplasmic microtubules in kip2 Delta were consistently short or absent throughout the cell cycle. In contrast, in kip3 Delta strains, the cytoplasmic microtubules were significantly longer than wild type at telophase. Furthermore, in the kip3 Delta cells with nuclear positioning defects, the cytoplasmic microtubules were misoriented and failed to extend into the bud. Localization studies found Kip2p exclusively on cytoplasmic microtubules throughout the cell cycle, whereas GFP-Kip3p localized to both spindle and cytoplasmic microtubules. Genetic analysis demonstrated that the kip2 Delta kar9 Delta double mutants were synthetically lethal, whereas kip3 Delta kar9 Delta double mutants were viable. Conversely kip3 Delta dhc1 Delta double mutants were synthetically lethal, whereas kip2 Delta. dhc1 Delta double mutants were viable. We suggest that the kinesin-related proteins, Kip2p and Kip3p, function in nuclear migration and that they do so by different mechanisms. We propose that Kip2p stabilizes microtubules and is required as part of the dynein-mediated pathway in nuclear migration. Furthermore, we propose that Kip3p functions, in part, by depolymerizing microtubules and is required for the Kar9p-dependent orientation of the cytoplasmic microtubules.