Electric birefringence study of the solution structure of chymotrypsin-cleaved Acanthamoeba myosin II

After removal of the 66 COOH-terminal amino acids from each of its two heavy chains by chymotrypsin digestion, Acanthamoeba myosin II forms only parallel dimers under conditions in which native myosin II forms bipolar filaments (Kuznicki, J., Cote, G. P., Bowers, B., and Korn, E. D. (1985) J. Biol. Chem. 260, 1967-1972). We have studied the solution structure of the chymotrypsin-cleaved myosin II by electric birefringence. Only two species, known to be monomer and parallel dimer from previous studies, were detected. The contribution to the birefringence decay from dimer increased from about 10 to 70% as the KCl concentration was lowered from 100 mM to 0 in 50% glycerol. At all ionic strengths, the monomer had a relaxation time corrected to water at 20 degrees C of 8.2 microseconds, whereas a relaxation time of 10.3 microseconds was expected for monomers with straight rigid rods. This strongly indicates that the myosin rod in solution is bent. On the assumption that there is a single bend 26 nm from the tip of the tail, as suggested by electron microscopy, it was calculated that the average bend angle would be 110 degrees, in solution, if as seems most likely, the average angle between the two globular heads were 180 degrees. The observed relaxation time of the dimer corrected to water at 20 degrees C was 25 microseconds, independent of ionic strength, which, if the motion of the heads were unrestricted, is consistent with a structure for a parallel dimer in which either the two monomer subunits have straight rigid rods and are staggered by about 28 nm or only one is bent and the stagger is 30 nm. As described in the accompanying Appendix, either of these dimers can be assembled into a bipolar filament compatible with the apparent structure of filaments of native myosin II (Pollard, T.D. (1982) J. Cell Biol. 95, 816-825).