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MBC in Press, published online ahead of print October 31, 2007
Mol. Biol. Cell 10.1091/mbc.E07-09-0879

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Submitted on September 10, 2007
Revised on October 12, 2007
Accepted on October 18, 2007

The Structure of the {gamma}-Tubulin Small Complex: Implications of Its Architecture and Flexibility for Microtubule Nucleation

Justin M. Kollman,* Alex Zelter,{dagger} Eric G.D. Muller,{dagger} Bethany Fox,{dagger} Luke M. Rice,* Trisha N. Davis,{dagger} and David A. Agard*

*Department of Biochemistry and Biophysics and the Howard Hughes Medical Institute, University of California, San Francisco, CA 94158; {dagger}Department of Biochemistry, University of Washington, Seattle, WA 98195

Monitoring Editor: Tim Stearns

The {gamma}-tubulin small complex ({gamma}-TuSC) is an evolutionarily conserved heterotetramer essential for microtubule nucleation. We have determined the structure of the S. cerevisiae {gamma}-TuSC at 25 Å resolution by electron microscopy. {gamma}-TuSC is Y-shaped, with an elongated body connected to two arms. Gold labeling showed that the two {gamma}-tubulins are located in lobes at the ends of the arms, and the relative orientations of the other {gamma}-TuSC components were determined by in vivo FRET. The structures of different subpopulations of {gamma}-TuSC indicate flexibility in the connection between a mobile arm and the rest of the complex, resulting in variation of the relative positions and orientations of the {gamma}-tubulins. In all of the structures the {gamma}-tubulins are distinctly separated, a configuration incompatible with the microtubule lattice. The separation of the {gamma}-tubulins in isolated {gamma}-TuSC likely plays a role in suppressing its intrinsic microtubule nucleating activity, which is relatively weak until the {gamma}-TuSC is incorporated into higher-order complexes or localized to microtubule organizing centers. We propose that further movement of the mobile arm is required to bring the {gamma}-tubulins together in microtubule-like interactions, and provide a template for microtubule growth.

Address correspondence to: David A. Agard (agard{at}msg.ucsf.edu)






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