Conformational Studies of G‐Actin Containing Bound Lanthanide

Abstract

The presence of bound Ln [lanthanide; Tb, Tm, Ho] ions gives rise to changes in the UV absorbance of [rabbit muscle] G-actin. Previously, Ln ions were shown to mobilize the adenosine moiety of ATP bound to the actin monomer. The changes in UV absorbance appear to be fully attributable to this mobilization. Any structural changes induced in G-actin by the substitution of Ln ions for Ca2+ must be small enough so as not to perturb the environment of any aromatic chromophores other than those exposed by the mobility of the nucleotide. This conclusion is supported by measurements of the rates of proteolytic digestion of actin with and without bound Ln ions. The effect of Ln ions on the actin-bound nucleotide is not simply due to the increase in positive charge on the actin monomer. This is demonstrated by the absence of any UV spectral changes on the addition of a 5-fold excess of Ca2+. Analysis of the UV difference spectrum obtained when Ln ions bind to G-actin indicates that at least 1 tryptophan residue is partially exposed to the solvent coincident with mobilization of the nucleotide. Tb3+ luminescence enhancement studies indicate that there are no Trp, Tyr or Phe residues in the vicinity of the high-affinity cation site. The exposed Trp may be close to the nucleotide.