Mechanism of GPR84 allosteric modulation at a helix 8-proximate site

Allosteric modulators offer opportunities for pathway-selective GPCR signalling, but the structural mechanisms enabling biased allosteric modulation remain unclear. Here we identify a helix 8-proximate allosteric site in the immune-metabolic receptor GPR84 and define how it achieves G _i -biased signalling. Cryo-EM structures of the GPR84-G _i complexes bound to the orthosteric agonist OX04539 alone or in combination with the positive allosteric modulator (PAM) PSB-16671 reveal that PSB-16671 binds at the interface of TM1, TM7, and helix 8, a location distinct from previously characterized GPCR allosteric pockets. Molecular dynamics simulations and mutagenesis uncover a polar interaction network linking orthosteric and allosteric sites through conserved residues including Asp66 ^2.50 , Asn104 ^3.36 , and Asn362 ^7.45 . Unexpectedly, disrupting this network enhances allosteric cooperativity, indicating that conformational flexibility within the network is essential for allosteric communication. PSB-16671 stabilizes a receptor conformation with pronounced TM6 displacement that favours Gi coupling while disfavouring β-arrestin recruitment. This G _i -biased profile sustains macrophage phagocytosis of cancer cells without the desensitization induced by balanced agonists. Sequence analysis suggests that helix 8-proximate allosteric sites may be broadly targetable across class A GPCRs, while receptor-specific contacts enable selective modulation. These findings establish structural and mechanistic principles for biased allosteric modulation applicable beyond GPR84.