Evidence for differential post-translational modifications of slow myosin heavy chain during murine skeletal muscle development

The contractile properties of muscle fibres are, in part, determined by the myosin heavy chain (MyHC) isoforms they express. Using monoclonal antibodi es, we show that at least three forms of slow twitch MyHC accumulate sequen tially during mouse fetal development and that slow MyHC maturation in slow fibres occurs before expression of the adult fast MyHCs in fast fibres. Ex pression of deletion derivatives of beta-cardiac MyHC cDNA shows that the s low MyHC epitopes that are detected in adult but not in young animals are l ocated near the N-terminus. The same N-terminal region of various fast MyHC molecules contains a conserved epitope that can, on occasions, be observed when slow MyHC cDNA is expressed in non-muscle cells. The results raise th e possibility that the N-terminal epitopes result from post-translational m odification of the MyHC and that a sequence of slow and fast MyHC isoform p ost-translational modifications plays a significant role during development of murine muscle fibres.