Proteinase-activated receptor 1 (PAR1) also known as Protease-activated receptor 1 or coagulation factor II (thrombin) receptor is a protein that in humans is encoded by the F2Rgene. PAR1 is a G protein-coupled receptor and one of four protease-activated receptors involved in the regulation of thrombotic response. Highly expressed in platelets and endothelial cells, PAR1 plays a key role in mediating the interplay between coagulation and inflammation, which is important in the pathogenesis of inflammatory and fibrotic lung diseases. It is also involved both in disruption and maintenance of endothelial barrier integrity, through interaction with either thrombin or activated protein C, respectively.
PAR1 is a transmembrane G-protein-coupled receptor (GPCR) that shares much of its structure with the other protease-activated receptors. These characteristics include having seven transmembrane alpha helices, four extracellular loops and three intracellular loops. PAR1 specifically contains 425 amino acid residues arranged for optimal binding of thrombin at its extracellular N-terminus. The C-terminus of PAR1 is located on the intracellular side of the cell membrane as part of its cytoplasmic tail.
Signal transduction pathway
This image gives an overview of the cleavage of PAR1 by thrombin. Thrombin, in red, binds to the cleavage site at the extracellular N-terminus of PAR1. Thrombin cleaves the peptide bond between Arg-41 and Ser-42 to reveal a tethered ligand at the new N-terminus and the cleaved peptide, in orange, is released outside of the cell.
PAR1 is activated when the terminal 41 amino acids of its N-terminus are cleaved by thrombin, a serine protease. Thrombin recognizes PAR1 by a Lysine-Aspartate-Proline-Arginine-Serine sequence at the N-terminal where it cuts the peptide bond between Arginine-41 and Serine-42. The affinity of thrombin to this specific cleavage site in PAR1 is further aided by secondary interactions between thrombin’s exosite and an acidic region of amino acid residues located C-terminal to Ser-42. This proteolytic cleavage is irreversible and the loose peptide, often referred to as parstatin, is then released outside of the cell. The newly revealed N-terminus acts as a tethered ligand that binds to a binding region between extracellular loops 3 and 4 of PAR1, therefore activating the protein. The binding instigates conformational changes in the protein that ultimately allow for the binding of G-proteins to sites on the intracellular region of PAR1.
Once cleaved, PAR1 can activate G-proteins that bind to several locations on its intracellular loops. For example, PAR1 in conjunction with PAR4 can couple to and activate G-protein G12/13 which in turn activates Rho and Rho kinase. This pathway leads to the quick alteration of platelet shape due to actin contractions that lead to platelet mobility, as well as the release of granules which are both necessary for platelet aggregation. Coupling can also occur with Gq, leading to phospholipase C-β activation; this pathway results in the stimulation of protein kinase C (PKC) which impacts platelet activation.
Additionally, both PAR1 and PAR4 can couple to G-protein q which stimulates intracellular movement for Calcium ions that serve as second messengers for platelet activation. This also activates protein kinase C which stimulates platelet aggregation and therefore blood coagulation further down the pathway.
The phosphorylation of PAR1’s cytoplasmic tail and subsequent binding to arrestin uncouples the protein from G protein signaling. These phosphorylated PAR1s are transported back into the cell via endosomes where they are sent to Golgi bodies. The cleaved PAR1s are then sorted and transported to lysosomes where they are degraded. This internalization and degradation process is necessary for the termination of receptor signaling.
In order to regain thrombin responsiveness, PAR1 must be replenished in the cell surface. Uncleaved PAR1 in the cell membrane gets bound by the AP2 adaptor complex at a tyrosine motif on the intracellular C-terminus, which stimulates the endocytosis of the unactivated PAR1. It is then stored in clathrin-coated vesicles within the cytosol and ultimately protected from proteolysis. This ensures that there is a constant supply of uncleaved PAR1 that can be cycled into the plasma membrane independent of PAR1 reproduction, thus resensitizing the cell to thrombin and resetting the signal transduction pathway.
This is a rendering of PAR1's structure when bound to an antagonist, Vorapaxar. The light blue structures represent the seven transmembrane alpha helices of PAR1. The green structures represent the extracellular loops and the orange structures represent the intracellular loops. The red molecule is Vorapaxar. C-terminal tail not pictured.
Finding selective agonists for PAR1 has also been a topic of interest for researchers. A synthetic SFLLRN peptide has been found to serve as an agonist for PAR1. The SFLLRN peptide mimics the first six residues of the N-terminal tethered ligand of activated PAR1 and binds to the same binding site on the second extracellular loop. So, even in the absence of thrombin, SFLLRN binding can garner a response from cleaved or uncleaved PAR1.
PAR1 is inhibited by Vorapaxar when the molecule binds to a binding pocket between extracellular loop 2 and 3 of the PAR1 where it stabilizes the inactivated protein structure and prevents the switch to the active conformation.
^Bahou WF, Nierman WC, Durkin AS, Potter CL, Demetrick DJ (September 1993). "Chromosomal assignment of the human thrombin receptor gene: localization to region q13 of chromosome 5". Blood. 82 (5): 1532–7. PMID8395910.
^Feistritzer C, Riewald M (April 2005). "Endothelial barrier protection by activated protein C through PAR1-dependent sphingosine 1-phosphate receptor-1 crossactivation". Blood. 105 (8): 3178–84. doi:10.1182/blood-2004-10-3985. PMID15626732.
^ abSpoerri PM, Kato HE, Pfreundschuh M, Mari SA, Serdiuk T, Thoma J, et al. (June 2018). "Structural Properties of the Human Protease-Activated Receptor 1 Changing by a Strong Antagonist". Structure. 26 (6): 829–838.e4. doi:10.1016/j.str.2018.03.020. PMID29731231.
^Hammes SR, Coughlin SR (February 1999). "Protease-activated receptor-1 can mediate responses to SFLLRN in thrombin-desensitized cells: evidence for a novel mechanism for preventing or terminating signaling by PAR1's tethered ligand". Biochemistry. 38 (8): 2486–93. doi:10.1021/bi982527i. PMID10029543.
Howell DC, Laurent GJ, Chambers RC (April 2002). "Role of thrombin and its major cellular receptor, protease-activated receptor-1, in pulmonary fibrosis". Biochemical Society Transactions. 30 (2): 211–6. doi:10.1042/BST0300211. PMID12023853.
Remillard CV, Yuan JX (May 2005). "PGE2 and PAR-1 in pulmonary fibrosis: a case of biting the hand that feeds you?". American Journal of Physiology. Lung Cellular and Molecular Physiology. 288 (5): L789–92. doi:10.1152/ajplung.00016.2005. PMID15821019.
Vu TK, Hung DT, Wheaton VI, Coughlin SR (March 1991). "Molecular cloning of a functional thrombin receptor reveals a novel proteolytic mechanism of receptor activation". Cell. 64 (6): 1057–68. doi:10.1016/0092-8674(91)90261-V. PMID1672265.
Wojtukiewicz MZ, Tang DG, Ben-Josef E, Renaud C, Walz DA, Honn KV (February 1995). "Solid tumor cells express functional "tethered ligand" thrombin receptor". Cancer Research. 55 (3): 698–704. PMID7834643.
Hein L, Ishii K, Coughlin SR, Kobilka BK (November 1994). "Intracellular targeting and trafficking of thrombin receptors. A novel mechanism for resensitization of a G protein-coupled receptor". The Journal of Biological Chemistry. 269 (44): 27719–26. PMID7961693.
Mathews II, Padmanabhan KP, Ganesh V, Tulinsky A, Ishii M, Chen J, et al. (March 1994). "Crystallographic structures of thrombin complexed with thrombin receptor peptides: existence of expected and novel binding modes". Biochemistry. 33 (11): 3266–79. doi:10.1021/bi00177a018. PMID8136362.
Hoffman M, Church FC (August 1993). "Response of blood leukocytes to thrombin receptor peptides". Journal of Leukocyte Biology. 54 (2): 145–51. doi:10.1002/jlb.54.2.145. PMID8395550.
Schmidt VA, Vitale E, Bahou WF (April 1996). "Genomic cloning and characterization of the human thrombin receptor gene. Structural similarity to the proteinase activated receptor-2 gene". The Journal of Biological Chemistry. 271 (16): 9307–12. doi:10.1074/jbc.271.16.9809. PMID8621593.
Li F, Baykal D, Horaist C, Yan CN, Carr BN, Rao GN, Runge MS (October 1996). "Cloning and identification of regulatory sequences of the human thrombin receptor gene". The Journal of Biological Chemistry. 271 (42): 26320–8. doi:10.1074/jbc.271.42.26320. PMID8824285.
Shapiro MJ, Trejo J, Zeng D, Coughlin SR (December 1996). "Role of the thrombin receptor's cytoplasmic tail in intracellular trafficking. Distinct determinants for agonist-triggered versus tonic internalization and intracellular localization". The Journal of Biological Chemistry. 271 (51): 32874–80. doi:10.1074/jbc.271.51.32874. PMID8955127.
Ogino Y, Tanaka K, Shimizu N (November 1996). "Direct evidence for two distinct G proteins coupling with thrombin receptors in human neuroblastoma SH-EP cells". European Journal of Pharmacology. 316 (1): 105–9. doi:10.1016/S0014-2999(96)00653-X. PMID8982657.
Molino M, Bainton DF, Hoxie JA, Coughlin SR, Brass LF (February 1997). "Thrombin receptors on human platelets. Initial localization and subsequent redistribution during platelet activation". The Journal of Biological Chemistry. 272 (9): 6011–7. doi:10.1074/jbc.272.9.6011. PMID9038223.
Renesto P, Si-Tahar M, Moniatte M, Balloy V, Van Dorsselaer A, Pidard D, Chignard M (March 1997). "Specific inhibition of thrombin-induced cell activation by the neutrophil proteinases elastase, cathepsin G, and proteinase 3: evidence for distinct cleavage sites within the aminoterminal domain of the thrombin receptor". Blood. 89 (6): 1944–53. PMID9058715.