WHAT is the origin of the forces generating chromosome and spindle movements in mitosis? Both microtubule dynamics1 and microtubule-dependent motors2 have been proposed as the source of these motor forces. Cytoplasmic dynein and kinesin are two soluble proteins3-6 that power membranous organelle movements on microtubules7,8. Kinesin directs movement of organelles to the 'plus' end of microtubules, and is found at the mitotic spindle in sea urchin embryos6, but not in mammalian cells9. Cytoplasmic dynein translocates organelles to the 'minus' end of microtubules, and is composed of two heavy chains and several light chains3,4. We report here that monoclonal antibodies to two of these subunits and to another polypeptide that associates with dynein localize the protein to the mitotic spindle and to the kinetochores of isolated chromosomes, suggesting that cytoplasmic dynein is important in powering movements of the spindle and chromosomes in dividing cells.
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