Noncanonical agonist-dependent and -independent arrestin recruitment of GPR1.

G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptors have diverse signaling properties with differential preferences for downstream pathways. Certain receptors, such as the chemerin receptor GPR1, undergo arrestin-mediated internalization but weak G protein signaling. However, the mechanisms of this unusual signaling pattern and its physiological relevance are unclear. We report the structures of GPR1 bound to chemerin and β-arrestin 1 or β-arrestin 2 and an agonist-free GPR1-β-arrestin 1 complex. Upon agonist stimulation, the receptor binds the two arrestins in distinct interaction patterns, which may account for their differential cellular responses. Agonist-independent internalization was mediated by an inactive, constitutively phosphorylated GPR1 that accommodates β-arrestin 1 in an unconventional pocket together with a fatty acid, which potentially provides a basis for GPR1 modulating lipid accumulation in lipid-overloaded adipocytes.