Embedding a Metal-Binding Motif for Copper Transporter into a Lipid Bilayer by Cu(I) Binding

Peptide-lipid interactions are widely involved with biologically significant phenomena, including the pathogenic mechanisms of protein misfolding diseases and transmembrane protein folding. In this paper, the interaction of the cysteine/tryptophan (Cys/Trp) motif, which is a metal-binding motif of copper transporter (Ctr) proteins, with a lipid bilayer was studied using fluorescence and circular dichroism (CD) spectroscopy. The binding of Cu(I) to the Cys/Trp motif induced a large red-edge excitation shift in the Trp fluorescence, indicating that the Trp residue is located inside the lipid bilayer following complexation of Cu(I) with the Cys/Trp motif. The Stern-Volmer quenching of the Trp fluorescence also supported the Cu(I) binding peptide embedding in the lipid bilayer. The measurement of the CD spectra indicated the increase in β-sheet content of the Cys/Trp motif peptide as a result of Cu(I) binding. These results lead to the conclusion that complexation with Cu(I) induces the change in the secondary structure of the Cys/Trp motif, which results in the peptide embedding in the lipid bilayer. Cu(I)-induced enhancement of the lipid affinity is discussed in terms of the mechanism for copper transport by Ctr.