On the mode of 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

Abstract

Evidence is presented that the removal of the alkali L chain subunit from [rabbit] myosin subfragment 1 results in the exposure of a site (or sites) at the carboxyl-terminal region of the H chain that is rapidly digested by trypsin and .alpha.-chymotrypsin. In the case of trypsin digestion, cleavage at this site proceeds at a much higher rate than cleavages at the 2 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 .apprx. 3000 daltons smaller than the corresponding fragment formed by digestion of subfragment 1. The ability of the H chain to reassociate with alkaline L chain at 4.degree. 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 L chain is binding to, or is capable of perturbing, a region of the H chain adjacent to the subfragment 1/subfragment 2 hinge region and support recent proposals that both the DTNB [5,5''-dithiobis(2-nitrobenzoic acid)] L chain and the alkali L chain may be interacting and may be modulating this flexible region of the cross bridge.