Thrombin is a serine protease that is involved in platelet aggregation and blood coagulation. It is cleaved from its precursor, prothrombin, and converts fibrinogen to fibrin in the final step of the clotting cascade. Thrombin mediates its regulatory effects by activating cell surface receptors. These receptors, including thrombin receptor (also designated PAR-1, for protease-activated receptor-1), PAR-2 and PAR-3 are members of the G-protein coupled receptor family, and share a similiar gene structure. Thrombin cleaves its receptor, releasing a 41 amino acid peptide which acts as a platelet agonist. Upon this activation by thrombin, the thrombin receptors trigger an increase in cytosolic Ca2+ concentration. Unactivated thrombin receptor cycles between the cell surface and an intracellular pool, while activated receptor internalizes rapidly and is degraded in the lysosomes. The human thrombin receptor is also known to be regulated by Sp1 and Sp3 transcription factors.Synonyms: CF2R, Coagulation factor II receptor, PAR-1, PAR1, Proteinase-activated receptor 1