Site-specific labelling of proteins with a rigid lanthanide-binding tag

This paper describes a generic method for the site-specific attachment of lanthanide complexes to proteins through a disulfide bond. The method is demonstrated by the attachment of a lanthanide-binding peptide tag to the single cysteine residue present in the N-terminal DNA-binding domain of the Escherichia coli arginine repressor. Complexes with Y³⁺, Tb ³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm ³⁺ and Yb³⁺ ions were formed and analysed by NMR spectroscopy. Large pseudocontact shifts and residual dipolar couplings were induced by the lanthanide-binding tag in the protein NMR spectrum, a result indicating that the tag was rigidly attached to the protein. The axial components of the magnetic susceptibility anisotropy tensors determined for the different lanthanide ions were similarly but not identically oriented. A single tag with a single protein attachment site can provide different pseudocontact shifts from different magnetic susceptibility tensors and thus provide valuable nondegenerate long-range structure information in the determination of 3D protein structures by NMR spectroscopy.