The structure of the acto-myosin subfragment 1 complex: Results of searches using data from electron microscopy and x-ray crystallography

Surmises of how myosin subfragment I (S1) interacts with actin filaments in muscle contraction rest upon knowing the relative arrangement of the two proteins, Although there exist crystallographic structures for both S1 and actin, as well as electron microscopy data for the acto-S1 complex (AS1), modeling of this arrangement has so far only been done ''by eye.'' Here we report fitted AS1 structures obtained using a quantitative method that is both more objective and makes more complete use of the data, Using undistorted crystallographic results, the best-fit AS1 structure shows significant differences from that obtained by visual fitting, The best fit is produced using the F-actin model of Holmes ef al, [Holmes, K, C,, Popp, D,, Gebhard, W. & Kabsch, W, (1990) Nature (London) 347, 44-49], S1 residues at the ASI interface are now found at a higher radius as well as being translated axially and rotated azimuthally. Fits using S1 plus loops missing from the crystal structure were achieved using a homology search method to predict loop structures, These improved fits favor an arrangement in which the Loop at the 50- to 20-kDa domain junction of S1 is located near the N terminus of actin, Rigid-body movements of the lower 50-kDa domain, which further improve the fit, produce closure of the large 50-kDa domain cleft and bring conserved residues in the lower 50-kDa domain into an apparently appropriate orientation for close interaction with actin, This finding supports the idea that binding of ATP to AS1 at the end of the ATPase cycle disrupts the actin binding site by changing the conformation of the 50-kDa cleft of S1.