Genetic evidence for interaction between eta- and beta-tubulins

The thermosensitive allelic mutations sm19-1 and sm19-2 of Paramecium tetraurelia cause defective basal body duplication: growth at the nonpermissive temperature yields smaller and smaller cells with fewer and fewer basal bodies. Complementation cloning of the SM19 gene identified a new tubulin, eta-tubulin, showing low homology with each of the other five tubulins, α to , characterized in P. tetraurelia. In order to analyze η-tubulin functions, we used a genetic approach to identify interacting molecules. Among a series of extragenic suppressors of the sm19-1 mutation, the su3-1 mutation was characterized as an E288K substitution in the β-PT2 gene coding for a β-tubulin, while the mutation noc^r1 conferring nocodazole resistance and localized in another β-tubulin gene, β-PT3, was shown to enhance the mutant phenotype. The interaction between η-tubulin and microtubules, revealed by genetic data, is supported by two further types of evidence: first, the mutant phenotype is rescued by taxol, which stabilizes microtubules; second, molecular modeling suggests that η-tubulin, like γ- and δ-tubulins, might be a microtubule minus-end capping molecule. The likely function of η-tubulin as part of a complex specifically involved in basal body biogenesis is discussed.