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Department of Biochemistry, Dartmouth Medical School, Hanover, New Hampshire 03755
The focusing of microtubules into mitotic
spindle poles in vertebrate somatic cells has been assumed to be the consequence of their nucleation from
centrosomes. Contrary to this simple view, in this article we show that an antibody recognizing the light intermediate chain of cytoplasmic dynein (70.1) disrupts both the focused organization of microtubule minus
ends and the localization of the nuclear mitotic apparatus protein at spindle poles when injected into cultured
cells during metaphase, despite the presence of centrosomes. Examination of the effects of this dynein-specific antibody both in vitro using a cell-free system for
mitotic aster assembly and in vivo after injection into
cultured cells reveals that in addition to its direct effect
on cytoplasmic dynein this antibody reduces the efficiency with which dynactin associates with microtubules, indicating that the antibody perturbs the cooperative binding of dynein and dynactin to microtubules during spindle/aster assembly. These results indicate
that microtubule minus ends are focused into spindle
poles in vertebrate somatic cells through a mechanism
that involves contributions from both centrosomes and
structural and microtubule motor proteins. Furthermore, these findings, together with the recent observation that cytoplasmic dynein is required for the formation and maintenance of acentrosomal spindle poles in
extracts prepared from Xenopus eggs (Heald, R., R. Tournebize, T. Blank, R. Sandaltzopoulos, P. Becker,
A. Hyman, and E. Karsenti. 1996. Nature (Lond.). 382:
420-425) demonstrate that there is a common mechanism for focusing free microtubule minus ends in both
centrosomal and acentrosomal spindles. We discuss these observations in the context of a search-capture-focus model for spindle assembly.
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