MITOTIC SPINDLE FUNCTION IN SACCHAROMYCES-CEREVISIAE REQUIRES A BALANCE BETWEEN DIFFERENT TYPES OF KINESIN-RELATED MOTORS

Two Saccharomyces cerevisiae kinesin-related motors, Cin8p and Kip1p, perform an essential role in the separation of spindle poles during sp indle assembly and a major role in spindle elongation. Cin8p and Kip1p are also required to prevent an inward spindle collapse prior to anap hase. A third kinesin-related motor, Kar3p, may act antagonistically t o Cin8p and Kip1p since loss of Kar3p partially suppresses the spindle collapse in cin8 kip1 mutants. We have tested the relationship betwee n Cin8p and Kar3p by overexpressing both motors using the inducible GA L1 promoter. Overexpression of KAR3 results in a shrinkage of spindle size and a temperature-dependent inhibition of the growth of wild-type cells. Excess Kar3p has a stronger inhibitory effect on the growth of cin8 kip1 mutants and can completely block anaphase spindle elongatio n in these cells. In contrast, overexpression of CIN8 leads to prematu re spindle elongation in all cells tested. This is the first direct de monstration of antagonistic motors acting on the intact spindle and su ggests that spindle length is determined by the relative activity of K ar3p-like and Cin8p/Kip1p-like motors.