An actin-binding site on the 20K fragment of myosin subfragment 1

Myosin subfragment 1 (S1) was covalently labeled with a fluorescent dye, N-7-(dimethylamino)-4-methyl-3-coumarinylmaleimide (DACM), and then digested by trypsin to cleave S1 heavy chain into fragments. The DACM-labeled and trypsin-treated S1 was complexed with F-actin and treated with a zero-length cross-linker, 1-ethyl-33-(dimethylamino)propyl carbodiimide (EDC). The cross-linking reaction generated a covalently linked complex of actin and the 20K fragment of S1 heavy chain, which exclusively incorporated the fluorescent dye, to form a fluorescent 65K cross-linked product. The 20K and 65K fluorescent peptides were isolated and purified and then subjected to cyanogen bromide and/or hydroxylamine cleavages. Mapping of fluorescent cleavage products on acrylamide gels revealed that the N-terminal 20 residues of the 20K fragment of S1 heavy chain contained a cross-linking site of actin.