By interacting with the C-terminal Phe of apelin, Phe$^{255}$ and Trp$^{259}$ in helix VI of the apelin receptor are critical for internalization
Résumé
Apelin is the endogenous ligand of the orphan seven-transmembrane domain (TM) G protein-coupled receptor APJ. Apelin is involved in the regulation of body fluid homeostasis and cardiovascular functions. We previously showed the importance of the C-terminal Phe of apelin 17 (K17F) in the hypotensive activity of this peptide. Here, we show either by deleting the Phe residue (K16P) or by substituting it by an Ala (K17A), that it plays a crucial role in apelin receptor internalization but not in apelin binding or in Gαi-protein coupling. Then we built a homology three-dimensional model of the human apelin receptor using the cholecystokinin receptor-1 model as a template, and we subsequently docked K17F into the binding site. We visualized a hydrophobic cavity at the bottom of the binding pocket in which the C-terminal Phe of K17F was embedded by Trp$^{152}$ in TMIV and Trp$^{259}$ and Phe$^{255}$ in TMVI. Using molecular modeling and site-directed mutagenesis studies, we further showed that Phe$^{255}$ and Trp$^{259}$ are key residues in triggering receptor internalization without playing a role in apelin binding or in Gαi-protein coupling. These findings bring new insights into apelin receptor activation and show that Phe$^{255}$ and Trp$^{259}$, by interacting with the C-terminal Phe of the pyroglutamyl form of apelin 13 (pE13F) or K17F, are crucial for apelin receptor internalization.