Characterization of the tropomyosin present in various chick embryo muscle types and in muscle cells differentiated in vitro.

Tropomyosin, present in various types of chick embryo muscle, has been characterized by two-dimensional gel electrophoresis. In skeletal muscle, it was found that both the alpha and beta subunits exist as two variants, alpha 1, alpha 2 and beta 1, beta 2. The most acidic variants (alpha 2 and beta 2) could be demonstrated to be phosphorylated and, based upon the facts that 1) after phosphatase treatment alpha 2 and beta 2 co-migrate with alpha 1 and beta 1 and 2) in vitro translation of skeletal muscle mRNA produces only alpha 1 and beta 1, we suggest that alpha 2 and beta 2 merely represent the phosphorylated forms of alpha 1 and beta 1. A similar situation is found in differentiated muscle cultures in vitro. In cardiac muscle or in cardiocytes, in culture, the only subunit of tropomyosin which is present (the alpha subunit) is also phosphorylated. However, in smooth muscle, none of the tropomyosin subunits is phosphorylated. The use of various modifications in the second dimension of two-dimensional gel electrophoresis has allowed us to separate completely the alpha subunits of slow and fast muscle tropomyosin and to show that: 1) the cardiac alpha subunit is distinct from either the slow alpha or the fast alpha subunit and 2) in vitro differentiated cells synthesize a tropomyosin which, by co-migration under various conditions, is identical with fast muscle tropomyosin.