Abstract
Monoclonal antibodies were used to identify and localize by immunoelectron microscopy epitopes on myosin isozymes. An antibody that reacts with an amino-terminal fragment of the myosin heavy chain maps on the myosin head 140 Å distal to the headrod junction. It identifies an epitope that is shared on adult and embryonic myosin, and detects two transitions in myosin expression during avian pectoralis myogenesis. Another antibody maps to the carboxyl terminus of the myosin rod. It is specific for an adult fast myosin epitope that is not detected in early developing pectoralis muscle. In contrast, an epitope that is present throughout development is identified by an antibody that reacts with a myosin light chain. This light chain epitope is localized at the head-rod junction. These results demonstrate structural changes in widely separated regions of the myosin molecule accompanying the sequential expression of developmental myosin isozymes.