MYOSIN SUBFRAGMENT-1 INHIBITS DISSOCIATION OF NUCLEOTIDE AND CALCIUM FROM G-ACTIN

The dissociation rates of 1,N6-ethenoadenosine 5'-triphosphate (epsilonATP) and of Ca2+ from G-actin and its complex with myosin subfragment 1 (S1) were measured by recording a large decrease in the fluorescence intensity of the dissociating nucleotide. Under the experimental conditions employed, the binary G-acto-S1A2 complex does not polymerize (Chaussepied, P., and Kasprzak, A. A. (1989) Nature 342, 950-953). The released nucleotide was hydrolyzed either by alkaline phosphatase or by apyrase; to trap Ca2+, EDTA was used. From the anisotropy of N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine (1,5-IAEDANS)-actin, it was established that during the dissociation of epsilonATP, the G-acto-S1 complex remained stable and the equilibrium of the system was unaltered. The reactions followed first order kinetics. The dissociation rate constant, k(d) for epsilonATP decreased from 5.5 x 10(-2) s-1 for free G-actin to 1 x 10(-4) s-1 for G-acto-S1A2; for Ca2+, k(d) was also similarly reduced from 2.8 x 10(-2) s-1 to 4 x 10(-3) s-1. Two proteolytically derived actin variants were also examined. For free subtilisin-cleaved actin, k(d) for epsilonATP was elevated 2-fold but was almost unchanged for Ca2+. In the complex of the cleaved G-actin with S1A2, k(d) for both epsilonATP and for Ca2+ were reduced. The removal of the last 3 amino acids from actin produced a derivative whose behavior in binding to S1, as well as in the kinetics of epsilonATP and Ca2+ dissociation, was undistinguishable from the unmodified protein.