Structural Basis for the Activation of Proteinase-Activated Receptors (PARs) by Endogenous Ligands

The proteinase-activated receptor (PAR) subfamily of G protein-coupled receptors (GPCRs) include four members, PAR1-PAR4, that play critical roles in hemostasis, thrombosis, embryonic development, wound healing, inflammation, and cancer progression. The PARs share a unique activation mechanism driven by proteinase cleavage at a specific site within the extracellular amino-terminus, exposing a tethered ligand that self-activates the receptor. Subsequent activation allows PAR family members to initiate complex intracellular signaling networks via traditional G protein-mediated pathways and beta-arrestin signaling and, in this way, the PARs link extracellular protease signaling molecules to cellular functions. Despite a primary reliance on biochemical studies for understanding tethered ligand recognition, direct structural visualization of these ligand-receptor complexes has been elusive. Here, we present structural snapshots of activated PAR1 and PAR2 bound to their endogenous tethered ligands, revealing, for the first time, shallow and constricted orthosteric binding pockets and highlighting critical residues involved in ligand recognition and receptor activation. Surprisingly, comparisons with antagonist-bound structures show minimal conformational changes in the TM6 helix, a typical signature of GPCR activation, with large movements of TM7 observed upon activation. These insights lead to the identification of a common mechanism for PAR1 and PAR2 activation and provide a structural template for designing novel antagonists targeting the orthosteric binding site, potentially opening new avenues for therapeutic interventions.