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.
|Original language||English (US)|
|Number of pages||9|
|Journal||European Journal of Biochemistry|
|State||Published - Jul 1978|