Isolation of separate mRNAs for alpha- and beta-tubulin and characterization of the corresponding in vitro translation products.

The messenger RNAs coding for alpha- and beta-tubulin have been isolated from embryonic chick brain. Although the mRNAs for the two tubulin subunits have been resolved on native gels, they are very similar in molecular weight (650,000 daltons) as judged by mobility on denaturing gels containing methy mercury. The mRNAs for beta- and gamma-actin have also been resolved on native gels, but migrate as an unresolved peak (molecular weight 6500,000-700,000 daltons) under denaturing conditions. Since the nonmuscle actins are substantially smaller proteins than alpha- and beta-tubulin, the large size of chick nonmuscle actin mRNAs suggests an unusually long untranslated region. Since tubulin and actin polypeptides are internal structural proteins, one would expect them to be synthesized only on free polysomes. Translation of mRNA derived directly from a purified membrane fraction or by puromycin release from that fraction, however, showed the synthesis of a small proportion of these proteins on polysomes that are membrane-associated. Peptide mapping has in all cases confirmed the identity of the products of cell-free synthesis with authentic alpha-tubulin, beta-tubulin and actin. Approximately 67% of the alpha- and 13% of the beta-tubulin chains produced by in vitro translation are competent for co-assembly into microtubules with added carrier microtubule protein.