Role of the PAR1 receptor 8th helix in signaling: the 7-8-1 receptor activation mechanism

The protease-activated receptors are tethered ligand G protein-coupled receptors that are activated by proteolytic cleavage of the extracellular domain of the receptor. The archetypic protease-activated receptor PAR1 strongly activates G(q) signaling pathways, but very little is known regarding the mechanism of signal transference between receptor and internally located G protein. The recent x-ray structure of rhodopsin revealed the presence of a highly conserved amphipathic 8th helix that is likely to be physically interposed between receptor and G protein. We found that the analogous 8th helix region of PAR1 was critical for activation of G(q)-dependent signaling. Engineering an 8th helix alpha-aneurysm with a downwards-directed alanine residue markedly interfered with signal transference to G(q). The 8th helix-anchoring cysteine palmitoylation sites were important for the affinity of ligand-dependent G protein coupling but did not affect the maximal signal. A network of H-bond and ionic interactions was found to connect the N-terminal portion of the 8th helix to the nearby NPXXY motif on transmembrane helix 7 and also to the adjacent intracellular loop-1. Disruption of these pairwise interactions caused additive defects in coupling to G protein, indicating that the transmembrane 7-8th helix-i1 loop may move in a coordinated manner to transfer the signal from PAR1 to G protein. This "7-8-1" interaction network was found to be prevalent in G protein-coupled receptors involved in endothelial signaling and angiogenesis.