Control of the Actomyosin Interaction - NCBI

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and its implications for muscle contraction. Science. 261:548-65. Rayment, I., W. R. Rypniewski, K. Schmidt-Base, R. Smith, D. R.. Tomchick, M. M. Benning, D. A. ...
348s

Biophysical Journal Volume 68 April 1995 348s

Control of the Actomyosin Interaction K. J. V. Poole,* K. C. Holmes,* G. Evans,* G. Rosenbaum,§ 1. Rayment,' and M. Lorenz* *Max Planck Institute for Medical Research, Heidelberg 69120, Germany; *European Molecular Biology Laboratory, Deutsches Elektronen Synchrotron, Hamburg 22603, Germany; §Argonne National Laboratory, Argonne, Illinois 60439; and 1lnstitute for Enzyme Research, University of Wisconsin, Madison, Wisconsin 53705 USA

An atomic model for the rigor complex of F-actin and the myosin head (Si) has been obtained by combining the molecular structures of the individual proteins (Holmes et al., 1990; Lorenz et al., 1993; Rayment et al., 1993b) with the low-resolution electron density maps of the actomyosin complex derived by cryoelectron microscopy (Rayment et al., 1993a; Schroeder et al., 1993). To investigate the role of the regulatory proteins in controlling the formation of this complex we have analyzed the low-angle x-ray diffraction from regulated thin filaments in rabbit muscle at nonoverlap in the presence and absence of Ca2" and have refined an atomic model of actin-tropomyosin (Lorenz et al., 1994) on the basis of these data. Even allowing for some contributions from extended portions of troponin, our modeling shows that tropomyosin must move up to 300 around the filament, when Ca2" binds. In the absence of Ca2" tropomyosin would clearly block myosin binding as visualized in the model of Rayment et al. (1993a); however, highly charged regions of the actin surface remain available for alternative weak electrostatic interactions with myosin. In the presence of Ca2" the model shows that most of the myosin binding surface on actin is uncovered but that tropomyosin still prevents the docking of the tip of the upper 50K domain of S1. We assume, therefore, that in this state of the filament myosin heads would be able to bind in a weakly bound but stereospecific way, which would represent the prepowerstroke complex. These findings are consistent with the results of biochemical kinetic studies (McKillop and Geeves, 1993), which show that the effect of Ca2" on the thin filament is to unblock sites for a weakly binding species of myosin head, and that full unblocking of the site for the strongly binding rigor bridge requires the accumulation of one or two strongly bound heads per tropo-

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myosin length. In fact, a comparison of the tropomyosin position in our calcium state of the filament with electronmicrographs of thin filaments decorated with rigor crossbridges (Milligan et al., 1990) does suggest a small further movement of tropomyosin is induced by the full binding of Si.

REFERENCES Holmes, K. C., D. Popp, W. Gebhard, and W. Kabsch. 1990. Atomic model of the actin filament. Nature. 347:44 49. 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 Bio. 234:826-836. Lorenz, M., D. Popp, K. V. Poole, G. Rosenbaum, and K. C. Holmes. 1994. An atomic model of the unregulated thin filament obtained by x-ray diffraction from orientated actin-tropomyosin gels. In Molecular Motors: Structure, Mechanics and Energy Transduction. Biophysical Society, Airlie, Virginia. Milligan, R. A., M. Whittaker, and D. Safer. 1990. Molecular structure of F-actin and location of surface binding sites. Nature. 348:217-21. Rayment, I., H. M. Holden, M. Whittaker, C. B. Yohn, M. Lorenz, K. C. Holmes, and R. A. Milligan. 1993a. Structure of the actomyosin complex and its implications for muscle contraction. Science. 261:548-65. 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. 1993b. The three-dimensional structure of a molecular motor, myosin subfragment-1. Science. 261:50-58. Schroeder, R. R., Manstein, D. J., Jahn, W., Holden, H. M., Rayment, I., Holmes, K. C., and Spudich, J. A. 1993. The interaction of the Dictyostelium myosin head with actin: a structural model of decorated actin. Nature. 364: 171-174.