X-ray scattering by myosin S-1: implications for the steric blocking model of muscle control

Shape and length of myosin heads

There is controversy concerning the shape and length of myosin heads. In the present paper we try to analyse the data and to draw clear conclusions in this field. When the myosin heads are isolated (S1) from the rest of the molecule, their length is approximately 12 nm and their shape is close to that of a prolate ellipsoid with an axial ratio approximately 2.3 (in solution) or close to that of a comma when attached to F-actin (with a length of 12-13 nm). When the myosin heads are observed on a whole molecule, their length is approximately 19 nm and they are pear-shaped. Here we suggest that all these observations are compatible. We believe that, for a whole myosin molecule, a large part of the head-rod joint (S1/S2 joint) is measured with the head, owing to a particularly heavy staining or shadowing of this joint. On the other hand, S1 is probably built up of a head part plus the S1/S2 joint, which is not revealed by the usual techniques (hydrodynamics, X-ray and neutron scattering). Finally, the comma shape would be related to a flexible part in the head region of S1, which is significantly bent when S1 is attached to F-actin, but which would be less bent for S1 in solution. A similar bending also occurs in crystalline S1.

Behaviour of the crossbridges in stretched or compressed muscle fibres

The maximum chord of the myosin heads is comparable to the closest surface-to-surface spacing between the myofilaments in a muscle at the slack length. Therefore, when the sarcomere length increases or when the fibre is compressed, the surface-to-surface myofilament spacing becomes lower than the head long axis. We conclude that, in stretched or compressed fibres, some crossbridges cannot attach, owing to steric hindrance. When the amount of compression is limited, this hindrance may be overcome by a tilting of the heads in the plane perpendicular to the filament axes; in this case, there is no consequence as concerns the crossbridge properties. In highly compressed fibres, the crossbridges become progressively hindered and all the crossbridges are hindered for an axis-to-axis spacing representing about 60% of the spacing observed under zero external osmotic pressure. In this case, both the isometric tension and the ATPase activity of the fibre are zero. In fibres stretched up to 3.77 microns (sarcomere length corresponding to the disappearance of the overlap between the thick and the thin filaments), the ratio of hindered crossbridges over the functional crossbridges may be estimated at about 55%. In stretched fibres, a noticeable proportion of crossbridges are sterically hindered and the crossbridges performance (e.g. constants of attachment and detachment) depends on filament spacing, i.e. on sarcomere length. Therefore, we think it is probably impossible to consider the crossbridges as independent force converters, since this idea requires that the crossbridge properties are independent of sarcomere length. In this connection, all the experiments performed on osmotically compressed fibres are of major importance for the understanding of the true mechanisms of muscle contraction.

Comparison of the structure of myosin subfragment 1 bound to actin and free in solution. A neutron scattering study using actin made "invisible" by deuteration

The structure of subfragment 1 (S1) bound to F-actin has been compared to the structure of free S1 using neutron scattering. The F-actin was rendered "invisible" to neutrons by selective deuteration and solvent contrast matching. Highly deuterated actin was purified from the slime mold Dictyostelium discoideum, which was fed deuterated Escherichia coli. The properties of this actin were found to be similar to those of protonated actin. The neutron-scattering pattern of S1 bound to this "invisible" actin was compared to that of free S1. At near-physiological ionic strength, a strong interference effect was observed, which arose from pairs of S1 molecules cross-linking actin filaments. However, at low ionic strength the only differences that could be observed were attributed to interference effects between neutrons scattered from S1s bound randomly to equivalent sites on an actin filament. These effects became negligible as the fraction of actin sites occupied by S1 approached zero. Thus, we conclude that the scattering by S1 attached to F-actin is identical with that of free S1, to a resolution of about 2.5 nm. The difference in apparent radii of gyration is less than 0.05 nm. Modeling calculations have been carried out to determine the sensitivity of neutron scattering to possible S1 deformations. The calculations showed that deformations of the structure of S1 that are large enough ultimately to produce a powerstroke of 5 nm or greater are only consistent with the data if they involve at most about 20% of the S1 mass. These results restrict the class of plausible models describing force generation in muscle contraction.

Structure of myosin decorated actin filaments and natural thin filaments

Negatively stained paracrystals of reconstituted thin filaments decorated with myosin subfragment 1 (S1), at high calcium concentrations (greater than or equal to 10(-5) M), exhibit pgg plane group symmetry with component filaments having 28 subunits in 13 turns of the actin genetic helix. Isolated S1 decorated F-actin filaments trapped in a stain film were also observed to form spontaneously paracrystals with pgg plane group symmetry. We conclude that a favourable S1-S1 interaction must exist in order to stabilize these structures. Three-dimensional helical reconstructions, calculated from these paracrystals show S1 to be curved, 12 to 14 nm long and tilted with respect to the helical axis, in broad agreement with previous reconstructions calculated from isolated particles. Reconstructions of S1 and HMM decorated filaments that resolve actin show a principal myosin binding site located on the side of the actin subunit reported by Taylor & Amos [J. molec. Biol. 147, 297-324 (1981)] and a possible small interaction on the opposite side. The appearance, symmetry and helical reconstructions of isolated F-actin filaments decorated with heavy meromyosin (HMM) were similar to those of S1 decorated filaments, except at high radii where extra mass was observed. This probably arose from the connection between the two heads of HMM bound to the same long-pitch strand of actin. In contrast to most studies on thin filaments, which use reconstituted filaments, we present data on natural I-segments of muscle homogenates. Individual filaments exhibited actin helical symmetry which on reconstruction gave a two-domain motif oriented consistently with its long axis approximately perpendicular to the helical axis, but inclined towards the 5.9 nm genetic helix. Our original interpretation of these maps [Seymour & O'Brien, Nature, Lond. 283, 680-2 (1980)] depended upon reconstructions from F-actin paracrystals, which suggested actin was rather symmetrical in shape. New data from two- and three-dimensional crystal studies and reconstructions of actin-tropomyosin filaments show that actin is rather elongated and consists of two domains. These results indicate that actin contributes towards both domains of our I-segment motif and are consistent with the monomer long axis lying approximately perpendicular to the helical axis. Although tropomyosin is not resolved, comparison of the actin-tropomyosin and I-segment reconstructions suggests that tropomyosin is strongly merged with the actin domain at a lower radius from the helical axis and that the domain at higher radius arises solely from actin.

Rabbit skeletal myosin heads in solution, as observed by ultracentrifugation and freeze-fracture electron microscopy: dimerization and maximum chord

The use of analytical ultracentrifugation and freeze-fracture electron microscopy in solution allowed us to observe the monomeric and dimeric forms of Mg.71. This subfragment of the myosin molecule contains the LC2 light chain and is comparable to a "native" myosin head. Sedimentation-diffusion equilibrium ultracentrifugation shows that it is necessary to use slightly different conditions in order to obtain a pure Mg.S1 dimer, as compared to the case of chymotryptic S1 (LC2-free S1). For example, in a buffer leading to a complete dimerization of chymotryptic S1, Mg.S1 is only in the form of a monomer-dimer mixture, with comparable proportions of monomer and dimer. The freeze-fracture technique, applied to solutions containing Mg.S1 or chymotryptic S1, revealed that the monomeric species both have the same maximum chord (about 120 A) and that both dimeric species also have the same maximum chord (about 250 A). The maximum chord of the monomer is comparable to the surface-to-surface spacing between the myosin and actin filaments, in a fiber at the slack length. In sharp contrast this chord is higher than this spacing in a stretched fiber. The consequences of this fact are discussed, with particular reference to the sarcomere length-tension relationship.

Negative staining of myosin molecules

A reproducible method has been developed for the negative staining of myosin molecules. The dimensions of stained molecules are in close agreement with those obtained by metal shadowing. Sharp bends in the tail, indicative of hinge regions, were observed at two positions 44 nm and 76 nm from the head-tail junction. The tail was often ill-defined at the position of the first (44 nm) bend. The bend positions may be sites of proteolytic cleavage that result in the production of long and short myosin subfragment S2. About half the molecules exhibited bending to various degrees at one or both of these positions, but cases where the tail folded back on itself in a 180 degrees bend were comparatively rare (approximately equal to 10%). However, in the absence of EGTA, a large fraction of the molecules (approximately equal to 80%) exhibited 180 degrees bends. A small region, approximately 20 nm long, at the tip of the tail often appears to be significantly different from the rest. The heads are about 19 nm long and roughly pear-shaped. Although sometimes straight, more often they show a pronounced curvature. Both senses of curvature were observed, but those curved in a clockwise manner were the most common, indicating preferential binding of one side of the head to the carbon substrate. An analysis of the different combinations of head shapes in individual molecules indicates that each head can rotate independently around its long axis. No preferred angle of orientation between the two heads in a molecule, or between either head and the tail could be found. Substructure has been observed within the heads.

Functionality of muscle proteins in gelation mechanisms of structured meat products

Recent advances in muscle biology concerning the discoveries of a large variety of proteins have been described in this review. The existence of polymorphism in several muscle proteins is now well established. Various isoforms of myosin not only account for the difference in physiological functions and biochemical activity of different fiber types or muscles, but also seem to differ in functional properties in food systems. The functionality of various muscle proteins, especially myosin and actin in the gelation process in modal systems which simulate structured meat products, is discussed at length. Besides, the role of different subunits and subfragments of myosin molecule in the gelation mechanism, and the various factors affecting heat-induced gelation of actomyosin in modal systems are also highlighted. Finally, the areas which need further investigation in this discipline have been suggested.

Structure of the myosin projections on native thick filaments from vertebrate skeletal muscle

Rabbit psoas muscle filaments, isolated in relaxing buffer from non-glycerinated muscle, have been applied to hydrophilic carbon films and stained with uranyl acetate. Electron micrographs were obtained under low-dose conditions to minimize specimen damage. Surrounding the filament backbone, except in the bare zone, is a fringe of clearly identifiable myosin heads. Frequently, both heads of individual myosin molecules are seen, and sometimes a section of the tail can be seen connecting the heads to the backbone. About half the expected number of heads can be counted, and they are uniformly distributed along the filament. The majority of heads appear curved. The remainder could be curved heads viewed from another aspect. Three times as many heads curve in a clockwise sense than in an anticlockwise sense, suggesting a preferential binding of one side of the head to the carbon film. The two heads of myosin molecules exhibit all the possible combinations of clockwise, anticlockwise and straight heads, and analysis of their relative frequencies suggests that the heads rotate freely and independently. The heads also adopt a wide range of angles of attachment to the tail. The lengths of heads cover a range of 14 to 26 nm, with a peak at 19 nm. The average maximum width is 6.5 nm. Both measurements are in excellent agreement with values for shadowed molecules. Since our data are from heads adsorbed to the film in relaxing conditions and the shadowed molecules were free of nucleotide, gross shape changes are not likely to be produced by nucleotide binding. The length of the link between the heads and the backbone was found to vary between 10 nm and 52 nm, with a broad peak at about 25 nm. Thus, the hinge point detected in the tail of isolated molecules was not usually the point from which the crossbridges swung out from the filament surface. The angle made by the link to the filament axis was between 20 degrees and 80 degrees, with a broad maximum around 45 degrees. These lengths and angles concur with our observation of an average limit of the crossbridges from the filament surface of 30 nm. This is sufficient to enable heads in the myofibril lattice to reach out beyond the nearest thin filament and should allow considerable flexibility for stereospecific binding to actin in active muscle.

X-ray scattering by single-headed heavy meromyosin. Cleavage of the myosin head from the rod does not change its shape

Low angle X-ray scattering from heavy meromyosin (HMM) and from single-headed heavy meromyosin (sHMM) have been examined to determine if the heads of myosin change shape when cleaved from the rod to form subfragment 1 (S1). The scattering intensities of intact HMM and sHMM were compared with those of their chymotryptic digestion products, S1 and subfragment 2 (S2). As the data with HMM were complicated by scattering between the two heads, the more extensive analysis was done with sHMM. Pseudo-Guinier plots of intact and digested sHMM, over the angular range used previously for S1, were linear and showed a difference in apparent radius of gyration (Rg) of only 0.07 +/- 0.04 nm. The absolute apparent Rg value of sHMM was 3.2 +/- 0.2 nm, which is comparable to the radius of gyration reported previously for S1 alone. A plot of the fractional differences in scattering intensities of intact and digested sHMM was flat to a reciprocal spacing of at least 1/3.5 nm-1. These results indicate that the head portions of sHMM and S1 have very similar structures at low resolution. Scattering curves for various models of sHMM and mixtures of S1 and S2 were calculated and the fractional difference plots of scattering intensities were made to determine how sensitive this type of analysis is to changes in the shape of the head. Changes in Rg of 0.1 nm or greater gave detectably non-flat difference plots. Thus, the X-ray scattering of sHMM (and HMM) demonstrated that differences in structure between the head of myosin and isolated S1 are likely to be small. Current controversies over myosin head structure are discussed in light of this result.

Small-angle X-ray scattering from myosin heads in relaxed and rigor frog skeletal muscles

Low-angle X-ray diffraction patterns from relaxed and non-overlap rigor muscles show a central region of diffuse scattering (disk) which is circularly symmetrical, behaves as solution scattering and comes predominantly from myosin heads. In full-overlap rigor the disk is compressed in the diagonal direction, indicating that the myosin heads have a bent shape and a preferred orientation consistent with a 45 degree angle of attachment to actin.