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Spindle assembly and accurate chromosome segregation require the proper regulation of microtubule dynamics. MCAK, a Kinesin-13, catalytically depolymerizes microtubules, regulates physiological microtubule dynamics, and is the major catastrophe factor in egg extracts. Purified GFP-tagged MCAK domain mutants were assayed to address how the different MCAK domains contribute to in vitro microtubule depolymerization activity and physiological spindle assembly activity in egg extracts. Our biochemical results demonstrate that both the neck and the C-terminal domain are necessary for robust in vitro microtubule depolymerization activity. In particular, the neck is essential for microtubule end binding, and the C-terminal domain is essential for tight microtubule binding in the presence of excess tubulin heterodimer. Our physiological results illustrate that the N-terminal domain is essential for regulating microtubule dynamics, stimulating spindle bipolarity, and kinetochore targeting; whereas the C-terminal domain is necessary for robust microtubule depolymerization activity, limiting spindle bipolarity, and enhancing kinetochore targeting. Unexpectedly, robust MCAK microtubule (MT) depolymerization activity is not needed for sperm-induced spindle assembly. However, high activity is necessary for proper physiological MT dynamics as assayed by Ran-induced aster assembly. We propose that MCAK activity is spatially controlled by an interplay between the N-and C-terminal domains during spindle assembly. INTRODUCTIONBipolar spindle assembly is essential for the faithful segregation of chromosomes into two daughter nuclei. Defects in this process are often associated with aneuploidy, which can lead to cancer or birth defects. The bipolar spindle is composed of a dynamic array of microtubules (MTs) and their associated proteins, including various MT-associated proteins and motor proteins, which are essential for proper spindle assembly (Gadde and Heald, 2004;Mogilner et al., 2006). Proper MT dynamics are highly regulated during the transition between interphase and mitosis, and many regulators of MT dynamics are critical for proper spindle assembly both in cells and in Xenopus laevis egg extracts (KlineSmith and Walczak, 2004). In egg extracts, spindle assembly is dominated by chromatin-mediated MT nucleation and organization (Sawin and Mitchison, 1991). During chromatin-mediated spindle assembly, randomly nucleated MTs around chromatin become organized into an astral array that leads to the formation of a bipolar spindle due to the generation of anti-parallel overlapping MTs that are extended and focused into poles by molecular motors and MT-associated proteins (Walczak et al., 1997(Walczak et al., , 1998Sharp et al., 2000;Gaetz and Kapoor, 2004;Mitchison et al., 2004Mitchison et al., , 2005Koffa et al., 2006). In addition to relying on molecular motors to organize and slide MTs, bipolarity also depends on the regulation of proper physiological MT dynamics by depolymerizing proteins that include depolymerizing kinesins and...

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We describe a novel interaction between HIV-1 Rev and microtubules (MTs) that results in the formation of bilayered rings that are 44–49 nm in external diameter, 3.4–4.2 MD (megadaltons) in mass, and have 28-, 30-, or 32-fold symmetry. Ring formation is not sensitive to taxol, colchicine, or microtubule-associated proteins, but requires Mg2+ and is inhibited by maytansine. The interaction involves the NH2-terminal domain of Rev and the face of tubulin exposed on the exterior of the MTs. The NH2-terminal half of Rev has unexpected sequence similarity to the tubulin-binding portion of the catalytic/motor domains of the microtubule-destabilizing Kin I kinesins. We propose a model wherein binding of Rev dimers to MTs at their ends causes segments of two neighboring protofilaments to peel off and close into rings, circumferentially containing 14, 15, or 16 tubulin heterodimers, with Rev bound on the inside. Rev has a strong inhibitory effect on aster formation in Xenopus egg extracts, demonstrating that it can interact with tubulin in the presence of normal levels of cellular constituents. These results suggest that Rev may interact with MTs to induce their destabilization, a proposition consistent with the previously described disruption of MTs after HIV-1 infection.