Flexibility of myosin rod, light meromyosin, and myosin subfragment-2 in solution

Myosin rod was prepared by papain proteolysis of myosin. The components of rod, light meromyosin (LMM) and subfragment-2 (S-2), were prepared by proteolysis of myosin and rod, respectively, using trypsin treated with tosylphenylalanine chloromethyl ketone. S-2, thus prepared, was of greater molecular weight than obtained previously, so that the combined molecular weights of LMM and S-2 were equal to that of rod, and S-2 contained virtually all of the region of the rod susceptible to trypsin. Electro-optical measurements were made on the three fragments in 2 mM sodium pyrophosphate, pH 9.3 at 3 degrees, over a large range of protein concentrations. Analysis of the relaxation of birefringence, at low protein concentration where there was no aggregation, showed that LMM (relaxation time 13.1 micros) behaves as a rigid cylinder. Rod (relaxation time 41.2 micros) and S-2 (relaxation time 6.0 micros) had relaxation rates that were too fast for rigid molecules of their dimensions, and therefore are not straight rods. This implies that myosin rod is flexible in the S-2 portion, presumably in the region susceptible to proteolysis. The implications of rod flexibility for the mechanism of muscle contraction are discussed.