Changes of lysine reactivities of actin in complex with myosin subfragment-1, tropomyosin and troponin.

The interactions of actin with myosin subfragment-1 and tropomyosin were explored by comparing the reactivities of lysine residues in F-actin alone with those in F-actin complexed to the other proteins. Limited reductive methylation was carried out on F-actin and the F-actin complexes with [14C]HCHO and [3H]HCHO, respectively. After dissociation from the other components, [3H]actin was combined with [14C]actin and 3H/14C of each lysine residue was measured. Myosin subfragment-1 reduced the reactivities of Lys-335 and Lys-372, while tropomyosin reduced those of Lys-237, -325, 327 and -335. When troponin was present in the absence of Ca2+, the effect of tropomyosin on Lys-335 remained the same, but its reactivity was completely restored upon the addition of Ca2+. Thus, the results suggest that different parts of actin are affected by the interaction with myosin subfragment-1 and tropomyosin, but the region containing Lys-335 is commonly affected by the presence of either of them. The change in reactivity is attributable either to a direct steric effect or to an induced conformational effect.