Effect of tryptic cleavage on the stability of myosin subfragment 1. Isolation and properties of the severed heavy-chain subunit.

The procedure of thermal ion-exchange chromatography has been used to examine the effect of prior tryptic cleavage on the stability of myosin subfragment 1 (SF1). Although it is found that digestion does destabilize the subunit interactions at physiological temperatures, the heavy-chain subunit can be isolated either as an equimolar complex comprised of 50K, 27K, and 21K fragments or as one comprised of 50K, 27K, and 18K peptides. Thus, the interactions within the heavy chain are considerably more stable than those between the two subunits. Both forms of the free severed heavy chain exhibit ATPase properties similar to those of the parent tryptic SF1. The Vmax for the actin-activated MgATPase of the free severed heavy chain is the same as that for both undigested and tryptic SF1 (A2). Since its Km for actin is similar to that of tryptic SF1(A2), it may be concluded that changes in the affinity of SF1 for actin induced by trypsin [Botts, J., Muhlrad, A., Takashi, R., & Morales, M. F. (1982) Biochemistry 21, 6903-6905] are not dependent on the presence of the associated alkali light chain. Furthermore, the communication between the SH1 site and the ATPase site is also shown to be independent of the associated alkali light chain, and it persists despite the cleavages present in the free heavy chain. Studies on the ability of these severed heavy chains to reassociate with free A1 and A2 chains indicate that the binding site is retained in the 21K-severed heavy chain but is lost in the 18K form.