On the mode of the alkali light chain association to the heavy chain of myosin subfragment 1. Evidence for the involvement of the carboxyl-terminal region of the heavy chain.

Evidence is presented that the removal of the alkali light chain subunit from myosin subfragment 1 results in the exposure of a site (or sites) at the carboxyl-terminal region of the heavy chain that is rapidly digested by both trypsin and alpha-chymotrypsin. In the case of trypsin digestion, cleavage at this site proceeds at a much higher rate than cleavage at the two other sensitive regions located in the interior of the primary structure of this chain. This initial cleavage is responsible for the generation, on further digestion with trypsin, of a carboxyl-terminal fragment about 3000 daltons smaller than the corresponding fragment formed by digestion of subfragment 1. The ability of the heavy chain to reassociate with alkali light chain at 4 degrees C in the presence of MgATP is essentially abolished by cleavage at this exposed site by either trypsin or chymotrypsin. These observations indicate that the alkali light chain is binding to, or is capable of perturbing, a region of the heavy chain adjacent to the subfragment 1/subfragment 2 "hinge" region and support recent proposals that both the DTNB light chain and the alkali light chain may be interacting and may be modulating this flexible region of the cross bridge.