Myosin subunit composition in human developing muscle.

Previous pyrophosphate-gel studies have reported the existence of embryonic neonatal myosin isoenzymes in human developing muscle. The present investigation was undertaken to characterize their subunit composition more precisely. Two immature muscle myosins are contrasted with adult myosin: neonatal myosin and foetal myosin. The neonatal form of myosin is weakly cross-reactive with rabbit slow myosin and contains only fast-type light chains (LC), LC1F and LC2F. The associated heavy chains consist of a single electrophoretic component that reacts exclusively with antibodies against human foetal myosin and has a mobility and peptide pattern distinct from that of adult fast and slow heavy chains. Foetal myosin is distinguished by the presence of low amounts of a heavy chain immunologically cross-reactive with the adult slow form and of two additional light-chain components: a LC2S light chain and a foetal-specific light chain (LCemb.). The foetal-specific light chain, as shown by one-dimensional-peptide-map analysis, is structurally unrelated to both LC1S and LC1F light chains of human adult myosin. We conclude from these results that the ontogenesis of human muscle myosin shares certain common features with that observed in other species, except for the persistence until birth of a foetal form of heavy chain (HCemb.).