Conformational changes in actin resulting from Ca2+/Mg2+ exchange as detected by proton NMR spectroscopy

Skeletal muscle actin can be maintained in a monomeric form in very low ionic strength solutions as well as in high concentrations (0.6 M) of MgCl2 or CaCl2. 400-MHz 1H-NMR spectra revealed characteristic changes which show that the conformation of actin alters by exchanging Ca2+ for Mg2+ in the single high-affinity cation binding site. When all low-affinity cation binding sites are filled (in the presence of high concentrations of Ca2+ or Mg2+), the spectra show that actin conformation differs from that in low-ionic-strength buffer. A comparison of actin in 0.6 M CaCl2 and 0.6 M MgCl2 revealed that the environment of only a small number of protons is affected by the exchange. A new proposal for the essential steps involved in actin polymerization is presented.