Interaction of Actin with Divalent Cations 1. The Effect of Various Cations on the Physical State of Actin.

The effect of various divalent cations on the state of aggregation of actin monomers has been studied at pH 7.6 by means of viscosity measurements, determination of the protein sedimenting at high and low centrifugal forces, dephosphorylation of the actin-bound ATP and by observation of the negatively stained preparations under the electron microscope. The metal concentration dependence of the degree of actin polymerization in the presence of Ca²⁺, Mg²⁺, Sr²⁺ and Mn²⁺ is the same. All these cations produce typical double-stranded F-actin filaments. Ni²⁺ and Zn²⁺ induce polymerization at lower concentrations than Mn²⁺ and alkaline earth metals, but the resultant polymers have lower viscosities. Examination in the electron microscope has shown that Ni²⁺ produces typical F-actin filaments, which, however, tend to brake into short fragments. In the presence of Zn²⁺ globular aggregates coexisting with the filaments have been observed. In the presence of Mn²⁺ or alkaline earth metals at millimolar concentrations the F-actin filaments assemble into net-like paracrystals which are transformed into side-by-side aggregates when the cation concentration is increased. The cation concentration dependences of polymerization and of paracrystal formation suggest that these two processes occur upon binding of these cations to distinct classes of sites and that the order of affinities to sites of weaker binding, involved in the paracrystal formation, is as follows: Mn²⁺ > Ca²⁺ > Mg²⁺ = Sr²⁺. Unlike the other cations, Zn²⁺ at concentrations higher than that necessary for maximum polymerization causes precipitation of G-actin without formation of any ordered structures. [ABSTRACT FROM AUTHOR]