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 filamen ts in muscle contraction rest upon knowing the relative arrangement of the two proteins, Although there exist crystallographic structures fo r 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 do ne ''by eye.'' Here we report fitted AS1 structures obtained using a q uantitative 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 H olmes 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 rota ted azimuthally. Fits using S1 plus loops missing from the crystal str ucture were achieved using a homology search method to predict loop st ructures, These improved fits favor an arrangement in which the Loop a t the 50- to 20-kDa domain junction of S1 is located near the N termin us of actin, Rigid-body movements of the lower 50-kDa domain, which fu rther improve the fit, produce closure of the large 50-kDa domain clef t and bring conserved residues in the lower 50-kDa domain into an appa rently 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 conforma tion of the 50-kDa cleft of S1.