Binding manner of actin to the lysine-rich sequence of myosin subfragment 1 in the presence and absence of ATP.

Actin was cross-linked to myosin subfragment 1 with a water-soluble carbodiimide both in the presence and in the absence of ATP, and the cross-linking of the N-terminal acidic sequence of actin to the lysine-rich sequence (--KKGGKKK--) at the junction between the 50K and the 20K fragments of lysines in the lysine-rich sequence were compared between the resulting acto-22K fragment and the uncross-linked 22K fragment by using a protein sequencer. It was found that, in the presence of ATP, a very small amount of cross-linked product was produced and, in the product, only one lysine residue which lies closest to the 50K fragment mainly decreased in its amount as compared to the corresponding lysine residue in un-cross-linked 22K. In the absence of ATP, on the other hand, the amounts of all five lysine residues in acto-22K were about 60% those of the corresponding residues in 22K. The results suggest that, in the so-called weakly binding state, the N-terminal acidic sequence of actin interacts infrequently and only at restricted sites of the lysine-rich sequence but it interacts fully over the whole length in the rigor state.