Site-selective protein editing by backbone extension acyl rearrangements

Protein and polypeptide heteropolymers containing non-α-backbone monomers are highly desirable as potential materials and therapeutics but many remain difficult or impossible to biosynthesize in cells using traditional genetic code expansion. Here we describe a next-generation approach to such materials that relies instead on proximity-guided intramolecular rearrangements that edit the protein backbone post-translationally. This approach relies on orthogonal aminoacyl-tRNA synthetase enzymes that accept α-hydroxy acid monomers whose side chains contain masked nucleophiles. Introduction of such an α-hydroxy acid into a protein translated in vivo, followed by nucleophile unmasking, sets up a thermodynamically favored intramolecular backbone extension acyl rearrangement (BEAR) reaction that edits the protein to install an extended-backbone monomer. In the examples described here, BEAR reactions are used to generate protein heteropolymers containing a β-backbone, γ-backbone or δ-backbone. This report represents a general strategy to install extended backbones into genetically encoded proteins and peptides expressed in cells.