Rapid combinatorial screening of peptide libraries for the selection of lanthanide-binding tags (LBTs)

Elucidating the structures and functions of novel proteins and their interaction partners is a ubiquitous challenge in laboratories worldwide, escalating the demand for tools for protein study. A small, versatile and multifaceted co-expression tag would therefore be extremely useful to protein scientists: Lanthanide-Binding Tags (LBTs) fulfill such criteria. These tags are remarkably short peptide sequences (15 to 20 residues) comprising only the encoded amino acids; post-expression addition of a lanthanide ion endows the LBTs with the abundant spectroscopic properties of lanthanides. Herein is reviewed a powerful combinatorial screen developed expressly for the synthetic evolution of short calcium-binding motifs into selective, potent binders of terbium and related lanthanide ions. A series of seven libraries resulted in peptides with dissociation constants nearly three orders of magnitude tighter than that of the parent sequence. These libraries utilized two orthogonal linkers on the solid phase support, enabling cleaved peptides to be screened "off-resin", while leaving a portion of the peptide intact on the synthesis resin for later sequencing. Terbium luminescence from the bound LBTs provides the positive selection criterion, and mass spectroscopy using limited capping was implemented for sequence deconvolution. The roles and importance of various residues within the later-generation LBTs are also discussed.