Atypical activation and molecular glue-like dimerization mechanism of an intrinsically-biased chemokine receptor

CXCR7, also known as atypical chemokine receptor 3 (ACKR3), is a naturally-biased, {beta}-arrestin-coupled seven transmembrane receptor (7TMR) that lacks productive coupling with heterotrimeric G-proteins. Despite a critical involvement in cancer metastasis, cardiovascular pathophysiology, and inflammatory disorders, the molecular basis of non-canonical activation and functional divergence of CXCR7 remains elusive. Here, we present a complete landscape of CXCR7 activation using a series of cryo-EM structures, and discover an atypical activation mechanism that is distinct from prototypical GPCRs. CXCR7 is maintained in a basal conformation by a unique tripartite ionic-lock involving TM5-TM6, in contrast to a broadly conserved TM3-TM6 ionic-lock in GPCRs, which is disrupted upon receptor activation. Importantly, activation of CXCR7 results in a constricted pocket and distinct surface topology on the intracellular side compared to prototypical GPCRs. Serendipitously, we capture novel dimeric arrangements of CXCR7 with an inter-protomer stitching by a native phospholipid serving as a molecular glue, and identify previously unanticipated extrahelical allosteric sites on the receptor. Surprisingly, in an intermediate state structure of CXCR7, the second extracellular loop (ECL2) displays a self-blocking conformation, in stark contrast to ECL2-mediated self-activating mechanism reported recently for some orphan GPCRs. Finally, we unequivocally establish CXCR7 as an atypical opioid receptor via a large peptide library screening and structure elucidation in complex with distinct opioid peptides imparting full receptor activation. In summary, our study elucidates an atypical mechanism of CXCR7 activation, and establishes it as an alternative, non-canonical opioid receptor target with potential for novel pain therapeutics.