RPtag as an Orally Bioavailable， Hyperstable Epitope Tag and Generalizable Protein Binding Scaffold.

Antibodies are the most prolific biologics in research and clinical environments because of their ability to bind targets with high affinity and specificity. However， antibodies also carry liabilities. A significant portion of the life-science reproducibility crisis is driven by inconsistent performance of research-grade antibodies， and clinical antibodies are often unstable and require costly cold-chain management to reach their destinations in active form. In biotechnology， antibodies are also limited by difficulty integrating them in many recombinant systems due to their size and structural complexity. A switch to small， stable， sequence-verified binding scaffolds may overcome these barriers. Here we present such a scaffold， RPtag， based on a ribose-binding protein (RBP) from extremophile Caldanaerobacter subterraneus. RPtag binds an optimized peptide with pM affinity， is stable to extreme temperature， pH， and protease treatment， readily refolds after denaturation， is effective in common laboratory applications， was rationally engineered to bind bioactive PDGF-β， and was formulated as a gut-stable orally bioavailable preparation.