Temperature-dependent conformational transition in the head-rod junctional region of the myosin molecule.

The effects of temperature, Mg2+, ATP, and actin on the conformation of the neck region of the myosin head were studied by limited proteolysis of heavy meromyosin (HMM) and subfragment 1 (S1) preparations obtained by papain digestion of myosin in the presence of Mg2+ (Mg-S1) or EDTA (EDTA-S1). The preparations were fluorescently labelled at the SH1 thiol group to enable identification of the COOH-terminal fragments of the head portion of the heavy chain where this group is located. The results indicate that the head-rod junctional region of the myosin heavy chain contains at least three different sites readily susceptible to trypsin at 25 degrees C if the light chain LC2 or its LC2' fragment are absent. The susceptibility of one of these sites dramatically decreases when the temperature is lowered to 0 degree C, indicating a temperature-dependent conformational transition in the head-rod junction. With the method used, this transition is detectable only in LC2/LC'2-deficient preparations since all three sites are protected, although to different extents, by LC2 and its LC'2 derivative. It is, however, most probable that the effect of the light chain is confined to steric hindrance of trypsin access and that the temperature-dependent structural transition in the head-rod junction can occur in the presence of intact LC2 as well and may contribute to the temperature sensitivity of force generation in muscle.