During cell division, correct positioning of chromosomes in mitotic and meiotic spindles depends on interactions of microtubules with kinetochores and, especially in higher eukaryotes, with the chromosome arms [1, 2]. Chromokinesins, highly concentrated on mitotic and meiotic chromatin, are thought to actively push the chromosome arms toward the spindle center, thereby contributing to chromosome alignment at the metaphase plate in early mitosis [1-9]. How many distinct classes of chromokinesins exist and how they cooperate to form a motile chromatin-microtubule interface are not known. Using a novel experimental assay with nonkinetochore chromatin reconstituted from Xenopus egg extract, we demonstrate that the microtubule motility generated on chromatin is continuous and plus-end directed. Using specific antibody depletions, we identify two distinct chromokinesins, kinesin-10 (Xkid) [8, 10, 11] and kinesin-4 (Xklp1) [12, 13], as the major activities mediating the interaction of meiotic chromatin with microtubules. Interestingly, we find that the slower motor, kinesin-10, more efficiently recruits microtubules and also dominates in collective microtubule transport both in the close-to-physiological environment of chromatin and also in a minimal in vitro assay. Our results provide an identification of the molecular activities involved in the generation of motor protein-mediated chromosome arm motility and yield mechanistic insight into the cooperation of the two major chromokinesins.
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