Helix-8 and Carboxyl Terminal Tail Regulation of Proteinase Activated Receptor-4 (PAR4) signalling
The 8th-helix (H8) and carboxyl terminal tail (CT) of GPCRs are crucial for interactions with intracellular signaling molecules and regulatory proteins. We investigated how H8 and CT residues influence signaling in PAR4, a tethered-ligand activated GPCR. Our analysis revealed that PAR4 activation by thrombin or AYPGKF-NH2 stimulates {beta}-arrestin-1/-2 recruitment and activates multiple G proteins (Gq, G11, Gz, G15, G12, G13, Gi1-3, GoA/B). Mutation of the H8 sequence (R352AGLFQRS359) significantly reduced G protein activation and {beta}-arrestin recruitment, with residues Leu355-Glu357 being particularly important. Mutations of specific lysine residues in H8 and CT also impaired signaling. Additionally, we identified a crucial TM7-H8 interaction and found that CT phosphorylation sites regulate {beta}-arrestin recruitment. Finally, using AlphaFold3 we predicted interactions between PAR4, {beta}-arrestins, and G proteins, revealing novel receptor regulatory sites in the intracellular loops, transmembrane domains, H8 and CT.