PAR1, 3, 4 PAR N-teminal fragments Thrombin-activated PAR Thrombin-mediated activation of Proteinase-activated receptors EC Number: 3.4.21 Pubmed1672265 Pubmed9087410 Pubmed9618465 Reactome Database ID Release 43114697 Reactome, http://www.reactome.org ReactomeREACT_72 Thrombin signaling is mediated at least in part by a small family of G protein-coupled Proteinase Activated Receptors (PARs). Human platelet activation by thrombin is mediated predominantly by PAR1; PAR4-induced platelet responses are less pronounced. PAR2 is not present in human platelets. PARs 1, 3 and 4 are activated when thrombin cleaves an N-terminal exodomain. This cleavage event unmasks a new N-terminus that serves as a tethered ligand that binds intramolecularly to the body of the receptor to effect transmembrane signaling. Intermolecular ligation of one PAR molecule by another can occur but, not surprisingly, appears to be less efficient than self-ligation. A synthetic peptide of sequence SFLLRN, the first six amino acids of the new N-terminus generated when thrombin cleaves PAR1, can activate PAR1 independent of protease and receptor cleavage. In addition to providing evidence for the tethered ligand mechanism, such tethered ligand-mimicking peptides have provided a convenient pharmacological tool for probing the effects of PAR activation in cells and tissues.