Pancreatic Polypeptide Is Recognized by Two Hydrophobic Domains of the Human Y4 Receptor Binding Pocket

Structural characterization of the human Y₄ receptor (hY₄R) interaction with human pancreatic polypeptide (hPP) is crucial, not only for understanding its biological function but also for testing treatment strategies for obesity that target this interaction. Here, the interaction of receptor mutants with pancreatic polypeptide analogs was studied through double-cycle mutagenesis. To guide mutagenesis and interpret results, a three-dimensional comparative model of the hY₄R-hPP complex was constructed based on all available class A G protein-coupled receptor crystal structures and refined using experimental data. Our study reveals that residues of the hPP and the hY₄R form a complex network consisting of ionic interactions, hydrophobic interactions, and hydrogen binding. Residues Tyr^2.64, Asp^2.68, Asn^6.55, Asn^7.32, and Phe^7.35 of Y₄R are found to be important in receptor activation by hPP. Specifically, Tyr^2.64 interacts with Tyr²⁷ of hPP through hydrophobic contacts. Asn^7.32 is affected by modifications on position Arg³³ of hPP, suggesting a hydrogen bond between these two residues. Likewise, we find that Phe^7.35 is affected by modifications of hPP at positions 33 and 36, indicating interactions between these three amino acids. Taken together, we demonstrate that the top of transmembrane helix 2 (TM2) and the top of transmembrane helices 6 and 7 (TM6–TM7) form the core of the peptide binding pocket. These findings will contribute to the rational design of ligands that bind the receptor more effectively to produce an enhanced agonistic or antagonistic effect.