THE A-TUBULE AND B-TUBULE OF THE OUTER DOUBLERS OF SEA-URCHIN SPERM AXONEMES ARE COMPOSED OF DIFFERENT TUBULIN VARIANTS

The alpha beta-tubulin heterodimer, the structural unit of microtubule s, comes in many variants, There are different alpha and beta isotypes encoded by multigene families, Additional heterogeneity is generated by a set of posttranslational modifications, Detyrosination of alpha-t ubulin, removal of the carboxy-terminal Glu-Tyr dipeptide of alpha-tub ulin, phosphorylation of some tubulins, polyglutamylation, and polygly cylation of alpha- and beta-tubulins all involve the acidic carboxy-te rminal region, We have investigated the distribution of tubulin varian ts in the axonemal microtubules of sea urchin sperm flagella by immuno logical procedures and by direct sequence and mass spectrometric analy sis of the carboxy-terminal peptides, The A and B tubules that compris e the nine outer doublers differ strongly in tubulin variants, A tubul es contain over 95% unmodified, tyrosinated alpha beta-tubulin. In B t ubules, alpha-tubulin is similar to 65% detyrosinated and both alpha-a nd beta-tubulin are 40-45% polyglycylated, These results show a segreg ation of tubulin variants between two different axonemal structures an d raise the possibility that posttranslational modifications of tubuli ns reflect or specify structurally and functionally distinct microtubu les.