Recent neutron scattering studies of muscle contraction and its control.

We have presented two applications of the method of neutron scattering utilizing selective deuteration of actin. In these experiments the actin was rendered effectively invisible to neutrons by matching the scattering-length densities of deuterated actin and the solvent. The scattering of neutrons by myosin S1 and by Tm bound to this actin was studied. For free chymotrypsin-generated S1 it was found that Rg = 4.0 +/- 0.15 nm, while for papain-generated S1 it was found that Rg = 4.6 +/- 0.2 nm. Upon binding of papain-generated S1 to actin at low NS1/N actin ratios, the change in Rg in difference experiments was delta Rg = 0.05 +/- 0.15 nm. This lack of significant change in Rg in the very low-s domain confirms and extends our earlier neutron scattering work in the higher-s domain. The longest chords of S1, as well as shorter ones, are not significantly altered upon actin binding. These results indicate that muscle contraction does not occur as a result of large-scale changes in S1 structure. In actin-Tm complexes, a measurement of the mean cross-helix separation, d, of Tm molecules has been made using neutron scattering. With deuterated actin matched out in 93% D2O buffer, it was found that d = 7.9 +/- 0.3 nm. This value is in good agreement with a model based on Tm crystallography and also with recent electron microscopy results. These experiments demonstrate the feasibility and value of neutron diffraction and scattering techniques in the study of muscle contraction and its control. One can expect that the further employment of emerging cell biology techniques for generating deuterated proteins will aid our understanding of muscle in the future.