The structural basis for PAR1 biased signaling

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PROJECT SUMMARY/ABSTRACT G-protein coupled receptors (GPCRs) elicit complex downstream signaling cascades by activating Gαq, Gα12/13, Gαi, or arrestin. There is a growing appreciation that biased agonists can dictate which signaling pathways are activated downstream of the receptor. Protease activated receptors (PARs) are the primary means by which proteases initiate intracellular signaling. PARs are activated by cleavage of the N-terminus to generate a tethered ligand. PAR1 has unique cleavage sites for multiple proteases that can lead to a panel of unique tethered ligands. These ligands are endogenous biased agonists that trigger specific signaling pathways. The molecular basis for this is not known. The long-term goals of this research program are to define how PARs mediate context specific signaling in endothelial cells, platelets and other cells. This project seeks to develop a foundation for understanding the molecular basis for PAR activation mechanisms that govern normal physiological responses. The overall objective of this proposal is to 1.) define the tethered ligand binding site(s) for PAR1 activated by three endogenous activators thrombin, APC, and MMP1 2.) uncover the structural basis for PAR1 biased signaling 3.) determine how these sites cooperate to mediate specific physiological signaling events. Our overall hypothesis is that PAR1 adopts specific conformations due to distinct ligand binding sites for each of the tethered ligands dictating which signaling pathways are activated. The scientific premise is based on the recent success of our experimental design that incorporates amide hydrogen/deuterium (H/D) exchange with purified PARs to determine how the tethered ligand influences the overall conformation. Molecular modeling will independently determine the ligand binding site(s). Finally, identified regions will be tested in cell signaling assays using a panel of PAR1 mutants to verify the importance on cell signaling. Our innovative approach will identify the previously unknown endogenous ligand binding sites for each of the tethered ligands generated by thrombin, APC, and MMP1. At the completion of these studies will define the potential conformations of PAR1 with endogenous activators. These experiments will provide the first structural insights as to how a single receptor can have opposite signaling outcomes under normal physiological conditions.