Structural basis of the 9-fold symmetry of centrioles Cell Kitagawa Laboratory, Centrosome Biology Laboratory Structural basis of the 9-fold symmetry of centrioles. Daiju Kitagawa, Ioannis Vakonakis, Natacha Olieric, Manuel Hilbert, Debora Keller, Vincent Olieric, Miriam Bortfeld, Michèle C. Erat, Isabelle Flückiger, Pierre Gönczy, Michel O. Steinmetz Cell 144, 364-375. February 4, 2011 DOI 10.1016/j.cell.2011.01.008 　 The centriole is an ancient organelle characterized by a universal nine-fold radial symmetry and which is fundamental for the assembly of cilia and flagella across eukaryotic evolution. In addition, the centriole is important for assembling the centrosome, the major microtubule organizing center of animal cells and, as such, is critical for genome stability. The genetic material duplicates once and only once per cell cycle, and so do centrioles. In contrast to the mechanisms governing DNA replication, however, those at the root of centriole formation are poorly understood and represent a fundamental open question in biology. In most species, the centriole is assembled around a cartwheel that comprises a central hub from which nine spokes radiate outwards and connect to nine microtubule blades. Here, using a combination of biophysical, biochemical, structural and cell biological approaches, we establish that self-assembly of SAS-6 is at the root of the universal nine-fold symmetry of the cartwheel and thus of centrioles. We demonstrate that the centriolar protein SAS-6 forms rod-shaped homodimers that interact in a pair-wise fashion through their N-terminal domains to form oligomers. We establish that such oligomerization is essential for centriole formation in C. elegans and human cells. We further generate a structural model of the related protein Bld12p from C. reinhardtii , in which nine homodimers assemble into a ring from which nine coiled-coil rod domains radiate outwards. Moreover, we demonstrate that recombinant Bld12p self-assembles into structures akin to the central hub of the cartwheel, which serves as a scaffold for centriole formation. Overall, our findings establish a structural basis for the universal nine-fold symmetry of centrioles. Fig. 1 The root of the nine-fold symmetry of centrioles. (A) A structural model establishes that nine homodimers of C. reinhardtii SAS-6 assemble into a ring. (B) Electron microscopy of C. reinhardtii SAS-6. Recombinant C. reinhardtii SAS-6 self-assembles into structures akin to the central hub of the cartwheel, which serves as a scaffold for centriole formation. Fig. 2 The higher order oligomerization of SAS-6 dimers is essential for centriole formation. In sas-6 ( RNAi ) embryos expressing RNAi-resistant wild-type SAS-6 fused to GFP (GFP-SAS-6RR wild-type), centriole formation was rescued, as demonstrated by the presence of the centriolar protein SAS-4 in each spindle pole. Importantly, however, there was no rescue of centriole formation in sas-6 ( RNAi) embryos expressing GFP-SAS-6RR[I154E] which can not form higher order oligomers. Fig. 3 SAS-6 homodimers dictate the universal nine-fold symmetry of centrioles. ←Back