Cryo-EM structures of GPR75 reveal an occluded orthosteric pocket challenging conventional drug discovery paradigms for an anti-obesity target.

The global obesity epidemic, affecting over 650 million adults, demands innovative therapeutics. GPR75 has emerged as a promising anti-obesity target, with genetic evidence linking loss-of-function variants to protection against obesity and type 2 diabetes. However, structural insights have remained elusive due to GPR75's inherent expression and stabilization challenges. Here we present the cryo-EM structures of human GPR75 in apo and Gq-coupled states, achieved through advanced stabilization techniques including NanoBiT and molecular glue approaches. Our structures reveal unique architectural features: a completely collapsed extracellular domain eliminates the traditional orthosteric binding pocket, raising critical questions about previously reported small molecule ligands. GPR75 assumes active-like conformation in both apo and G protein complexed structures through unique molecular switches-the canonical DRY motif is replaced by HRL, abolishing the ionic lock, while a distinctive Lys134-Asp210 salt bridge stabilizes the active conformation without ligand binding. This dramatic structural divergence from conventional GPCRs necessitates alternative therapeutic strategies targeting allosteric sites or protein-protein interactions rather than orthosteric pockets. Our findings establish a crucial structural framework for developing next-generation anti-obesity therapeutics.