Identification of Human Bone Morphogenetic Protein‐2 <i>Elbow</i> Epitope as Its Coreceptor Recognition Site and Design of <i>Elbow</i> ‐Derived Hairpin Peptides to Specifically Target the Coreceptor

Human bone morphogenetic protein-2 (BMP2) is an effective osteoinductor in bone formation and growth via binding to its cognate receptors and coreceptors. Traditionally, the protein is identified to possess a conformational wrist epitope and a linear knuckle epitope that can be recognized by type I and type II receptors, respectively. In this study, we defined an additional epitope termed elbow as the specific binding site of its coreceptor, which is concentrated into a small helical hairpin region of BMP2 monomer 1 and partially overlaps with wrist but is far away from knuckle. A linear lEEP peptide is derived from the elbow epitope, which, however, is intrinsically disordered and cannot be maintained in native helical hairpin conformation due to lack of BMP2 protein context support, thus only exhibiting a low affinity to coreceptor. Disulfide stapling strategy is then used to stabilize the helical hairpin conformation of linear lEEP, which results in its two cyclic counterparts. The stapling is demonstrated to effectively improve peptide affinity and also exhibits a high selection for coreceptor over type-I receptor. The cyclic peptides are found to maintain a well-ordered, native-like conformation, which can be readily recognized by coreceptor through a conformational selection mechanism.