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Department of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
The Saccharomyces cerevisiae kinesin-related
motor Kar3p, though known to be required for karyogamy, plays a poorly defined, nonessential role during
vegetative growth. We have found evidence suggesting
that Kar3p functions to limit the number and length of
cytoplasmic microtubules in a cell cycle-specific manner. Deletion of KAR3 leads to a dramatic increase in
cytoplasmic microtubules, a phenotype which is most
pronounced from START through the onset of
anaphase but less so during late anaphase in synchronized cultures. We have immunolocalized HA-tagged
Kar3p to the spindle pole body region, and fittingly,
Kar3p was not detected by late anaphase. A microtubule depolymerizing activity may be the major vegetative role for Kar3p. Addition of the microtubule polymerization inhibitors nocodazol or benomyl to the
medium or deletion of the nonessential 
-tubulin
TUB3 gene can mostly correct the abnormal microtubule arrays and other growth defects of kar3 mutants,
suggesting that these phenotypes result from excessive
microtubule polymerization. Microtubule depolymerization may also be the mechanism by which Kar3p acts
in opposition to the anaphase B motors Cin8p and
Kip1p. A preanaphase spindle collapse phenotype of
cin8 kip1 mutants, previously shown to involve Kar3p,
is markedly delayed when microtubule depolymerization is inhibited by the tub2-150 mutation. These results
suggest that the Kar3p motor may act to regulate the
length and number of microtubules in the preanaphase
spindle.
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