A bipolar kinesin

C hromosome segregation during mitosis depends on the action of the mitotic spindle, a self-organizing, bipolar protein machine which uses microtubules (MTs) and their associated motors 1 , 2 . Members of the BimC subfamily of kinesin-related MT–motor proteins are believed to be essential for the formation and functioning of a normal bipolar spindle 3 – 14 . Here we report that KRP 130 , a homotetrameric BimC-related kinesin purified from Drosophila melanogaster embryos 13 , has an unusual ultrastructure. It consists of four kinesin-related polypeptides assembled into a bipolar aggregate with motor domains at opposite ends, analogous to a miniature myosin filament 15 . Such a bipolar ‘minifilament’ could crosslink spindle MTs and slide them relative to one another. We do not know of any other MT motors that have a bipolar structure