Protease-activated receptor
A protease-activated receptor is a cell receptor that is activated by cleavage of part of its extracellular domain. It is located on platelets. There are 4 known protease-activated receptors or PAR's, numbered from one to four. These receptors are members of the seven transmembrane G-protein coupled receptor superfamily, and are expressed throughout the body. PAR's are activated by the action of serine based proteases such as thrombin (acts on PAR's 1, 3 and 4) and trypsin (PAR 2). These enzymes act upon the N-terminus of the receptor, and cause the formation of a tethered ligand. In this state, part of the receptor itself acts as the agonist, causing a physiological response. Most of the PAR family act through the actions of G-proteins i (cAMP inhibitory) and q (calcium signalling) to cause cellular actions. Recent research has implicated these novel receptors in the inflammatory response (including arthritis), muscle growth, and bone cell differentiation and proliferation.