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Genetic Evidence for Interaction between - and ß-Tubulins

Centre de Génétique Moléculaire, CNRS, 91198 Gif-sur-Yvette,¹ Ecole Supérieure de Physique et Chimie Industrielles, 75005 Paris,² Institut de Biologie Moléculaire des Plantes, CNRS, 67083 Strasbourg Cedex, France³

Received 3 October 2003/ Accepted 12 December 2003

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, to , characterized in P. tetraurelia. In order to analyze -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 ß-PT2 gene coding for a ß-tubulin, while the mutation noc^r1 conferring nocodazole resistance and localized in another ß-tubulin gene, ß-PT3, was shown to enhance the mutant phenotype. The interaction between -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 -tubulin, like - and -tubulins, might be a microtubule minus-end capping molecule. The likely function of -tubulin as part of a complex specifically involved in basal body biogenesis is discussed.