The Mg2+-induced conformational change in rabbit skeletal muscle G-actin

The divalent cation Mg2+ binds specifically and tightly to a specific site on rabbit muscle G-actin, and it has been shown previously that such binding involves a conformational change in the monomeric actin as measured by a time-dependent change in the fluorescence of G-actin labeled with the fluorescent probe N-iodoacetyl N'-(5-sulfo-1-naphthyl)ethylenediamine (Frieden, C., Lieberman, D., and Gilbert, H. R. (1980) J. Biol. Chem. 255, 8991-8993). The characteristics of specific Mg2+-induced conformational changes using this labeled G-actin have been examined. The data are consistent with an initial poor binding of Mg2+ followed by an isomerization process resulting in tight binding. Mg2+ and Ca2+ compete for this site and the isomerization induced by Mg2+ can be reversed with high Ca2+ concentrations. The overall dissociation constant for Mg2+ binding is highly pH-sensitive, becoming larger with decreasing pH, implying that ionizable groups may control Mg2+ and Ca2+ binding and that these cations bind preferentially to the unprotonated form. Mg2+ binding to this site increases the off rate constant of the tightly bound ATP relative to Ca2+ by about 6-fold. It is suggested that Mg2+ binding to this site may be related to hydrolysis of the ATP.