Protease-activated receptor

Protease-activated receptors are a subfamily of related G protein-coupled receptors that are activated by cleavage of part of their extracellular domain. They are highly expressed in platelets, but also on endothelial cells, myocytes and neurons.

Classification
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.

Activation
PAR's are activated by the action of serine proteases such as thrombin (acts on PAR's 1, 3 and 4) and trypsin (PAR 2). These enzymes cleave the N-terminus of the receptor, which in turn acts as a tethered ligand. In the cleaved 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), 12/13 (Raf/Ras activation) and q (calcium signalling) to cause cellular actions.

Function
Recent research has implicated these novel receptors in the inflammatory response (including arthritis), muscle growth, and bone cell differentiation and proliferation.