A tight-binding interaction between smooth-muscle native thin filaments and heavy meromyosin in the presence of MgATP.

The binding of the Ca2+-regulated native thin filaments from vascular smooth muscle to vascular smooth-muscle heavy meromyosin was measured in the presence of 3 mM-MgATP. At 25 degrees C and I 0.25 binding had an affinity of 1 X 10(-6)-0.3 X 10(-6) M-1 with a stoichiometry of one molecule bound to one actin monomer. The Km for the activation of heavy-meromyosin ATPase was 20-50 microM. Thin filament-heavy meromyosin binding was not altered by Ca2+ (pCa 9-4) or the extent of myosin phosphorylation. With skeletal-muscle heavy meromyosin affinity was 0.023 X 10(6) M-1 in parallel with activation of the ATPase (Km 54 microM). It is concluded that tight binding is specific to smooth-muscle proteins and that it is not related to the ATPase activation site.