PAR4 activation requires a coordinated rearrangement of ECL3 and Thr153 in the tethered ligand binding site formed by TM3 and TM7.
PAR4 Leu310 (rs2227376) in ECL3 has lower receptor reactivity compared to Pro310 and correlates with reduced VTE risk in GWAS analysis.
Protease activated receptor 4 (PAR4) mediates sustained thrombin signaling in platelets and is required for a stable thrombus. PAR4 is activated by proteolysis of the N-terminus to expose a tethered ligand. The structural basis for PAR4 activation and the location of its ligand binding site (LBS) are unknown. Using hydrogen/deuterium exchange (H/D exchange), computational modeling and signaling studies, we determined the molecular mechanism for tethered ligand-mediated PAR4 activation. H/D exchange identified that the LBS is composed by transmembrane domain 3 (TM3) and TM7. Unbiased computational modeling further predicted an interaction between Gly48 from the tethered ligand and Thr153 from the LBS. Mutating Thr153 significantly decreased PAR4 signaling. H/D exchange and modeling also showed that extracellular loop 3 (ECL3) serves as a gatekeeper for the interaction between the tethered ligand and LBS. A naturally occurring sequence variant (P310L, rs2227376) and two experimental mutations (S311A and P312L) determined that the rigidity conferred by prolines in ECL3 are essential for PAR4 activation. Finally, we examined the role of the polymorphism at position 310 in venous thromboembolism (VTE) using the INVENT consortium multi-ancestry GWAS meta-analysis. Individuals with the PAR4 Leu310 allele had a 15% relative risk reduction for VTE (odds ratio [OR]: 0.85; 95% CI: 0.77-0.94) compared to the Pro310 allele. These data are consistent with our H/D exchange, molecular modeling, and signaling studies. In conclusion, we have uncovered the structural basis for PAR4 activation and identified a previously unrecognized role for PAR4 in VTE.