Immunochemical analysis of C-protein isoform transitions during the development of chicken skeletal muscle

Isoforms of C-protein in adult chickens which differ in fast (pectoralis major, PM) and slow (anterior latissimus dorsi, ALD) skeletal muscles can be distinguished immunochemically with monoclonal antibodies (McAbs) specific for the respective fast (MF-1) and slow (ALD-66) protein variants (Reinach et al., 1982 and 1983). The expression of these C-proteins during chick muscle development in vivo has been analyzed by immunoblot and immunofluorescence procedures. Neither MF-1 nor ALD-66 reacted with whole-cell lysates or myofibrils from PM of 12-day-old embryos. However, both McAbs bound to peptides of 145 kDa in PM from late embryonic and young posthatched chickens. All of the myofibers in these muscles reacted with both antibodies, but the binding of the anti-slow McAb (ALD-66) diminished progressively with age and was completely negative with PM by 2 weeks after hatching. In contrast, the ALD muscle from 17 days in ovo thru adulthood only reacted with ALD-66; no binding of MF-1 could be detected at these stages. Since both fast and slow myosin light chains (LC) coexist within embryonic pectoralis and ALD muscles (e.g., G. F. Gauthier, S. Lowey, P. A. Benfield, and A. W. Hobbs, 1982, J. Cell Biol. 92, 471-484) yet segregate to specific fast and slow muscle fibers at different stages of development, the temporal transitions of C-protein and myosin LC were compared during myogenesis. "Slow-type" C-protein appeared after the disappearance of slow myosin light chains, whereas the accumulation of the "fast-type" light chains occurred before the expression of "fast-type" C-protein. The pattern of isoform transitions appears to be far more complex than previously suspected.