A BIPOLAR KINESIN

CHROMOSOME Segregation during mitosis depends on the action of the mit otic spindle, a self-organizing, bipolar protein machine which uses mi crotubules (MTs) and their associated motors(1,2). Members of the BimC subfamily of kinesin-related MT-motor proteins are believed to be ess ential for the formation and functioning of a normal bipolar spindle(3 -14). Here we report that KRP(130), a homotetrameric BimC-related kine sin purified from Drosophila melanogaster embryos(13), has an ultrastr ucture. It consists of four kinesin-related polypeptides assembled int o a bipolar aggregate with motor domains at opposite ends, analogous t o a miniature myosin filament(15). Such a bipolar 'minifilament' could crosslink spindle MTs and slide them relative to one another. We do n ot know of any other MT motors that have a bipolar structure.