Crystalline actin tubes

The effect of the trivalent cations scandium (Sc³⁺) and yttrium (Y³⁺) on the conformation of G-actin was examined using ultraviolet difference and high resolution ¹H-NMR spectroscopy. A comparison was made with data obtained previously with the trivalent lanthanide cations (Ln³⁺). These results indicate that the first and subsequent Y ions (ionic radius 101.9 pm) behave exactly like Ln³⁺. Sc³⁺ is a smaller ion (87 pm) than any of the Ln³⁺. The first Sc³⁺ binds to a site on actin that is inaccessible to Mg²⁺, Y³⁺ and Ln³⁺. However, the second Sc³⁺ to bind behaves like an Ln³⁺. On replacing the native divalent cation (Mg²⁺), both Y³⁺ and Sc³⁺ mobilize the adenine ring of ATP bound to actin, thus exposing underlying residues to the solvent. When Y³⁺ and Sc³⁺ saturate their binding sites on actin, and when the ionic strength is raised to 0.1 M with KCI at pH 6.9, the actin aggregates. Y³⁺ binds to actin with a ratio of 6 : 1 and induces the aggregation of actin into crystalline actin tubes, whilstSc³⁺ Sc³⁺binds with a ratio of 8 : 1 and induces amorphous actin aggregates. These results are consistent with the suggestion that actin tubes are induced by trivalent cations, principally on the basis of their binding stoichiometry, which in turn is determined by ionic radius.