title: The structure of the gamma-tubulin small complex: implications of its architecture and flexibility for microtubule nucleation.
abstract: The gamma-tubulin small complex (gamma-TuSC) is an evolutionarily conserved heterotetramer essential for microtubule nucleation. We have determined the structure of the Saccharomyces cerevisiae gamma-TuSC at 25-A 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.
authors.affiliation: Department of Biochemistry and Biophysics and the Howard Hughes Medical Institute, University of California, San Francisco, San Francisco, CA 94158, USA.
authors.forename: Justin M
authors.initials: JM
authors.lastname: Kollman
authors.forename: Alex
authors.initials: A
authors.lastname: Zelter
authors.forename: Eric G D
authors.initials: EG
authors.lastname: Muller
authors.forename: Bethany
authors.initials: B
authors.lastname: Fox
authors.forename: Luke M
authors.initials: LM
authors.lastname: Rice
authors.forename: Trisha N
authors.initials: TN
authors.lastname: Davis
authors.forename: David A
authors.initials: DA
authors.lastname: Agard
mesh_terms: D031541:Fluorescence Resonance Energy Transfer; D008854:Microscopy, Electron; D008870:Microtubules / Q000737:chemistry*; D008958:Models, Molecular; D009711:Nucleotides / Q000378:metabolism; D018583:Pliability; D012441:Saccharomyces cerevisiae / Q000737:chemistry* / Q000648:ultrastructure; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry* / Q000648:ultrastructure; D014404:Tubulin / Q000737:chemistry* / Q000648:ultrastructure
chemical_list: D009711:Nucleotides; D029701:Saccharomyces cerevisiae Proteins; C100764:TUB4 protein, S cerevisiae; D014404:Tubulin