微管是细胞骨架的一个重要组成部分，在活体中通常是在一个由γ-微管蛋白复合物构成的核心周围以13个原始细丝(protoflilament)为一组的形式来组织的。
这一精确的几何是怎样控制的仍不清楚。现在，由普遍保守的核心微管核化复合物“γ-微管蛋白小复合物” (γTuSC)构成的高级组合体（从酿酒酵母中分离出）的结构已利用冷电子显微镜被确定。该结构为了解γTuSC是怎样建立13重的微管蛋白对称性的提供了线索。（生物谷Bioon.com）
生物谷推荐原文出处：
Nature doi:10.1038/nature09207
Microtubule nucleating γ-TuSC assembles structures with 13-fold microtubule-like symmetry
Justin M. Kollman,Jessica K. Polka,Alex Zelter,Trisha N. Davis& David A. Agard
Microtubules are nucleated in vivo by γ-tubulin complexes. The 300-kDa γ-tubulin small complex (γ-TuSC), consisting of two molecules of γ-tubulin and one copy each of the accessory proteins Spc97 and Spc98, is the conserved, essential core of the microtubule nucleating machinery1, 2. In metazoa multiple γ-TuSCs assemble with other proteins into γ-tubulin ring complexes (γ-TuRCs). The structure of γ-TuRC indicated that it functions as a microtubule template2, 3, 4, 5. Because each γ-TuSC contains two molecules of γ-tubulin, it was assumed that the γ-TuRC-specific proteins are required to organize γ-TuSCs to match 13-fold microtubule symmetry. Here we show that Saccharomyces cerevisiae γ-TuSC forms rings even in the absence of other γ-TuRC components. The yeast adaptor protein Spc110 stabilizes the rings into extended filaments and is required for oligomer formation under physiological buffer conditions. The 8-? cryo-electron microscopic reconstruction of the filament reveals 13 γ-tubulins per turn, matching microtubule symmetry, with plus ends exposed for interaction with microtubules, implying that one turn of the filament constitutes a microtubule template. The domain structures of Spc97 and Spc98 suggest functions for conserved sequence motifs, with implications for the γ-TuRC-specific proteins. The γ-TuSC filaments nucleate microtubules at a low level, and the structure provides a strong hypothesis for how nucleation is regulated, converting this less active form to a potent nucleator.
