Introduction
Bipolar myosin filaments in the striated muscle A-band1 are built from myosin molecules, each of which has a 1600 -Å-long rod region (a two-chain coiled-coil α-helical structure), on one end of which are two elongated globular myosin heads (cross-bridges). The heads are ATPases and interact with actin filaments to produce force and movement.2 In vertebrate striated muscle myosin filaments, the myosin molecules pack together into bipolar filaments, with myosin rods forming the filament backbone and with myosin heads in quasi-helical arrays on the filament surface.1,3 The backbone also has on its surface additional protein components such as titin4,5 and C-protein.6 The central region of myosin filaments where rod packing is anti-parallel is called the bare zone or M-region (Fig. 1a).1,7,8 It was originally called the bare zone, since there are no myosin heads in this region (especially in synthetic filaments). However, in muscle, this part of the filament is not at all bare; there are other proteins on the filament surface, such as the M-band proteins and titin, that make the term M-region more appropriate. A possible molecular packing scheme for the myosin molecules in the M-region was proposed by Squire,9 as was a general structure with dihedral 32-point group (D32) symmetry proposed by Luther et al.10
The M-band in the middle of the M-region in vertebrate striated muscles contains myomesin, M-protein, and the C-terminal region of titin. Each of these three proteins is an assembly of IgI-like and fibronectin-3-like domains.4,5,11-23 The M-band also contains the muscle-specific form of creatine kinase (MM-CK17) and the proteins obscurin and obscurin-like-1.24,25 Since the M-region part of titin also contains a kinase domain perhaps near M6,19,26 it is clear that the M-band possesses not only a structural role but also important metabolic and regulatory properties. In addition, there are diseases associated with mutations in the M-band proteins (e.g., Lange et al.,18 Fukuzawa et al.,24 Ehler and Gautel,27 and Ottenheijm et al.28).
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Myosin filaments have two main structural regions: the cross-bridge region and the bare zone or M-region. We have previously investigated the three-dimensional (3D) structure of the cross-bridge region of myosin filaments from fish skeletal muscle in the relaxed state by both X-ray modelling29 and electron microscopy, combined with single-particle analysis.30 Electron microscopy and single-particle analysis have also been used to study the structure of vertebrate cardiac myosin filaments.31,32
In the present study, the M-regions of myosin filaments isolated from goldfish skeletal muscle under relaxing conditions and as viewed in negative stain by electron microscopy were selected, treated as segments, and subjected to 3D single-particle analysis. The final 3D reconstruction reveals details of density distributions in the five main nonmyosin densities at the M-band (M6′, M4′, M1, M4 and M6). The analysis maintained the well-documented axial spacing of the M-band density peaks and also revealed additional densities (e.g., M3, M3′, M8, M8′ and projecting density at M9). Two crown levels at the M-region edge (probably P2 and P3) were identified clearly by comparison with the corresponding crown levels in our previously published bridge region reconstructions.30,31 Another crown level was tentatively identified as P1. The centres of mass of opposite P1 (P1,P1′) levels are about 1540 Å apart, with the 1600-Å rods probably fully overlapped and with the projecting heads tilting back slightly towards the M-band. In addition, there are densities (e.g., M3, M8 and M9) whose protein composition has yet to be identified.
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