Prostanoids [prostaglandins (PGs) and thromboxanes (TXs)] are a series of bioactive lipid metabolites that function in an autacoid manner via activation of cognate G protein-coupled receptors (GPCRs). The nine subtypes of prostanoid receptors (DP1, DP2, EP1, EP2, EP3, EP4, FP, IP, TP) are involved in a wide range of functions, including inflammation, immune response, reproduction, and homeostasis of the intestinal mucosa and cardiovascular system. Among the prostanoid receptors, the structure of antagonist-bound DP2, which belongs to the chemoattractant receptor family, was previously determined. However, the mechanisms of prostanoid recognition and receptor activation remained elusive. To address this issue, we determined the crystal structures of antagonist-bound EP4 and PGE2-bound EP3. The EP3-PGE2 complex exhibits an active-like conformation, including outward movement of the cytoplasmic end of transmembrane (TM) 6 relative to the cytoplasmic end of TM6 of the EP4 complex. The carboxyl moiety of PGE2 is recognized through three hydrogen bonds formed by highly conserved residues: Y1142.65, T206Extracelluar loop 2 (ECL2), and R3337.40 (superscripts denote Ballesteros-Weinstein numbering). In addition, the v-chain of PGE2 orients toward TM6, which appears to contribute to receptor activation. The structure reveals important insights into the activation mechanism of prostanoid receptors and provides a molecular basis for the binding modes of endogenous ligands. These findings should facilitate the development of subtype-selective and non-PG-like ligands.
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