We propose to investigate structural features of the interaction of myosin with actin by studying the structure of rigor muscles at the molecular level. The structure analysis will be performed by a variety of complementary techniques designed to analyze electron micrographs of embedded, sectioned muscles and to interpret the X-ray diffraction patterns of intact muscles. Information about the actin-myosin interaction in muscle will be derived both from the physical shape of the attached cross-bridges per se and also from their spatial distribution throughout the muscle. The results from different muscle-types will be used to investigate the importance of different structural features in determining the force producing characteristics of muscles. Electron micrographs of longitudinal and transverse sections of insect rigor muscle will be analyzed by Fourier techniques to give a 3D reconstruction of an "average" cross-bridge structure and compared with the structure derived from model building studies of X-ray diffraction patterns from vertebrate-striated muscles in rigor. The structures of individual cross-bridges will be determined by real-space analyses of tilt-series of electron micrographs of thin and thick sections of insect flight muscle in rigor and will be used to determine if all cross-bridges have the same structure, and to what extent the structure of actin filaments labelled with myosin subfragment-1 describes the cross-bridge structure in muscle. The positions at which cross-bridges attach to actin in rigor-insect muscle will be modelled to investigate the effect of such parameters as myosin-filament symmetry and the two-heads of myosin which at present can not be imaged directly.