Perspectives on tubulin isotype function and evolution based on the observation that Tetrahymena thermophila microtubules contain a single α‐ and β‐tubulin

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Abstract
We have cloned and sequenced the two β‐tubulin genes of the ciliated protozoan Tetrahymena thermophila. The two genes encode identical 443 amino acid peptides which are 99.7% identical to the β‐tubulin proteins of T. pyriformis and 95% identical to human β1 tubulin. T. thermophila contains only one β‐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 α‐ and a single β‐tubulin peptide. We have also carried out a phylogenetic analysis of 84 complete β‐tubulin peptide sequences. This analysis supports two hypotheses regarding β‐tubulin evolution and function: (1) Multifunctional β‐tubulins are under greater evolutionary constraint than β‐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.