title: SAS-6 is a cartwheel protein that establishes the 9-fold symmetry of the centriole.
abstract: Centrioles consist of nine-triplet microtubules arranged in rotational symmetry. This structure is highly conserved among various eukaryotic organisms and serves as the base for the ciliary axoneme. Recently, several proteins such as SAS-6 have been identified as essential to the early process of centriole assembly, but the mechanism that produces the 9-fold symmetry is poorly understood. In C. elegans and Drosophila, SAS-6 has been suggested to function in the formation of a centriolar precursor, a central tube that then assembles nine-singlet microtubules on its surface. However, the generality of the central tube is not clear because in many other organisms, the first structure appearing in the centriole assembly is not a tube but a flat amorphous ring or a cartwheel-a structure with a hub and nine radiating spokes. Here we show that in Chlamydomonas the SAS-6 protein localizes to the central part of the cartwheel and that a null mutant of SAS-6, bld12, lacks that part. Intriguingly, this mutant frequently has centrioles composed of 7, 8, 10, or 11 triplets in addition to 9-triplet centrioles. We presume that, in many organisms, SAS-6 is an essential component of the cartwheel, a structure that stabilizes the 9-triplet structure.
authors: Department of Biological Sciences, University of Tokyo, Tokyo 113-0033, Japan.
authors: Yuki
authors: Y
authors: Nakazawa
authors: Madoka
authors: M
authors: Hiraki
authors: Ritsu
authors: R
authors: Kamiya
authors: Masafumi
authors: M
authors: Hirono
mesh_terms: D020418:Algal Proteins / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D002502:Centrioles / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D002696:Chlamydomonas
chemical_list: D020418:Algal Proteins