Binding Toxic Metals to New Calmodulin Peptides

Abstract

A 33-amino acid peptide corresponding to the helix-turn-helix motif of the calcium binding site I of Paramecium tetraurelia calmodulin has been synthesized and its binding loop stabilized by a specific disulphide bond. Analysed by electrospray mass spectrometry (ES-MS), circular dichroism (CD) and fluorescence, such a cyclic peptide is found to bind calcium, cadmium, terbium and europium ions with native-like affinity, and with 30±1 µM and 8±4 µM dissociation constants for calcium and cadmium ions, respectively. Metal binding induces an ordered conformation in the peptide, resembling that of the calmodulin site I. Interestingly, uranium, in the uranyl form, binds this peptide, as revealed by ES-MS, CD and fluorescence spectroscopy. Sequence mutation aiming to increase the binding cavity suppresses binding of calcium, cadmium and uranium ions, but preserves binding of lanthanide ions, showing that metal selectivity can be modulated by specific mutation in the binding loop. Such disulphide-stabilized peptide may represent a useful model to engineer new metal specificity in calmodulin variants. These novel proteins may be useful in the development of new biosensors to monitor metal pollution and to augment metal binding capability of bacterial and plant cells that can be used in biosorption techniques to (bio)remediate soils and waters contaminated by heavy metals.