The interaction of actin with various cations has been studied at pH 7.6, at 20 °C, by means of gel filtration and centrifugation techniques. In addition to the single high-affinity site, five low-affinity sites with apparent association constants of 5–6 × 103 M−1 for Ca2+Mn2+ and Sr2+ and of 1.6 × 104 M−1 for Ni2+ have been found in the actin molecule. Unlike the other cations, Zn2+ appears to have three high-affinity sites from which it is not removed by Dowex 50 treatment, and eight low-affinity sites with an apparent association constant of 1.3 × 104 M−1.

Competition experiments suggest that Ca2+, Sr2+, Mn2+ and Ni2+ are bound to the same low-affinity sites, some of which are also available for Zn2+. Zn2+ also binds to additional sites which are not available for the other cations.

Binding of 4 mol of either Ca2+, Mn2+, Sr2+, Ni2+ or Zn2+ per mol of actin monomer at the low-affinity sites characterized above causes a complete transformation of the monomeric actin into polymers.

The nonidentity of some of the Zn2+-binding sites with those for the other cations seems to explain structural differences between the polymers formed in the presence of this cation and those produced upon binding of Mn2+, Ni2+ or alkaline earth metals, described in the preceding paper in this journal. The results suggest that formation of the double-stranded F-actin filaments requires the neutralization of negative charge at specific sites in the monomer molecules.

The aggregation of F-actin filaments into paracrystals observed at pH 7.6 in the presence of Ca2+, Mg2+, Sr2+ or Mn2+ at millimolar concentrations seems to involve the binding of these cations to a third class of binding sites with apparent association constants of the order of 102 M−1. The binding parameters to this class of sites could not be determined by the methods used in this work.