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

• Published in: Journal of muscle research and cell motility 2000, Vol.21 (7), p.681-690
• Main Authors: Cantino, M E, Brown, L D, Chew, M, Luther, P K, Squire, J M
• Format: Article
• Language: English
• Subjects: Animals; Binding sites; Goldfish; Muscle, Skeletal - metabolism; Muscle, Skeletal - ultrastructure; Muscular system; Myosins - metabolism; Myosins - ultrastructure; Proteins
• ISSN: 0142-4319; 1573-2657
• DOI: 10.1023/A:1005661123914

Despite extensive knowledge of many muscle A-band proteins (myosin molecules, titin, C-protein (MyBP-C)), details of the organization of these molecules to form myosin filaments remain unclear. Recently the myosin head (crossbridge) configuration in a relaxed vertebrate muscle was determined from low-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 A-spaced crossbridge level particularly prominent. However, X-ray diffraction cannot determine the polarity of the crossbridge array relative to the neighbouring actin filaments; information crucial to a proper understanding of the contractile event. Here, electron micrographs of negatively-stained goldfish A-segments and of fast-frozen, freeze-fractured plaice A-bands have been used to determine the resting myosin head polarity relative to the M-band. In agreement with the X-ray data, the prominent 429 A-spaced striations are seen outside the C-zone, where no non-myosin proteins apart from titin are thought to be located. The head orientation is with the concave side of the curved myosin heads (containing the entrance to the ATP-binding site) facing towards the M-band and the convex surface (containing the actin-binding region at one end) facing away from the M-band.