Mapping Human Protease-activated Receptor 4 (PAR4) Homodimer Interface to Transmembrane Helix 4

Thrombin activates platelets by binding and cleaving protease-activated receptors 1 and 4 (PAR1 and PAR4). Because of the importance of PAR4 activation on platelets in humans and mice and emerging roles for PAR4 in other tissues, experiments were done to characterize the interaction between PAR4 homodimers. Bimolecular fluorescence complementation and bioluminescence resonance energy transfer (BRET) were used to examine the PAR4 homodimer interface. In bimolecular fluorescence complementation experiments, PAR4 formed homodimers that were disrupted by unlabeled PAR4 in a concentration-dependent manner, but not by rhodopsin. In BRET experiments, the PAR4 homodimers showed a specific interaction as indicated by a hyperbolic BRET signal in response to increasing PAR4-GFP expression. PAR4 did not interact with rhodopsin in BRET assays. The threshold maximum BRET signal was disrupted in a concentration-dependent manner by unlabeled PAR4. In contrast, rhodopsin was unable to disrupt the BRET signal, indicating that the disruption of the PAR4 homodimer is not due to nonspecific interactions. A panel of rho-PAR4 chimeras and PAR4 point mutants has mapped the dimer interface to hydrophobic residues in transmembrane helix 4. Finally, mutations that disrupted dimer formation had reduced calcium mobilization in response to the PAR4 agonist peptide. These results link the loss of dimer formation to a loss of PAR4 signaling.