Melting of myosin rod as revealed by electron microscopy. II. Effects of temperature and pH on length and stability of myosin rod and its fragments.

Effects of temperature and pH on intact rabbit and chicken myosin, isolated myosin rods, rabbit subfragment-2 (61 kDa, 53 kDa, and 34 kDa) and chicken light meromyosin (LMM) fragments were tested to induce a phase transition from alpha-helix to coil conformation, within the hinge region. The influence of temperature and pH were studied directly with length determination by electron microscopy. An increase of temperature to 50 degrees C yielded a shortening of 16 nm, 8 to 9 nm and 7 to 11 nm for intact myosin, isolated rods and long S-2 fragments, respectively. The length of the 34 kDa short S-2 and LMM fragments were unchanged. An increase of pH from neutral to pH 8.0 yielded values that were somewhat smaller, e.g. 12 nm, 6 nm and 6 to 8 nm for intact myosin, isolated rods and long S-2 fragments, respectively, whereas the 34 kDa short S-2 LMM fragments were also unaffected. Thus, melting and subsequent shortening is confined to the region between LMM and short S-2 segment, that is the hinge region. Alteration of temperature had a stronger shortening effect than alteration of pH, and shortening of long S-2 was more pronounced under physiological salt conditions as compared with high (0.3 M) salt. The shortening of rods in intact myosin amounted to twice the value observed with isolated rods. The amount of contraction was somewhat smaller in rods than in the 61 kDa and 53 kDa long S-2 fragments.