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

Abstract

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°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 K(m) for the activation of heavy-meromyosin ATPase was 20-50 μM. 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 106 M-1 in parallel with activation of the ATPase (K(m) 54 μM). It is concluded that tight binding is specific to smooth-muscle proteins and that it is not related to the ATPase activation site.