The First Step in the Polymerisation of Actin

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Abstract
In the presence of certain cations (e.g., K+ or Mg2+) actin [from rabbit skeletal muscle] polymerizes. Below a certain concentration (the critical concentration) the monomer G-actin does not polymerize on the addition of K+ or Mg2+. The proteolysis experiments of Rich and Estes strongly suggest that cations induce a change in conformation of G-actin leading to a novel form of actin, G*-actin. This conformational change may be the first step in the polymerization of actin. G*-actin induced by K+ was studied by difference spectroscopy. G*-actin is a monomer; bound ATP is not cleaved. G-actin .tautm. G*-actin equilibrium at 4.degree. C was studied as a function of K+ or Mg2+ concentration. With KCl, the transformation is evidently a screening effect. The effect of Mg2+ is more specific and the change in conformation of the G-actin could result from the binding of 2-3 Mg2+ ions/molecule. G-actin .tautm. G*-actin transformation evidently results from the neutralization of a polyanionic region on the actin surface; this region may be the highly negatively charged N terminus.