PERSPECTIVES ON TUBULIN ISOTYPE FUNCTION AND EVOLUTION BASED ON THE OBSERVATION THAT TETRAHYMENA-THERMOPHILA MICROTUBULES CONTAIN A SINGLE ALPHA-TUBULIN AND BETA-TUBULIN

We have cloned and sequenced the two beta-tubulin genes of the ciliated protozoan Tetrahymena thermophila. The two genes encode identical 443 amino acid peptides which are 99.7% identical to the beta-tubulin proteins of T. pyriformis and 95% identical to human beta1 tubulin. T. thermophila contains only one alpha-tubulin gene (Callahan et al., 1984: Cell 36:441-445). Thus, all of the extremely diverse microtubule structures in this unicellular organism can be formed from a single alpha- and a single beta-tubulin peptide. We have also carried out a phylogenetic analysis of 84 complete beta-tubulin peptide sequences. This analysis supports two hypotheses regarding beta-tubulin evolution and function: 1) Multifunctional beta-tubulins are under greater evolutionary constraint than beta-tubulins present in specialized cells or in cells with very few microtubule related functions, which can evolve rapidly; and 2) Cells which form axonemes maintain a homogeneous population of tubulins.