Bld10p (gold particles) localizes to the proximal part of the basal body (bb).

Centrioles and surrounding centrosomal material nucleate both mitotic spindles and interphase microtubule asters, but centrioles are not an easy structure to dissect with either biochemistry or genetics. There are budding yeast mutants that arrest at various stages of maturation of the spindle pole body (SPB), but the SPB and mammalian centriole bear no physical resemblance to each other despite sharing some proteins and fulfilling the same function.

Matsuura et al. (page 663) therefore resort to a trick of Chlamydomonas biology. In this unicellular green algae, the basal body has its normal function of nucleating flagellar microtubules during interphase, but it also switches to act as a centriole during cell division. This dual role gives the authors an easily identifiable phenotype for the isolation of mutants defective in centriole function.

Using insertional mutagenesis, the team isolated 74 Chlamydamonas mutants lacking flagella. Most of the mutants lack flagellum-specific proteins, but one mutant, bld10, which grew very slowly and exhibited abnormal cell division, piqued their interest. Upon closer examination, they found that bld10 mutants had no basal bodies. Before this discovery, it was not known if the complete elimination of basal bodies would result in death. These findings suggest that insertional mutagenesis could be used to find and characterize the more than 200 other potential players involved in basal body assembly.

Immunogold labeling showed that Bld10p localizes to the cartwheel, a complex circular structure where microtubules originate. The authors speculate that Bld10p's localization means that the cartwheel plays a critical part in putting together basal bodies. ▪