Protease-activated receptor-4 (PAR4) is a G protein-coupled receptor (GPCR) for thrombin and is proteolytically activated, similar to the prototypical PAR1. Due to the irreversible activation of PAR1, receptor trafficking is intimately linked to signal regulation. However, unlike PAR1, the mechanisms that control PAR4 trafficking are not known. Here, we sought to define the mechanisms that control PAR4 trafficking and signaling. In HeLa cells depleted of clathrin by siRNA, activated PAR4 failed to internalize. Consistent with clathrin-mediated endocytosis, expression of a dynamin dominant-negative K44A mutant also blocked activated PAR4 internalization. However, unlike most GPCRs, PAR4 internalization occurred independently of -arrestins and the receptor's C-tail domain. Rather, we discovered a highly conserved tyrosine-based motif in the third intracellular loop of PAR4 and found that the clathrin adaptor protein complex-2 (AP-2) is important for internalization. Depletion of AP-2 inhibited PAR4 internalization induced by agonist. In addition, mutation of the critical residues of the tyrosine-based motif disrupted agonist-induced PAR4 internalization. Using Dami megakaryocytic cells, we confirmed that AP-2 is required for agonist-induced internalization of endogenous PAR4. Moreover, inhibition of activated PAR4 internalization enhanced ERK1/2 signaling, whereas Akt signaling was markedly diminished. These findings indicate that activated PAR4 internalization requires AP-2 and a tyrosine-based motif and occurs independent of -arrestins, unlike most classical GPCRs. Moreover, these findings are the first to show that internalization of activated PAR4 is linked to proper ERK1/2 and Akt activation.Thrombin is a coagulant protease that plays an essential role in hemostasis and thrombosis in response to tissue injury (1).Thrombin is locally generated at sites of vascular injury, where it initiates responses in various cell types including platelets, endothelial, and smooth muscle cells. Thrombin elicits cellular responses through members of the protease-activated receptor (PAR) 5 family of G protein-coupled receptors (GPCRs). Unlike classic GPCRs, which are activated by a diffusible ligand, PARs are activated by the proteolytic cleavage of the receptor N-terminal domain (2, 3). The newly exposed N terminus, termed the "tethered ligand," then binds intramolecularly to the receptor's second extracellular loop, inducing a conformational change that facilitates coupling to heterotrimeric G proteins and downstream signaling. Studies have shown that a six-amino acid synthetic peptide mimic of a PAR's tethered ligand is sufficient to activate the receptor and evoke cellular responses similar to those resulting from proteolytic cleavage of the receptor (4). Because of the irreversible mechanism of PAR1 activation with the generation of a tethered ligand that cannot diffuse away, the mechanisms that regulate PAR1 trafficking are important for maintaining proper signaling and appropriate cellular responses (5).After activa...