Distribution of developmental myosin isoforms in isolated A-segments.

Immunogold labelling was used to determine the distribution of myosin isoforms within the A-bands of developing chicken pectoralis muscles. Previous localization studies led to the suggestion that neonatal myosin is preferentially located in the centre of heterogeneous thick filaments that contain either embryonic or adult myosin in addition to neonatal myosin. To further explore the possibility that neonatal myosin may serve to nucleate thick filament assembly, a method was developed to isolate A-segments (arrays of myosin filaments) from myofibrils in the presence of MgATP. A-bands usually dissociate into thick and thin filaments in a relaxing buffer, but the inclusion of an antibody against M-line protein prevented separation of the thick filament array. Well-ordered A-segments, approximately 1.5 microns in length, were prepared from muscles 12, 29, 40 days, and approximately 1 year after hatching. After reaction with monoclonal antibodies specific for neonatal and adult myosins, the A-segments were labelled with gold-conjugated secondary antibodies prior to negative staining. An antibody which cross-reacts with embryonic myosin was used to localize that epitope in A-bands of myofibrils from day 1 and day 3 posthatch muscles. At ages where expression of neonatal myosin was high, extensive gold labelling of A-segments was observed in the electron microscope. However, no preferential distribution of antibodies was observed at any age, independent of whether embryonic or adult myosin was coexpressed with the neonatal myosin, suggesting that neonatal myosin is not segregated to any particular region in the A-bands of developing muscles.