Different polymeric forms of actin detected by the fluorescent probe terbium ion

Publisher Website
Google Scholar
Abstract
The interaction of actin [from rabbit skeletal muscle] with Tb3+ was studied by following the fluorescence emitted at 545 nm when the protein was excited at 285 nm in the presence of Tb3+. At low ionic strength, each actin monomer binds, at saturation, 6 Tb3+ with an association constant of 0.8 .mu.M-1. In the presence of 0.1 M KCl the association constant decreases to 0.15 and 0.24 .mu.M-1 at subcritical and overcritical actin concentrations, respectively; the number of the binding sites remains 6. When polymeric actin is formed by the addition of 2 mM MgCl2, the association constant drops to 0.008 .mu.M-1 and the number of the binding sites to 4. The lower number of the Tb3+ binding sites (4) in the actin polymerized by MgCl2 as compared to the number of binding sites (6) of the actin polymerized by KCl is taken as evidence of a looser structure of this latter polymer. Tb3+ does not replace 45Ca2+ at the single, high-affinity site of G-actin. Removal of this Ca2+, in the presence of Tb3+, destroys the typical G- and F-actin structures.