An enzyme-probe method to detect structural changes in the myosin rod.

The temperature-dependence of local melting within the alpha-helical, coiled-coil structure of rabbit myosin rod has been investigated by following changes in the rate constants of proteolytic digestion. The kinetics of fragmentation of the rod by three different enzymes (alpha-chymotrypsin, trypsin and papain) over the temperature range 5 to 40 degrees C (pH 7, I = 0.5) has been monitored by electrophoresis of the digestion products on sodium dodecyl sulfate/polyacrylamide gels. All rate constants were corrected for the intrinsic temperature-dependence of the enzyme by comparison with model substrates. Results from the three enzyme-probes are similar in showing that local melting within the rod occurs in two distinct stages. At temperatures between 5 and 25 degrees C, melting is confined to a restricted segment of the rod structure near the light meromyosin/heavy meromyosin junction. At temperatures between 25 and 40 degrees C, a wider segment of the rod lysing between the junction and the short subfragment-2 segment (the hinge domain) appears to be melting, judging from the broad spectrum of cleavage sites observed in this region. Results are compared with those from other physicochemical methods that measure the hinging or opening of the coiled-coil structure of the rod.