Both carboxy-terminal tails of α- and β-tubulin are essential, but either one will suffice

Microtubules (MTs) are organized into distinct systems essential for cell shape, movement, intracellular transport, and division [1]. Electron crystallographic analyses [2] provide little information about how MTs produce diverse structures and functions, perhaps because they failed to visualize the last 10 residues of the alpha- and the last 18 of the beta-tubulin C-terminal tails (CTTs) (3], which likely play a role in MT diversity. CTTs define conserved, nonallelic isotypes in mammals [4], are major sites of posttranslational modifications (PTMs) (5-7], are binding sites for microtubule-associated proteins (MAPS) [3], and determine MT motor processivity [8]. Using mutagenesis and homologous gene replacement in Tetrahymena thermophila, we analyzed mutations, deletions, tail switches, and tail duplications of alpha- and beta-tubulin CTTs. We demonstrate that a tail is required for the essential function of both alpha- and beta-tubulin. However, the two tails are interchangeable, and cells grow normally with either an alpha or a beta tail on both tubulins. In addition, an alpha gene containing a duplicated alpha C terminus rescues a lethal mutant lacking all known posttranslational modification sites on the beta C terminus but cannot rescue deletion of the beta tail. Thus, tubulin tails have a second essential function that is not associated with posttranslational modification.