Cryo-EM structure of orphan G protein-coupled receptor GPR21

GPR21 belongs to class A orphan G protein-coupled receptor (GPCR). The endogenous ligands for human GPR21 remain unidentified. GPR21 expression is associated with developing type 2 diabetes (T2DM), a multifactorial metabolic disease caused by pancreatic beta-cell dysfunction, decreasing insulin production, insulin resistance, and obesity. Animal studies suggested that GPR21 is a potential therapeutic target for T2DM treatment. The underlying mechanisms leading to GPR21 self-activation remain unknown. In our co-expression analysis, we noted that GPR21 could also form a stable complex with an unreported G alpha protein subtype, G alpha s. To gain further insights into the structural mechanisms of GPR21 activation, we employed cryo-electron microscopy (cryo-EM) and single-particle analysis to resolve the high-resolution structure of GPR21-G alpha s complexes. The clear electron density map of the GPR21-G alpha s provided direct evidence that GPR21 could couple to G alpha s protein at physiological conditions. Thus, GPR21 might mediate previously unexplored pathways in normal or pathological conditions, which warrants further investigation. Structure-guided mutagenesis and biochemical analysis revealed that extracellular loop 2 (ECL2) of GPR21 is essential for the receptor transducing intracellular signal via cAMP. Together, the new structure data reveal a novel signaling cascade of human GPR21 mediated by ECL2 and provide fundamental information for future structure-based drug development.

Automated summary

GPR21 is a class A orphan GPCR associated with developing T2DM. Cryo-EM and single-particle analysis revealed the high-resolution structure of GPR21-G alpha s complexes, providing direct evidence of their coupling at physiological conditions. Mutagenesis and biochemical analysis showed that ECL2 of GPR21 is essential for transducing intracellular signals via cAMP.