Actin monomer conformation under polymerizing conditions studied by proton nuclear magnetic resonance and circular...

Skeletal muscle actin above a critical concentration polymerizes in physiological concentrations of KCl. Earlier studies have concluded that evidence exists for a monomeric species of actin with a conformation distinct from that of G-actin. Re-investigations of these earlier studies, however, have cast doubt on the concept of a new monomeric actin species. In this study we have adopted two methods, high-resolution proton nuclear magnetic resonance and near ultraviolet circular dichroism spectroscopy, to investigate the existence or otherwise of the putative monomer conformation variously called F-monomer, G-actin or KCl-monomer. For proton nuclear magnetic resonance spectroscopy, unmodified actin maintained below its critical concentration as well as higher concentrations of two chemically modified, unpolymerizable actin samples were studied in the absence and presence of KCl. No difference was found in the environment of even a single proton within the entire actin structure. For circular dichroism we studied actin maintained below its critical polymerization concentration and found very little change in ellipticities when KCl was added to the G-actin solution. We therefore conclude that there is no species of actin monomer with a conformation distinct from that of G-actin.