Different roles of two gamma-tubulin isotypes in the cytoskeleton of the Antarctic ciliate Euplotes focardii: remodelling of interaction surfaces may enhance microtubule nucleation at low temperature

In ciliates, different microtubular structures are nucleated from diverse Microtubule Organizing Centers (MTOCs). c-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 c-tubulin on different MTOCs and during the cell cycle. In Tetrahymena thermophila, the a- and b-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 a- and b-tubulin transcription. This study deals with the characterization of c-tubulin in the amicronuclear ciliate Tetrahymena pyriformis. Sequence analysis revealed some specific substitutions in nucleotidebinding loops characteristic of the Tetrahymena genus and putative conserved phosphorylation sites located on the external surface of the c-tubulin molecule. c-Tubulin expression during the cell cycle, in the presence of microtubular poisons and after deciliation, was also characterized. We found that c-tubulin mRNA levels are correlated with basal body proliferation and c-tubulin nuclear localization. We also found that c-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 c-tubulin transcription.