Studies of the cross-bridge movement were undertaken by time resolved studies of contracting frog muscle using low angle x-ray fibe diffraction (Huxley et al. 1981; Irving et al. 1992). While these results are fully consistent with the swinging cross-bridge theory, the complexity of the system and low resolution of the method precludes an unambiguous interpretation. Therefore a simpler model system has been studied - "decorated actin" - an actin filament with a myosin cross-bridge bound to each actin. While the structure of decorated actin in the rigor state (no bound nucleotide) has been extensively studied (Moore et al. 1970) (Milligan and Flicker 1987) corresponding studies in the presence of ATP are difficult since the binding of ATP leads to rapid dissociation of the cross-bridges from actin. Time-resolved electron micrograph studies in fact show no bulk change of the cross bridge orientation on binding ATP before dissociation takes place(Pollard et al. 1993) whereby a reorientation of the lever arm would not have been detected at the resolution attainable. High resolution electron micrographs of actin decorated with smooth muscle myosin, however, show a 30-35° rotation of the lever arm on binding ADP (Jontes et al. 1995; Whittaker et al. 1995). Although the main movement of the lever arm would be expected to be associated with phosphate release since this is a step associated with a large change in free energy, some fraction of the movement could arise from ADP binding and release. Moreover, this movement should be recoverable on adding ADP to actomyosin, which indeed it is. Although the effect has only been found in smooth muscle myosins it is generally important in providing the first direct demonstration of a nucleotide-induced lever-arm swing. More recent work using a spin label attached to a light chain also supports this result (Gollub et al. 1996). Moreover, x-ray diffraction studies of muscle fibres loaded with exogenous smooth muscle cross-bridges show the predicted changes in the fibre diffraction pattern resulting from the lever arm swing on binding ADP (Poole et al. 1997).
Purified myosin cross bridges (S1) can be attached to a substrate and used to transport actin filaments in vitro in the presence of ATP. A study by Spudich et al (Uyeda et al. 1996) shows that the speed of actin transport in motility assays is proportional to the length of the lever arm. Moreover, the fulcrum appears to lie near the broken helix (gg690-710) which contains the especially reactive thiols (SH1, SH2) of myosin. A similar result has been obtained by Manstein and coworkers using a "synthetic" lever arm made from repeating [alpha]-actinin repeats in place of the light chain binding region (Anson et al. 1996).
Mutagenesis studies also indicate the importance of the SH1-SH2 region in controlling movement of the lever arm (Patterson and Spudich 1996). A ggG699A mutation, between the SH1 and SH2 groups, slows myosin transport of actin 100-fold (Kinose et al. 1996).
Specific fluorescent markers attached to the "regulatory" light chain show a small angular movement on contraction (Allen et al. 1995; Irving et al. 1995), whereas the "lever arm hypothesis" expects about 50° rotation. However, if only a fraction (ca. 10-20%) of the cross bridges in active muscle take part in contraction at any one time, the magnitude of this apparent rotation can be proportionally scaled up towards the anticipated value.
The Myosin Cross Bridge has two Conformations
According to the Lymn-Taylor scheme (fig. 2) the myosin cross bridge would be expected to have two discernible conformations: (1) when it first attaches to actin with the products of hydrolysis still bound with the lever at the beginning of the working stroke; and (2) at the end of the working stroke when the phosphate and ADP are released. This sequence is often referred to as the "power stroke". The end state is referred to as "rigor", since it is the state muscle enters on ATP depletion. It is also called "strong" because it binds to actin quite tightly. The initial state is called the "weak binding state" because of its low affinity for actin (see (Geeves and Conibear 1995) ). We might anticipate that these two states of the myosin cross bridge might exist independently from actin and indeed protein crystallography shows this to be the case.
The chicken S1 structure was solved without bound nucleotide. Furthermore, the chicken S1 crystal structure fits excellently into the electron micrograph reconstructions of the strong actin-myosin nucleotide-free interaction (decorated actin) . Therefore the crystal structure of chicken S1 would appear to represent the end of the power stroke or rigor state.
In addition, Rayment et al have studied a crystalline fragment of the dictyostelium myosin II cross-bridge which has been truncated after residue 761 (equivalent to gg781). The truncation eliminates the lever arm and the associated light chains (which aids crystallization). However, the converter domain is still present. The crystal structures of the 761 construct have been determined with a number of ATP analogs, particularly ADP.BeFx (Fisher et al. 1995)and ADP.vanadate (Smith and Rayment 1996). ADP.vanadate complexes are used as analogs of the transition state or possibly of the ADP.Pi state. While the ADP.BeFx state looks similar to rigor, the ADP.vanadate structure shows, compared to the chicken rigor structure dramatic changes in shape of the S1 structure, There is a closing of the 50K upper/lower domain cleft, particularly around the [gamma]-phosphate binding pocket, and large movements in the C-terminal region. The 50K upper/lower domains rotate a few degrees w.r.t. each other around the helix gg648-666 in a way which closes the nucleotide binding pocket (Fig 5) - a movement of some 5Å. At the same time the outer end of the long helix (the so called switch 2 helix, residues gg475-507) bends out 24° at residue V497. This is coupled to a rotation of the converter domain (gg711-781) by 70°. The fulcrum is provided by the mutual rotation of the distal part of the SH1-SH2 helix around the distal part of the switch 2 helix.
Figs 6
and 7A model of this new state is shown in Fig. 6. The coordinates of the missing lever arm have been generated from the chicken coordinates by superimposing the converter domains. For comparison Fig. 7) we have generated the corresponding diagram from the coordinates with ADP.BeFx bound in the active site(Fisher et al. 1995) which, for reasons stated below, we take to be the ADP state. On comparing Figs. 6 and 7 one sees that the end of the lever arm has moved through 12nm along the actin helix axis, which is greater than most estimates of the size of the power stroke. Therefore, it would appear that the ADP.vanadate state is indeed a model of the anticipated "beginning of power stroke" state.
The mechanism for coupling the movement of the lever arm with the status of the nucleotide binding pocket revealed by this structure suggests that the two events are tightly coupled: pocket closed, lever up (beginning); , pocket open, lever down (end).
This state could be influenced by the missing light chains
Since the experiments on dictyostelium myosin have used a truncated form, Smith and Rayment have urged caution in interpreting the results incase they should be caused in some way by the absense of the light chains or effects of crystal packing. In addition, the converter domain is partly disordered. Infact, the model building shows no conflict between the essential light chain and the N-terminal part of the myosin heavy chain. Moreover, a sequence comparison between 80 different myosin sequences (Cope, et al 1996) shows that there is a core myosin sequence which terminates exactly at the point where Smith and Rayment's truncation occured. Beyond this point myosin sequences vary according to the number and type of light chains bound. Therefore, it would appear that there is a good chance that the results with bound ADP vanadate are to be taken at their face value. Moreover, an independent determination of the ADP.BeFxstructure (see below) shows very similar effects to those found with ADP.vanadate although the crystal packing is quite different.
For ATP-hydrolysis the 50k upper/lower domain cleft should close
Fig 5Smith and Rayment (Smith and Rayment 1996) point out the similarity of the active site of myosin in the closed form with ras p21 and other G-proteins and that the enzymes probably share a common mechanism for hydrolysis (Schweins et al. 1995). The differences between the open and closed forms of the myosin cross-bridge in the neighborhood of the active site reside almost entirely in the conformation of the linker region (gg465-470) which joins the 50K upper and lower domains. Smith and Rayment point out that this region is structurally equivalent to the so-called switch 2 region in ras p21 with which it also has a very strong sequence homology. The mutual rotation and closing of the 50k upper/lower domains cleft causes this region to move by about 5Å. In the chicken crystal structure (open), which has no bound nucleotide and should therefore be in the rigor conformation, the switch 2 region is pulled away from the nucleotide binding pocket. A similar movement of the switch-2 region depending on whether di- or tri-nucleotide is bound is also found in the G-proteins. Only in the closed form (ADP.vanadate) can the hydrogen bond between the carbonyl of ggG466 and the [gamma]-phosphate (Fig 5), which is an invariant characteristic of the G-protein active sites, be formed. Because of the importance of ggG466 (and other residues) for hydrolysis it is difficult to see how hydrolysis can proceed in the open (rigor) form which would therefore appear not to be an MgATPase: the closing would appear to be essential for hydrolysis.
ADP.BeFx can produce both open and closed states
ADP.BeFx is thought to be an analogue for ATP. Fisher et al (Fisher et al. 1995) solved the structure of dictyostelium S1 truncated at 761 with ADP.BeFx bound in the active site and found it to be remarkably similar to chicken S1 without nucleotide. This result appears to show that the structure of the ATP state is "open", which is puzzling since it would not be able to hydrolyze the ATP. Moreover, the attitude of the converter domain (and hence the "neck") is close to rigor which is also unexpected for the ATP state (c.f. Figs. 6 and 4). More recently Schlichting et al (Schlichting et al. 1997) have solved the structure of an ADP.BeFx complex of truncated S1 and find it to be essentially identical to the ADP.vanadate complex. The active site is closed and the converter domain is in the rotated configuration. The construct used in this case was truncated at position 754 and is therefore 7 residues shorter than that used by Fisher et al. This results in a tighter binding of ADP (Kurzawa et al. 1997). Apparently, on account of this difference , in the shorter construct the binding energy of ADP.BeFx is adequate to tip the scales for the closed structure, whereas in the longer construct it was not. Therefore one can picture the transition between the two forms of myosin as being sensitively poised (but well determined - intermediate states have yet to show up): the structure solved by Fisher et al apparently corresponds to the ADP-bound state whereas the structure solved by Schlichting et al corresponds to the ATP-bound state. Comparing the Fisher et al ADP.BeFx state (Fig 6 ) with chicken rigor (Fig 4) there is in fact a 10° movement of the lever arm. This may reflect small changes in the angle of the lever arm induced by the binding of ADP but it could also reflect species differences.
Phosphate release: Actin binds to the open form of the 50k upper/lower cleft and thereby facilitates phosphate release.
The closed structure found with the ADP.vanadate generates a tight hydrogen bonding pattern for the [gamma]-phosphate which probably explains the high phosphate affinity. This interaction in turn is important for stabilizing the closed form. Opening the cleft destroys the [gamma]-phosphate binding pocket. Energy-filter cryo electron microscopy of decorated actin (Schroeder et al. 1997) shows that the cleft may be open in the actin-myosin complex. Therefore it seems very likely that actin binding opens the cleft rather than closes the cleft as was suggested earlier (Rayment et al. 1993). Opening the cleft destroys the phosphate binding site and facilitates [gamma]-phosphate release.by a different route to that by which it entered (a "back door enzyme" (Yount et al. 1995) ).
Although kinetic studies provide evidence that the actin myosin binding in the presence of nucleotide is a multi-step process, there is no structural data on an initial weak binding of the closed form to actin. A consistent scheme may be developed by postulating that there is an additional transitory state, a bent closed form. We suppose that actin binds myosin with one main set of contacts at approximately constant geometry, namely as is seen in the rigor actin-myosin complex (i.e. the open form of myosin) . The 50K lower domain probably forms the invariant contacts to actin: the switch from weak to strong binding probably involves the recruitment of loops (the 50K- 20K loop and the "404" loop) from the 50K upper domain to form the strong binding state . When confronted with myosin in the closed form actin probably binds the 50K lower domain first, which binds actin weakly. The subsequent binding of the loops produces an open form which releases the [gamma]-phosphate and binds actin strongly.
Summary
Crystallographic studies show two distinct structural states for myosin S1: the "open" or "end" conformation which is characterized by the absence of nucleotide (rigor); and the "closed" or "beginning" state, which is favored by binding ATP or the products complex (ADP.Pi). Myosin transports actin by switching between these two states. "Open" and "closed" refer to the status of the ATP binding site which extends from the 50K upper domain across to the 50K lower domain. This in turn is coupled to the rotation of a C-terminal lever arm. In the "closed" form the lever arm is at the beginning of the power stroke whereas in the "open" form it is at the end of the power stoke. The preference for "open" or "closed" is also controlled by binding to actin. We hypothesize that the closed state binds only weakly to actin. On this basis we can correlate the structural states with the Lymn-Taylor cycle.
Starting from an actin-myosin complex at the end of the power stroke, the binding of ATP brings about rapid closure of the cleft and concomitant release from actin. The closed state hydrolyses ATP to ADP. Pi without attaching to actin. Thereafter, the rebinding of myosin in the closed or "beginning" conformation of the products complex to actin opens the cleft to facilitate release of the [gamma]-phosphate. Release of phosphate induces an isomerisation to the open "end" conformation since it is the presence of the [gamma]-phosphate which stabilizes the closed form. The isomerisation results in large changes of angle of the "lever arm" (at the distal part of the myosin head). Since the S1 is strongly attached to actin at this stage this results in a 12nm transport of actin past myosin.
Figure legends
Fig 1. The sarcomere consists of interdigitating myosin (thick) filaments and actin (thin) filaments. As muscle contracts the sets of filaments move past each other. The movement is produced by the myosin cross-bridges which interact cyclically with the actin filaments and move them by a kind of "rowing" action. A muscle fibril consists of many thousand sacomeres arranged in series.
Fig 2. The Lymn-Taylor cycle (Lymn and Taylor 1971): the myosin cross bridge is bound to actin in rigor 45°- position - "down" [1]. ATP binds which leads to very fast dissociation from actin [2]. The hydrolysis of ATP to ADP and Pi leads to a return of the myosin cross bridge to the 90° "up" position whereupon it rebinds to actin [3]. This leads to release of the products and return to [1]. In the last step actin is "rowed" past myosin.
Fig 3. Numerous experiments (mostly negative) indicated that the scheme shown in Fig 1 needed revision: only the distal part of the cross bridge moves (Cooke 1986).
Fig 4 . The structure of the actin myosin complex (Rayment et al. 1993; Schroeder et al. 1993) : shown are (right) five actin molecules in an actin helix (Holmes et al. 1990) and (left) a myosin cross-bridge (S1)(Rayment et al. 1993): shown are: 25K fragment (green); 50K upper fragment (red); 50K lower fragment (white); the disordered chain between the 50K domain and the 20K domain is shown as a yellow loop - note this loop has been modeled; the first part of the 20K domain including the SH2 helix (until 699) is shown light blue; the SH1 helix, converter domain and the C-terminal helix - "the neck" (dark blue): the regulatory light chain (magenta) and the essential light chain (yellow). Figures prepared with GRASP (Nicholls et al. 1991).
Fig 5 A view of the ATP binding site looking out from the actin helix. Shown are: the P-loop (green); an MgATP molecule with the base at the back and the three phosphate groups in the front (carbon yellow, nitrogen blue, phosphate light-blue, oxygen red, magnesium green); parts of the 50K upper domain (red) including the so called "switch-1" region (right); the switch-2 region in the open "ATP" (white) and closed "ADP" (gray) conformations - the conserved glycine (gg466) is shown in gray or white- note that this residue moves about 5Å between the two conformations; the helix (gg648-666) (which acts as fulcrum for the relative rotation of the 50K upper and lower domains (blue)
Fig 6 The "end" state: modeled from the crystallographic data on the dictyostelium myosin motor domain truncated at residue 761 and complexed with ADP.BeFx (Fisher et al. 1995). To establish the orientation w.r.t. the actin helix (right) the 50K upper and lower domains have been superimposed on the corresponding domains in the rigor structure shown in Fig3. Although the motor domain has bound nucleotide, it appears to be close to the rigor state. The missing "neck" region or lever arm (light blue) has been modeled from the chicken S1 data by superimposing the converter domains.
Fig 7 A reconstruction of the "beginning" state from the crystallographic data on the dictyostelium construct truncated at 761 and complexed with ADP.vanadate (Smith and Rayment 1996). Note the 70° rotation of the converter domain. The missing "neck" or lever arm has been modeled from chicken S1 data by superimposing the converter domains. The rotation of the converter domain is controlled by the bending out of the "switch-2" helix.
The end of the lever arm moves about 12nm between the two states.
References
Allen, T. S., C. Sabido-David, N. Ling, M. Irving and Y. E. Goldman (1995). "Transients of fluorescence polarization in skeletal muscle fibers labeled with rhodamine on the regulatory light chain." Biophysical Journal 68: 85S-86S.
Anson, M., M. A. Geeves, S. E. Kurzawa and D. Manstein (1996). "Myosin Motors with artificial lever arms." EMBO. J 15: 6069-6074.
Cooke, R. (1986). "The Mechanism of Muscle Contraction." CRC Crit Rev. Biochem 21: 53-118.
Cope, M. Jamie T. V., Whisstock, J., Rayment, I., and Kendrick-Jones, J. (1996). "Conservation within the myosin motor domain: Implications for structure and function" Structure 4: 969-987.
Engelhardt, W. A. and M. N. Ljubimowa (1939). "Myosine and Adenosinetriphosphate." Nature 144: 668-669.
Fisher, A. J., C. A. Smith, J. B. Thoden, R. Smith, K. Sutoh, H. M. Holden and I. Rayment (1995). "X-ray structures of the myosin motor domain of Dictyostelium discoideum complexed with MgADP.BeFx and MgADP.AlF4-." Biochemistry 34: 8960-72.
Geeves, M. A. and P. B. Conibear (1995). "The role of three-state docking of myosin S1 with actin in force generation." Biophysical Journal 68: 199S-201S.
Gollub, J., C. R. Cremo and R. Cooke (1996). "ADP release produces a rotation of the neck region of smooth myosin but not skeletal myosin." Nature Structural Biology 3: 796-802.
Holmes, K. C. (1996). "Muscle proteins - their actions and interactions." Current Opinion in Structural Biology 6: 781-789.
Holmes, K. C. (1997). "The swinging lever arm hypothesis of muscle contraction." Current Biology 7: R112-R118.
Holmes, K. C., D. Popp, W. Gebhard and W. Kabsch (1990). "Atomic model of the actin filament." Nature 347: 44-49.
Houdusse, A. and C. Cohen (1996). "Structure of the regulatory domain of scallop myosin at 2Å resolution: implications for regulation." Structure 4: 21-32.
Huxley, A. F. and R. M. Niedergerke (1954). "Structural changes in muscle during contraction. Interference microscopy of living muscle fibres." Nature 173: 971-973.
Huxley, A. F. and R. Simmons (1971). "Proposed Mechanism of Force Generation in Striated Muscle." Nature (Lond.) 233: 533-538.
Huxley, H. E. (1957). "The double array of filaments in cross-striated muscle." Biophysic. and Biochem. Cytol. 3: 631-648.
Huxley, H. E. (1958). "The contraction of muscle." Scientific American 199: 66-82.
Huxley, H. E. (1969). "The Mechanism of Muscular Contraction." Science 164:
Huxley, H. E. and J. Hanson (1954). "The cross-striations of muscle during contraction and stretch and their structural interpretation." Nature 173: 973-976.
1356-1366.
Huxley, H. E., R. M. Simmons, A. R. Faruqi, M. Kress, J. Bordas and M. H. J. Koch (1981). "
Millisecond Time-Resolved changes in X-ray Reflections from Contracting Muscle during Rapid Mechanical Transients, Recorded using Synchrotron Radiation." Proc. Natl. Acad. Sci. USA 78: 2297-2301.
Irving, M., V. Lombardi, G. Piazzesi and M. A. Ferenczi (1992). "Myosin Head Movements are Synchronous with the Elementary Force-Generating Process in Muscle." Nature (Lond.) 357: 156-158.
Irving, M., T. St Claire Allen, C. Sabido-David, J. S. Craik, B. Brandmeier, J. Kendrick-Jones, J. E. Corrie, D. R. Trentham and Y. E. Goldman (1995). "Tilting of the light-chain region of myosin during step length changes and active force generation in skeletal muscle." Nature 375: 688-91.
Jontes, J. D., E. M. Wilson-Kubalek and R. A. Milligan (1995). "The brush border myosin-1 tail swings through a 32° arc upon ADP release." Nature 378: 751-753.
Kabsch, W., H. G. Mannherz, D. Suck, E. F. Pai and K. C. Holmes (1990). "Atomic structure of the actin:DNase I complex." Nature 347: 37-44.
Kinose, F., S. X. Wang, U. S. Kidambi, C. L. Moncman and D. A. Winkelmann (1996). "Glycine 699 is pivotal for the motor activity of skeletal muscle myosin." J. Cell Biol. 134: 895-909.
Kühne, W. (1864). Untersuchungen über das Protoplasma und die Contractilität. Leipzig, Verlag von Wilhelm Engelmann.
Kurzawa, S. E., D. J. Manstein and M. A. Geeves (1997). "Dictyostelium discoideum myosin II: characterization of functional myosin motor fragments." Biochemistry 36: 317-23.
Lohmann, K. (1931). "Darstellung der Adenylpyrophosphatsäure aus Muskulatur." Biochem. Z. 233: 460-469.
Lorenz, M., D. Popp and K. C. Holmes (1993). "Refinement of the F-Actin Model against X-Ray Fiber Diffraction Data by the Use of a Directed Mutation Algorithm." J. Mol. Biol. 234: 826-836.
Lymn, R. W. and E. W. Taylor (1971). "Mechanism of Adenosine Triphosphate Hydrolysis of Actomyosin." Biochemistry 10: 4617-4624.
Margossian, S. S. and S. Lowey (1973). "Substructure of the myosin molecule: III Preparation of single- headed derivatives of myosin." J. Mol. Biol. 74: 301-311.
Margossian, S. S. and S. Lowey (1973). "Substructure of the myosin molecule; IV Interactions of myosin and its subfragments with adenosine triphosphate and actin." J. Mol. Biol. 74: 313-330.
Milligan, R. A. and P. F. Flicker (1987). "Structural relationships of actin, myosin, and tropomyosin revealed by cryo-electron microscopy." J Cell Biol 105: 29-39.
Moore, P. B., H. E. Huxley and D. J. DeRosier (1970). "Three-dimensional reconstruction of F-actin, thin filaments, and decorated thin filaments." J. Mol. Biol. 50: 279-295.
Nicholls, A., K. A. Sharp and B. Honig (1991). "Protein folding and association: Insights from the interfacial and thermodynamic properties of hydrocarbons." Proteins 11: 281-296.
Patterson, B. and J. A. Spudich (1996). "Cold-sensitive mutations of dictyostelium myosin heavy-chain highlight functional domains of the myosin motor." Genetics 143: 801-810.
Pollard, T. D., D. Bhandari, P. Maupin, D. Wachstock, A. Weeds and H. Zol (1993). "Direct visualization by electron microscopy of the weakly-bound intermediates in the actomyosin ATPase cycle." Biophys. J 64: 454-471.
Poole, K. J. V., J. Evans, G. Rosenbaum, M. Lorenz and C. R. Cremo (1997). "Micromolar ADP causes large-scale conformational changes in smooth-muscle myosin-S1bound to actin." in preparation :
Rayment, I., H. M. Holden, M. Whittaker, C. B. Yohn, M. Lorenz, K. C. Holmes and R. A. Milligan (1993). "Structure of the actin-myosin complex and its implications for muscle contraction [see comments]." Science 261: 58-65.
Rayment, I., W. R. Rypniewski, K. Schmidt-Base, R. Smith, D. R. Tomchick, M. M. Benning, D. A. Winkelmann, G. Wesenberg and H. M. Holden (1993). "Three-dimensional structure of myosin subfragment-1: a molecular motor." Science 261: 50-8.
Reedy, M. K., K. C. Holmes and R. T. Tregear (1965). "Induced changes in the orientation of the cross-bridges of glycerinated insect flight muscle." Nature (Lond) 207: 1276-1280.
Rosenbaum, G., K. C. Holmes and J. Witz (1971). "Synchrotron radiation as a source for x-ray diffraction." Nature 230: 434-437.
Schlichting, I., A. Becker, D. Manstein and K. C. Holmes (1997). "The structure of dictyostelium myosin truncated at position 754 with bound ADP.BeF3." (in preparation) :
Schroeder, R., I. Angert, W. Jahn and K. C. Holmes (1997). "The structure of the actin-myosin complex determined by energy-filtered cryo-electronmicroscopy." (in preparation) :
Schroeder, R. R., D. J. Manstein, W. Jahn, H. Holden, I. Rayment, K. C. Holmes and J. A. Spudich (1993). "Three-dimensional atomic model of F-actin decorated with Dictyostelium myosin S1." Nature 364: 171-4.
Schweins, T., M. Geyer, K. Scheffzek, A. Warschel, H. R. Kalbitzer and A. Wittinghofer (1995). "Substrate-assisted catalysis as a mechanism for GTP hydrolysis of p21ras and other GTP-binding proteins." Structural Biology 2: 36-43.
Smith, C. A. and I. Rayment (1996). "Active site comparisons highlight structural similarities between myosin and other P-loop proteins." Biophys. J. 70: 1590-1602.
Smith, C. A. and I. Rayment (1996). "X-ray structure of the magnesium(ii).adp.vanadate complex of the dictyostelium-discoideum myosin motor domain to 1.9Å resolution." Biochemistry 35: 5404-5417.
Straub, F. B. (1943). "Actin." Studies from the Inst. of Med. Chem. Univ. Szeged (reprinted by S. Karger, Basel-New York) 2: 3.
Szent-Gyorgyi, A. G. (1953). "Meromyosins, the subunits of myosin." Arch. Biochem. Biophys. 42: 305.
Uyeda, T., P. D. Abramson and J. A. Spudich (1996). "The neck region of the myosin motor domain acts as a lever arm to generate movement." Proceedings Of The National Academy Of Sciences Of The United States Of America 93: 4459-4464.
Whittaker, M., E. M. Wilson-Kubalek, J. E. Smith, L. Faust, R. A. Milligan and H. L. Sweeney (1995). "Smooth muscle myosin moves 35Å upon ADP release." Nature 378: 748-751.
Yount, R. G., D. Lawson and I. Rayment (1995). "Is myosin a "back door" enzyme?" Biophysical Journal 68: 47S-49S.