PAR family members interact with one another and other GPCRs as homodimers and heterodimers. (A) In addition to homodimers, PAR1 and PAR4 form heterodimers that influence the rate of PAR4 activation by thrombin. (B) Thrombin-cleaved PAR1 can transactivate an adjacent PAR2. The PAR1-PAR2 heterodimer traffic within the cell as a unit and activate Rac1 pathways. (C) PAR1-PAR3 heterodimers may regulate cytoprotective signaling on endothelial cells. (D) In platelets, PAR4-P2Y12 form heterodimers that recruit β-arrestin-2 to PAR4 where it serves as a scaffold for AKT signaling.