A-band architecture in vertebrate skeletal muscle: polarity of the myosin head array

Despite extensive knowledge of many muscle A-band proteins (myosin molecule s, titin, C-protein (MyBP-C)), details of the organization of these molecul es to form myosin filaments remain unclear. Recently the myosin head (cross bridge) configuration in a relaxed vertebrate muscle was determined from lo w-angle X-ray diffraction (Hudson et al. (1997), J Mol Biol 273: 440-455). This showed that, even without C-protein, the myosin head array displays a characteristic polar pattern with every third 143 Angstrom -spaced crossbri dge level particularly prominent. However, X-ray diffraction cannot determi ne the polarity of the crossbridge array relative to the neighbouring actin filaments; information crucial to a proper understanding of the contractil e event. Here, electron micrographs of negatively-stained goldfish A-segmen ts and of fast-frozen, freeze-fractured plaice A-bands have been used to de termine the resting myosin head polarity relative to the M-band. In agreeme nt with the X-ray data, the prominent 429 Angstrom -spaced striations are s een outside the C-zone, where no non-myosin proteins apart from titin are t hought to be located. The head orientation is with the concave side of the curved myosin heads (containing the entrance to the ATP-binding site) facin g towards the M-band and the convex surface (containing the actin-binding r egion at one end) facing away from the M-band.