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CHROMOSOME 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 KRP₁₃₀, a homotetrameric BimC-related kinesin purified from Drosophila melanogaster embryos¹³, 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 filament¹⁵. 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.