Fiber type-specific distribution of M-band proteins in chicken muscle.

The functions of two myofibrillar proteins, myomesin (Mr 185,000) and M-protein (Mr 165,000), associated with the M-band are as yet unknown. To extend our knowledge of these proteins, we have examined chicken striated muscles with fast and slow contractile properties, e.g., pectoralis major, PLD, ALD, medial adductor, and lateral adductor, to determine the expression and isoform composition of myomesin and M-protein in various muscles and fiber types. The high molecular weight M-band proteins were characterized and quantitated using monoclonal antibodies in immunoblotting and double-antibody sandwich ELISA. Fiber specificity was determined by immuno- and enzyme histochemistry. In addition to the previously reported Mr 195,000 and 190,000 isoforms of myomesin in heart [Grove et al. (1985): J Cell Biol 101:1431], the Mr 185,000 myomesin in skeletal muscles may represent different isoforms in fast and slow muscles on the basis of distinctive degradation patterns. M-protein has the same molecular weight in striated chicken muscles and degradation patterns indicate only one isoform. The low quantities of M-protein in slow muscles were shown to be due to the absence of M-protein in two of the generally recognized slow fiber types, types I and III. Thus, M-protein was present only in fast type II fibers, whereas myomesin was ubiquitous in all fiber types. Whatever the causal relationship, M-protein appears to function in fast motor units composed of type II fibers.