| Centrioles have a highly conserved structure consisting of nine triplet microtubules arranged in a rotational symmetry. How this arrangement is determined is poorly understood. By analyzing two centriole mutants, bld10 and bld12, we found that the cartwheel, a sub-centriolar structure with a hub and nine radiating spokes, is crucial for stabilizing the 9-fold symmetry. bld10 has a null mutation in a coiled-coil protein, Bld10p, that localizes to the cartwheel, and totally lacks the centrioles. Intriguingly, when a truncated Bld10p (ΔC2) that lacks the C-terminal 35% is expressed in bld10, centrioles with eight symmetrically arranged triplets are assembled, in addition to others with the normal nine triplets. From the morphology of the cartwheel spoke in the ΔC2 centriole and immunoelectron microscope localization, we conclude that Bld10p is a major component of the spoke tip that extends the cartwheel diameter, and that the 8-triplet centrioles were formed because the diameter became smaller by the truncation. The other mutant bld12 frequently has centrioles composed of 7, 8, 10, 11 triplets in addition to the normal 9-triplet centrioles. Positional cloning revealed that the gene product of BLD12 codes for a homolog of SAS-6, an essential protein for centriole formation in C. elegans. Evidence suggests that Chlamydomonas SAS-6 functions in arranging the radial array of the cartwheel spokes as a component of the central part of the cartwheel. The cartwheel must be crucial for stable formation of the 9-fold symmetrical structure of the centriole.
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