Interaction of myosin subfragment 1 with F-actin studied by differential scanning calorimetry

Nucleotide-induced and actin-induced structural changes in SH1-SH2-modified myosin subfragment 1

We compared the structural properties of myosin subfragment 1 (S1) modified at both reactive SH-groups, SH1 (Cys707) and SH2 (Cys697), with the properties of unmodified S1 and SH1-modified S1. It is shown using differential scanning calorimetry (DSC) that SH1 modification has no noticeable influence on the changes in S1 thermal unfolding induced by the formation of S1 ternary complexes with ADP and P(i) analogs (V(i), AlF(4)(-), and BeF(x)). These changes, however, normally expressed in a significant increase of S1 thermal stability, are almost fully prevented by modification of both SH1 and SH2. In contrast, SH2 modification had no effect on the changes induced by the formation of the ternary complexes S1-ADP-V(i), S1-ADP-AlF(4)(-), and S1-ADP-BeF(x) in EPR spectra of S1 spin-labeled at SH1 group. Interaction of S1 with F-actin substantially increased the thermal stability of S1; a similar effect was observed by DSC with both SH1- and SH1-SH2-modified S1. Overall, our results demonstrate that modification of both reactive SH-groups on S1 has no influence on the actin-induced changes of S1 and on the local nucleotide-induced conformational changes in the SH1 group region, but strongly prevents the global nucleotide-induced structural changes in the entire S1 molecule. The results suggest that modification of SH1 and SH2 impairs the spread of nucleotide-induced conformational changes from the ATPase site throughout the structure of the entire S1 molecule, thus disturbing a coupling between the motor and regulatory domains in the myosin head.

Actin and temperature effects on the cross-linking of the SH1-SH2 helix in myosin subfragment 1

Past biochemical work on myosin subfragment 1 (S1) has shown that the bent alpha-helix containing the reactive thiols SH1 (Cys(707)) and SH2 (Cys(697)) changes upon nucleotide and actin binding. In this study, we investigated the conformational dynamics of the SH1-SH2 helix in two actin-bound states of myosin and examined the effect of temperature on this helix, using five cross-linking reagents that are 5-15 A in length. Actin inhibited the cross-linking of SH1 to SH2 on both S1 and S1.MgADP for all of the reagents. Because the rate of SH2 modification was not altered by actin, the inhibition of cross-linking must result from a strong stabilization of the SH1-SH2 helix in the actin-bound states of S1. The dynamics of the helix is also influenced by temperature. At 25 degrees C, the rate constants for cross-linking in S1 alone are low, with values of approximately 0.010 min(-1) for all of the reagents. At 4 degrees C, the rate constants, except for the shortest reagent, range between 0.030 and 0.070 min(-1). The rate constants for SH2 modification in SH1-modified S1 show the opposite trend; they increase with the increases in temperature. The greater cross-linking at the lower temperature indicates destabilization of the SH1-SH2 helix at 4 degrees C. These results are discussed in terms of conformational dynamics of the SH1-SH2 helix.