Sequence of a highly divergent beta tubulin gene reveals regional heterogeneity in the beta tubulin polypeptide.

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Abstract
The nucleotide sequence of a chicken genomic DNA segment containing the chicken .beta.4 tubulin gene was determined. The predicted amino acid sequence of .beta.4 is surprisingly divergent from that of the chicken .beta.2 gene that encodes the dominant neural .beta. tubulin. .beta.4 differs from .beta.2 at 36 residue positions and encodes a polypeptide that is 4 amino acids longer, yielding a divergence of 8.9% between the 2 .beta. tubulin isotypes. While many of the amino acid substitutions are conservative, several involve significant alteration in the physiochemical properties of the residue. The amino acid substitution positions are not randomly located within the primary sequence but are distinctly clustered; major divergence occurs in the carboxy-terminal region beyond residue 430 and within the 2nd protein coding exon segments of the genes. Large regions of absolute sequence conservation are also present. Certain sequences within the heterogeneous regions are conserved in other species, indicating that these regions are under positive evolutionary selection pressure and are therefore probably essential for some aspect of .beta. tubulin function. Regional amino acid sequence heterogeneity may play an important role in the establishment of functionally differentiated .beta. tubulin polypeptides.