Mechanistic insights into the versatile stoichiometry and biased signaling of the apelin receptor-arrestin complex.

The apelin receptor (APJR) plays a pivotal role in regulating cardiovascular and metabolic health. Understanding the mechanisms of biased agonism at APJR is crucial for drug discovery, as stimulation of the β-arrestin pathway may lead to some adverse effects. Structural analyses of APJR-Gi complexes have clarified the structural basis of receptor dimerization and activation, yet the absence of structural data on APJR-arrestin complexes has impeded a comprehensive understanding of APJR stoichiometry in the dual signaling pathways and biased agonism. Here, we present APJR-β-arrestin1 structures bound to a clinical drug analog, revealing 2:2 and 2:1 stoichiometries associated with differential β-arrestin recruitment. Through comparison of the two transducer-coupled APJR structures bound to the same ligand, we identify key residues and motifs crucial for directing biased signaling. These findings highlight APJR's versatile stoichiometry in coupling with β-arrestin and Gi proteins, establishing a framework for understanding biased agonism and guiding the development of therapeutics.