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Vol. 16, Issue 6, 2836-2847, June 2005
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Department of Biochemistry, University of Wisconsin–Madison, Madison, WI 53706
Submitted October 26, 2004;
Revised March 9, 2005;
Accepted March 16, 2005
Monitoring Editor: Trisha Davis
Maskin is the Xenopus homolog of the transforming acidic coiled coil (TACC)-family of microtubule and centrosome-interacting proteins. Members of this family share a 
200 amino acid coiled coil motif at their C-termini, but have only limited homology outside of this domain. In all species examined thus far, perturbations of TACC proteins lead to disruptions of cell cycle progression and/or embryonic lethality. In Drosophila, Caenorhabditis elegans, and humans, these disruptions have been attributed to mitotic spindle assembly defects, and the TACC proteins in these organisms are thought to function as structural components of the spindle. In contrast, cell division failure in early Xenopus embryo blastomeres has been attributed to a role of maskin in regulating the translation of, among others, cyclin B1 mRNA. In this study, we show that maskin, like other TACC proteins, plays a direct role in mitotic spindle assembly in Xenopus egg extracts and that this role is independent of cyclin B. Maskin immunodepletion and add-back experiments demonstrate that maskin, or a maskin-associated activity, is required for two distinct steps during spindle assembly in Xenopus egg extracts that can be distinguished by their response to "rescue" experiments. Defects in the "early" step, manifested by greatly reduced aster size during early time points in maskin-depleted extracts, can be rescued by readdition of purified full-length maskin. Moreover, defects in this step can also be rescued by addition of only the TACC-domain of maskin. In contrast, defects in the "late" step during spindle assembly, manifested by abnormal spindles at later time points, cannot be rescued by readdition of maskin. We show that maskin interacts with a number of proteins in egg extracts, including XMAP215, a known modulator of microtubule dynamics, and CPEB, a protein that is involved in translational regulation of important cell cycle regulators. Maskin depletion from egg extracts results in compromised microtubule asters and spindles and the mislocalization of XMAP215, but CPEB localization is unaffected. Together, these data suggest that in addition to its previously reported role as a translational regulator, maskin is also important for mitotic spindle assembly.
Abbreviations used: CPEB, cytoplasmic polyadenylation element binding protein; GST, glutathione S-transferase; MT, microtubule; TACC, transforming acidic coiled coil; XMAP, Xenopus microtubule associated protein.
The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).
* These authors contributed equally to this work.
Present address: School of Life Sciences, Peking University, Beijing 100871, China
Present address: Department of Neurology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02115.
Address correspondence to: Christiane Wiese (wiese{at}biochem.wisc.edu).
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