Synthesis of adult myosin light chains by embryonic muscle cultures.

Myosin light chain synthesis has been analyzed in cultures of fast and slow muscles from chicken and quail embryos. Synthesis was assayed by [35S]methionine incorporation and two-dimensional electrophoresis of total cell extracts. Our results show that differentiated cultures of embryonic anterior latissimus dorsi and pectoral muscles synthesize proteins that comigrate on two-dimensional gels with the five myosin light chains of adult fast (pectoral) and slow (anterior latissimus dorsi) muscle. Partial proteolytic digestion and peptide analyses further confirm the identity of these proteins as adult light chains. Cultures of dividing myoblasts do not synthesize any of these fiber type isozymes, and synthesis of the isozymes is initiated at myoblast fusion. Also, myogenic clones drived from single myoblasts differentiate to synthesize these five myosin light chains, indicating that individual myoblasts have the potential to express the synthesis of all fiber type light chain isozymes. We conclude that the primary events in muscle differentiation include the initiation of synthesis of the entire set of adult fast and slow myosin light chain isozymes. The developmental and physiological implications of these results for the establishment of fiber type specificity are discussed.