J. Am. Chem. Soc. 138, 3553–3561 (2016)

Membrane-spanning proteins are notoriously challenging to study owing to their hydrophobic nature and tendency to form aggregates; this makes them difficult to purify and recalcitrant to solid-phase peptide synthesis (SPPS), the primary tool for the production of proteins with precise post-translational modifications or site-specific labeling. To facilitate the synthesis of membrane-spanning proteins by SPSS, Zheng et al. developed a removable backbone modification (RBM), consisting of one or more arginine residues, that is straightforward to install during the synthesis of transmembrane regions and facile to remove once synthesis of the peptide is complete. The RBM tag is compatible with a variety of amino acid sequences and improves handling of the synthesized peptide by increasing its aqueous solubility and disrupting aggregation. Using this method, the authors achieved the synthesis and purification of a selectively 15N-labeled p7 ion channel from hepatitis C virus, enabling them to study its ligand interactions by NMR, as well as the synthesis of the four-transmembrane Escherichia coli protein EmrE. While this method is currently limited to use in small- to medium-length peptides, the compatibility of this RBM with native chemical ligation suggests that it could in future be used to synthesize larger membrane-spanning proteins.