The first step in the polymerisation of actin.
In the presence of certain cations (e.g. K+ or Mg2+) actin polymerizes. Below a certain concentration (the critical concentration) the monomer G-actin does not polymerize on the addition of K+ or Mg2+. However, the proteolysis experiments of Rich and Estes [J. Mol. Biol. 104, 777--792 (1976)] 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. We have studied G*-actin induced by K+, by difference spectroscopy. We show that G*-actin is a monomer and we confirm that the bound ATP is not cleaved. We also studied the G-actin in equilibrium with G*-actin equilibrium at 4 degrees C as a function of K+ or Mg2+ concentration. With KCl, the transformation can be accounted for as 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 two or three Mg2+ ions/molecule. We suggest that the G-actin in equilibrium with G*-actin transformation results from the neutralization of a polyanionic region on the actin surface and that this region could be the highly negatively charged N terminus.
D000199:Actins / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D007700:Kinetics; D046911:Macromolecular Substances; D008274:Magnesium / Q000494:pharmacology; D009132:Muscles / Q000378:metabolism; D011189:Potassium Chloride / Q000494:pharmacology; D011817:Rabbits; D013056:Spectrophotometry, Ultraviolet; D014783:Viscosity
D000199:Actins; D046911:Macromolecular Substances; D011189:Potassium Chloride; D000251:Adenosine Triphosphatases; D008274:Magnesium