We have prepared antibodies specific for HSET, the human homologue of the KAR3 family of minus end-directed motors. Immuno-EM with these antibodies indicates that HSET frequently localizes between microtubules within the mammalian metaphase spindle consistent with a microtubule cross-linking function. Microinjection experiments show that HSET activity is essential for meiotic spindle organization in murine oocytes and taxol-induced aster assembly in cultured cells. However, inhibition of HSET did not affect mitotic spindle architecture or function in cultured cells, indicating that centrosomes mask the role of HSET during mitosis. We also show that (acentrosomal) microtubule asters fail to assemble in vitro without HSET activity, but simultaneous inhibition of HSET and Eg5, a plus end-directed motor, redresses the balance of forces acting on microtubules and restores aster organization. In vivo, centrosomes fail to separate and monopolar spindles assemble without Eg5 activity. Simultaneous inhibition of HSET and Eg5 restores centrosome separation and, in some cases, bipolar spindle formation. Thus, through microtubule cross-linking and oppositely oriented motor activity, HSET and Eg5 participate in spindle assembly and promote spindle bipolarity, although the activity of HSET is not essential for spindle assembly and function in cultured cells because of centrosomes.

Accurate segregation of genetic material during both mitosis and meiosis is essential for the viability of future cellular generations. Genetic material is packaged in the form of chromosomes during cell division, and chromosomes are segregated equally into two daughter cells by a dynamic, microtubule-based structure known as the spindle. Molecular motor proteins of the kinesin and dynein superfamilies are essential players in the functional microanatomy of cell division. They power various aspects of spindle assembly and function, including establishing spindle bipolarity, spindle pole organization, chromosome alignment and segregation, regulating microtubule dynamics, and cytokinesis. This review highlights the roles that various members of the kinesin and dynein motor superfamilies play during mitosis and meiosis. Understanding how microtubule motors function during cell division will unravel how the spindle precisely segregates chromosomes, and may offer insights into the molecular basis of disease states that arise from spindle malfunctions. For example, chromosome non-disjunction during meiosis causes such disorders as Klinefelter, Turner, and Down Syndromes. Chromosome non-disjunction during mitosis is an important contributing mechanism for tumor progression. In addition, since motor proteins are essential for spindle assembly and function, they provide obvious targets for intervention into the cell division cycle, and compounds that specifically block motor functions during mitosis may prove to be valuable chemotherapeutic agents. Anat Rec (New Anat) 261:14-24, 2000.

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