Changes in tropomyosin during development of chick embryonic skeletal muscles in vivo and during differentiation of chick muscle cells in vitro.

We have studied the changes in tropomyosin subunits and their pattern of phosphorylation during embryonic development and also during in vitro differentiation of cultured muscle cells. We have found that in 10-day-old embryos tropomyosin consists of 75% beta subunit and 25% alpha subunit. However, only 60% of the beta subunit is phosphorylated while the alpha subunit is almost exclusively present in its phosphorylated form. During subsequent development, the amount of alpha-tropomyosin increases so that by hatching it represents 50% of the total subunits. During the same time, the amount of phosphorylated variants for both the alpha and the beta subunits decreases so that after hatching they represent less than 20% of each subunit. In cultures of differentiating myoblasts, the alpha subunit is synthesized before the beta subunit, and within each subunit, the unphosphorylated variants are the first to be synthesized. The reason for this discrepancy is not known at the present time.