A divergent testis-specific alpha-tubulin isotype that does not contain a coded C-terminal tyrosine

On the basis of analysis of cDNA clones of alpha-tubulin RNAs expressed during spermiogenesis in chickens, we report the identification of a novel alpha-tubulin which is expressed exclusively in chicken testes. Comparison of its sequence with those previously determined not only demonstrates that the encoded polypeptide is significantly divergent from other alpha-tubulins but also supports the hypothesis that alpha-tubulin isotypes are distinguished by a carboxy-terminal variable region sequence and, to a lesser extent, by a domain near the amino terminus. Since essentially all previously known alpha-tubulins undergo a unique cycle of removal and posttranslational readdition of a tyrosine residue at the extreme carboxy terminus, the presence in this testes alpha-tubulin of a very divergent carboxy terminus that does not contain an encoded tyrosine raises the possibility that this polypeptide does not participate in the usual cycle of tyrosination/detyrosination.