Skeletal muscle is the primary tissue involved in athe movements and production of force in man and all higher animals. Although the number of major pathologies affecting skeletal muscle are relatively few (multiple sclerosis, muscular dystrophy for example), the severity of these pathologies underlines the importance of muscle to the healthy individual, and th need to understand the normal functioning of this tissue. Contraction is known to involve an interaction between two sets of microscopic filaments; the myosin-containing thick filaments and the actin- containing thin filaments. In order to fully understand the contractile cycle it is essential to understand the structure of these filaments. The broad, long term objective of the research is to understand the structure to the contractile cycle. The specific aim of the proposed studies is to elucidate the molecular arrangement of myosin and C-protein (an accessory protein) in the vertebrate thick filament. The studies to accomplish this goal involve; (1) the use of a combination of electron microscopy and computer image analysis to compute a three-dimensional reconstruction of the arrangement of myosin and accessory proteins in filaments isolated from goldfish and chickens; and (2) the use of SDS gel electrophoresis and immunolabelling with antibodies to chicken C-protein to determine whether C-protein is still present on the isolated thick filaments. In addition, the potential of immunolabelling for C-protein in freeze-substituted muscle will be examined for its ability to determine the location of C-protein relative to the crossbridge array. A comparison of the structure of these filaments with the earlier results on the frog thick filament should yield significant insights into athe structure of the vertebrate thick filament.