M-protein from chicken pectoralis muscle: isolation and characterization.

Proteins of chain molecular weights 170,000, 90,000 and 43,000 have been ascribed to the M-line in the literature; whether all the components are indeed from this region remains to be determined. We present here studies on the 170,000 and 90,000 molecular weight components isolated from high-salt extracts of washed chicken pectoralis muscle. Three components have been purified and characterized, two of chain molecular weight 170,000 and one of 90,000.The 90,000 chain molecular weight protein is identified as the enzyme glycogen phosphorylase b; it has physical characteristics and values of specific activity and pyridoxal-5′-phosphate content similar to those reported for this enzyme. Antibodies to the purified protein do not bind to the M-line region.The two 170,000 chain molecular weight proteins comigrate on sodium dodecyl sulphate/polyacrylamide gels in a band between myosin and C-protein designated “b” by Starr &; Offer (1971) in their classification of myosin contaminants. Both proteins are single chain molecules with a low α-helix content. They are distinguished by sedimentation coefficients of 5.1 S and 7.1 S. Antibodies to the 5 S protein bind strongly to the M-line, whereas those to the 7 S protein weakly stain the Z-line. The 7 S protein is identified as glycogen debranching enzyme. We conclude from these studies that of the three components isolated, only the 5 S protein is a likely constituent of the M-line.