Structural basis of oligomerization-modulated activation and autoinhibition of orphan receptor GPR3.

G protein-coupled receptor 3 (GPR3) is a class A orphan receptor characterized by high constitutive activity in the G signaling pathway. GPR3 has been implicated in Alzheimer's disease and the regulation of thermogenesis in human adipocytes, yet the molecular mechanisms underlying its self-activation and potential endogenous modulators remain unclear. In this study, we present cryo-electron microscopy (cryo-EM) structures of GPR3 in different oligomerization states, both in the absence and presence of G protein. Notably, in addition to the monomeric form of GPR3, our findings reveal a functional GPR3 dimer with an extensive dimer interface-a feature rarely observed in class A GPCRs. Moreover, oligomerization appears to be linked to a unique autoinhibition mechanism involving intracellular loops, which may regulate GPR3 signaling. Collectively, these results provide new insights into the oligomerization-modulated activation of orphan GPCRs, advancing our understanding of their signaling properties.