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

Abstract

Myosin rod was prepared by papain proteolysis of rabbit 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 was of greater MW than obtained previously, so that the combined MW 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 3 fragments in 2 mM sodium pyrophosphate, pH 9.3 at 3.degree., over a large range of protein concentrations. Analysis of the relaxatin of birefringence, at low protein concentration where there was no aggregation, showed that LMM (relaxation time 13.1 .mu.s) behaved as a rigid cylinder. Rod (relaxation time 41.2 .mu.s) and S-2 (relaxation time 6.0 .mu.s) had relaxation rates that were too fast for rigid molecules of their dimensions and therefore were not straight rods. Myosin rod was flexible in the S-2 portion, presumably in the region susceptible to proteolysis. The implications of rod flexibility for the mechanism of muscle contraction were discussed.