Prostaglandin E₂ (PGE₂), a bioactive lipid derived from arachidonic acid, mediates a broad range of physiological processes through four G protein-coupled receptor (GPCR) subtypes: EP1–EP4. While high-resolution structures of EP2, EP3 and EP4 have been resolved, EP1 remained structurally uncharacterized due to its intrinsic instability, hindering detailed understanding of its Gq-mediated signaling.

In a study published in PNAS, a research team led by Eric H. Xu (XU Huaqiang) and XU Youwei from the Shanghai Institute of Materia Medica of the Chinese Academy of Sciences reported cryo-electron microscopy (cryo-EM) structure of the human EP1 receptor in complex with PGE₂ and the heterotrimeric Gq protein, completed structural atlas of EP receptor family, and revealed EP1-specific mechanisms of ligand recognition and signal transduction.

To overcome the instability of EP1, the researchers employed a multi-pronged engineering strategy, including BRIL fusion, truncation of flexible loops, incorporation of a mini-Gq chimera, and NanoBiT-assisted complex stabilization. They resolved the structure of the EP1–PGE₂–Gq complex at 2.55 Å resolution using single-particle cryo-EM, enabling detailed analysis of both ligand binding and G protein coupling interfaces.

The researchers observed that the activation of EP1 induced a modest outward shift of transmembrane helix 6 (TM6) by approximately 12°, which was notably smaller than the nearly 18° displacement seen in EP2–EP4, suggesting a subtype-specific activation mechanism. Besides, they identified a unique constellation of residues (S42^1.42, H88^2.54, G92^2.58, and F334^7.36) that formed a distinct binding motif for PGE2, absent in other EP receptors.

Functional assays confirmed that these residues are critical for ligand-induced activation. Notably, the researchers found that EP1 diverged from canonical class A GPCR motifs: It lacks conserved DRY sequence and features an unusual cysteine at position 3.51, further highlighting its unique signaling profile.

On the intracellular side, EP1 receptor engaged Gq protein through both conserved and receptor-specific interactions. Residues such as R63^ICL1, E294^6.32, and Q298^6.36 contributed to the precise orientation of Gα5 helix within the cytoplasmic cavity. Additional compensatory interactions such as those involving S69^2.35 appeared to stabilize G protein coupling, compensating for missing contacts found in related receptors FP and TP.

The findings of this study not only complete the structural framework of PGE₂ signaling through its receptor family, but also provide a blueprint for developing selective EP1-targeted therapies. Given EP1's involvement in pain, cardiovascular disease, and certain cancers, structure-based drug design informed by this study holds significant translational potential.