CYTOPLASMIC DYNEIN PLAYS A ROLE IN MAMMALIAN MITOTIC SPINDLE FORMATION

The formation and functioning of a mitotic spindle depends not only on the assembly/disassembly of microtubules but also on the action of mo tor enzymes. Cytoplasmic dynein has been localized to spindles, but wh ether or how it functions in mitotic processes is not yet known. We ha ve cloned and expressed DNA fragments that encode the putative ATP-hyd rolytic sites of the cytoplasmic dynein heavy chain from HeLa cells an d from Dictyostelium. Monospecific antibodies have been raised to the resulting polypeptides, and these inhibit dynein motor activity in vit ro. Their injection into mitotic mammalian cells blocks the formation of spindles in prophase or during recovery from nocodazole treatment a t later stages of mitosis. Cells become arrested with unseparated cent rosomes and form monopolar spindles. The injected antibodies have no d etectable effect on chromosome attachment to a bipolar spindle or on m otions during anaphase. These data suggest that cytoplasmic dynein pla ys a unique and important role in the initial events of bipolar spindl e formation, while any later roles that it may play are redundant. Pos sible mechanisms of dynein's involvement in mitosis are discussed.