Tubulin sequence conservation.

Various aspects of the primary structure of tubulin are discussed and tubulin sequence data are compared. A hypothesis concerning the evolution of tubulin is also presented. The main points raised are: (1) Identical glycyl rich regions in alpha- and beta-tubulin, which are similar in both sequence and predicted secondary structure to a region in several nucleotide binding enzymes, may be involved in binding GTP. (2) Small regions of homology are present to actin, myosin and troponin T. These homologous regions may have the same function, resulting in a convergence of their sequences, or they may have arisen by a pathway of protein evolution which is still only very poorly understood. (3) The mutation rate between pig and chick brain tubulin is 0.22 PAMs (accepted point mutations/100 residues) per hundred million years, which is comparable to that of the histones. At an early time in its history, however, the tubulin heterodimer appears to have had a relatively high rate of mutation. This may have been during the evolution of the first eukaryotes.