The Mr 165,000 M-protein myomesin: a specific protein of cross-striated muscle cells.

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Abstract
The tissue specificity of chicken 165,000 M-protein, tentatively named myomesin, a tightly bound component of the M-line region of adult skeletal and heart myofibrils, was investigated by immunological techniques. Besides skeletal and heart muscle, only thymus (known to contain myogenic cells) contained myomesin. No myomesin was detected in smooth muscle or any other tissue tested. This was confirmed in vitro on several cultured embryonic cell types. Only skeletal and heart muscle cells, but not smooth muscle or fibroblast cells, showed the presence of myomesin. When the occurrence and the distribution of myomesin during differentiation of breast muscle cells in culture were studied by the indirect immunofluorescence technique, this protein was first detected in postmitotic, nonproliferating myoblasts in a regular pattern of fluorescent cross-striations. EM of sections through young myotubes showed it within the forming H-zones of nascent myofibrils. In large myotubes the typical striation pattern in the M-line region of the myofibrils was observed. Synthesis of myomesin measured by incorporation of [35S]methionine into immunoprecipitable protein of differentiating cells increased sharply after .apprx. 48 h in culture, i.e., at the time when the major myofibrillar proteins are accumulated. No significant amounts of myomesin were found in cells prevented from undergoing normal myogenesis by 5''-bromodeoxyuridine. Myomesin is apparently a myofibrillar protein specific for cross-striated muscle which represents a highly specific marker for cross-striated muscle cell differentiation. Myomesin may play an important role in myofibril assembly and/or maintenance.