Protease-activated receptor 1 (PAR1) is an unusual GPCR that interacts with multiple G protein subfamilies (G(q/11), G(i/o), and G(12/13)) and their linked signaling pathways to regulate a broad range of pathophysiological processes. However, the molecular mechanisms whereby PAR1 interacts with multiple G proteins are not well understood. Whether PAR1 interacts with various G proteins at the same, different, or overlapping binding sites is not known. Here we investigated the functional and specific binding interactions between PAR1 and representative members of the G(q/11), G(i/o), and G(12/13) subfamilies. We report that G(q/11) physically and functionally interacts with specific amino acids within the second intracellular (i2) loop of PAR1. We identified five amino acids within the PAR1 i2 loop that, when mutated individually, each markedly reduced PAR1 activation of linked inositol phosphate formation in transfected COS-7 cells (functional PAR1-null cells). Among these mutations, only R205A completely abolished direct G(q/11) binding to PAR1 and also PAR1-directed inositol phosphate and calcium mobilization in COS-7 cells and PAR1-/- primary astrocytes. In stark contrast, none of the PAR1 i2 loop mutations disrupted direct PAR1 binding to either G(o) or G(12), or their functional coupling to linked pertussis toxin-sensitive ERK phosphorylation and C3 toxin-sensitive Rho activation, respectively. In astrocytes, our findings suggest that PAR1-directed calcium signaling involves a newly appreciated G(q/11)-PLCε pathway. In summary, we have identified key molecular determinants for PAR1 interactions with G(q/11), and our findings support a model where G(q/11), G(i/o) or G(12/13) each bind to distinct sites within the cytoplasmic regions of PAR1.
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