Cell Cycle-dependent Expression of γ-Tubulin in the Amicronuclear Ciliate Tetrahymena pyriformis

In ciliates, different microtubular structures are nucleated from diverse Microtubule Organizing Centers (MTOCs). γ-Tubulin is a tubulin superfamily member that plays an essential role in microtubule nucleation at the MTOCs. However, little is known about mechanisms regulating the activity of γ-tubulin on different MTOCs and during the cell cycle. In Tetrahymena thermophila, the α- and β-tubulin expression is regulated mainly at the transcriptional level, and changes in the ratio of polymerized/unpolymerized tubulin dimers lead to an increase or decrease of α- and β-tubulin transcription. This study deals with the characterization of γ-tubulin in the amicronuclear ciliate Tetrahymena pyriformis. Sequence analysis revealed some specific substitutions in nucleotide-binding loops characteristic of the Tetrahymena genus and putative conserved phosphorylation sites located on the external surface of the γ-tubulin molecule. γ-Tubulin expression during the cell cycle, in the presence of microtubular poisons and after deciliation, was also characterized. We found that γ-tubulin mRNA levels are correlated with basal body proliferation and γ-tubulin nuclear localization. We also found that γ-tubulin expression changes during anti-microtubular drugs treatment, but does not changes during reciliation. These findings suggest a relationship between the level of unpolymerized tubulin dimers and γ-tubulin transcription.