Essential for yeast nuclear fusion during mating. KAR3 is a bifunctional protein having a kinesin-like motor domain joined to a distinct microtubule binding domain. It may mediate microtubule sliding during nuclear fusion and possibly mitosis. May interact with spindle microtubules to produce an inwardly directed force acting upon the poles. KAR3 function antagonizes CIP8 and KIP1 outward force action. KAR3 motor activity is directed toward the microtubule's minus end. {ECO:0000269|PubMed:11729143; ECO:0000269|PubMed:2138512}.