Phosphorylated and unphosphorylated forms of cardiac tropomyosin

Cardiac tropomyosin from 20-day-old chick embryos is composed of three different polypeptides with the same molecular weight but different isoelectric points. These polypeptides, which are designated as alpha 1, alpha 2, and alpha 3, have identical peptide maps. In vitro, however, only polypeptide alpha 1 is synthesized in a reticulocyte lysate programmed with cardiac RNA. These results, together with the observations indicating that tropomyosin alpha 2 corresponds to a phosphorylated polypeptide, suggest that only tropomyosin alpha 1 corresponds to a primary translational product and that forms alpha 2 and alpha 3 are derived from alpha 1 as a consequence of posttranslational modifications.