A-band architecture in vertebrate skeletal muscle: polarity of the myosin head array.
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.
Department of Physiology and Neurobiology, University of Connecticut, Storrs 06269-2131, USA. cantino@uconnvm.uconn.edu
M E
ME
Cantino
L D
LD
Brown
M
M
Chew
P K
PK
Luther
J M
JM
Squire
D000818:Animals; D006054:Goldfish; D018482:Muscle, Skeletal / Q000378:metabolism* / Q000648:ultrastructure*; D009218:Myosins / Q000378:metabolism* / Q000648:ultrastructure*
D009218:Myosins