Tubulin sequence homology

The building block of a microtubule is the tubulin subunit, a heterodimer of a- and -tubulin. Both of these 55,000-MW monomers are found in all eukaryotes, and their sequences are highly conserved. Although a third tubulin, y-tubulln, is not part of the tubulin subunit, it probably nucleates the polymerization of subunits to form ctB-mlcro-tubules. Encoded by separate genes, the three tubulins exhibit homology with a 40,000-MW bacterial GTPase, called FtsZ (see Figure 5-30b). Tike tubulin, this bacterial protein... 

Fig. 3. Comparison of the amino acid sequences predicted from the a- and j3-tubulin mRNA sequences. Open circles indicate homologous, conservative substitutions, and boxed regions indicate areas of high homology. For completeness, the sequence of the first 25 amino acids of chick brain a-tubulin, deduced by the work of Luduena and Woodward (1973), is also included. Note that the asterisks indicate identical amino acids common to both polypeptide chain sequences. [Reproduced from Valenzuela et al. (1981). Nature (London) 289, 650-655.]...

In contrast to S. cerevisiae, the microtubule cytoskeleton in the fission yeast Schizosacaromyces pombe is more complex and the amino acid sequences of the tubulin polypeptides are closer to mammalian sequences. The fission yeast homologs of cofactors B, D, and E are essential for survival in this organism. It seems highly likely that the cofactors are also essential for life in ciliates, flagellates, and multicellular organisms, in which the sequence of tubulins are also highly conserved. 

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