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Kinetochores are proteinacious structures formed at the centromere of each chromosome. Kinetochores have three primary functions in that they serve as the primary attachment site for spindle microtubules to chromosomes, they generate poleward force, and they are involved in checkpoint signalling to couple cell cycle progression to spindle organization (C.L. Rieder and E.D. Salmon. (1998). Trends Cell Biol. 8, 310-318). In vertebrate cells, each kinetochore binds between 10 and 40 microtubules, which form relatively stable bundles that can be visualized by immunofluorescence microscopy for tubulin (green). At metaphase, chromosomes (blue) align at the spindle equator and kinetochore fibers emanating from sister kinetochores toward opposite spindle poles have equal length. Simultaneous perturbation of the spindle proteins Kid and NuMA interferes with chromosome alignment, and kinetochore fibers extending toward opposite spindle poles have unequal length. Kid is a kinesin-related protein that associates with chromosome arms and generates polar ejection force (A.A. Levesque and D.A. Compton. (2001). J. Cell Biol. 154, 1135-1146). NuMA is a large coiled-coil-containing protein that is responsible for cross-linking microtubule minus ends at spindle poles and for tethering centrosomes to the spindle (T. Gaglio, A. Saredi, and D.A. Compon. (1995). J. Cell Biol. 131, 693-708). These results indicate that chromosome alignment in vertebrate cells is directed by postional cues derived from both Kid on chromosome arms and NuMA at spindle poles. An important question raised by these results is how the motility of kinetochores is controlled by positional cues generated by polar ejection forces and spindle poles.

-Aime A. Levesque, Louisa Howard, Michael B. Gordon, and Duane A. Compton.