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 G␣ i -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-

Background: Apelin receptor represents a therapeutic target for cardiovascular diseases. Results: Apelin 17 activates ERK1/2 in a ␤-arrestin-dependent and G protein-dependent manner, whereas apelin 17 with deleted C-terminal phenylalanine only signals through the G protein. Conclusion:Biased signaling promoted by an apelin fragment lacking the C-terminal phenylalanine is favoring G i over ␤-arrestin. Significance: Apelin receptor ␤-arrestin signaling may account for apelin hypotensive activity.

Zhang et al . (Research Articles, 11 November 2005, p. 996) reported that obestatin, a peptide derived from the ghrelin precursor, activated the orphan G protein–coupled receptor GPR39. However, we found that I 125 -obestatin does not bind GPR39 and observed no effects of obestatin on GPR39-transfected cells in various functional assays (cyclic adenosine monophosphate production, calcium mobilization, and GPR39 internalization). Our results indicate that obestatin is not the cognate ligand for GPR39.