Figure 2.

Protease-activated receptor family. Four PARs are known. Amino acid sequence identity between human (h-) and mouse (m-) homologues of each is approximately 60%, but identity between different PARs within a single species falls to approximately 30%. Xen indicates Xenopus. Human PAR1, PAR3, and PAR4 can be activated by thrombin, and sensing thrombin is likely, at least in part, their role in vivo (see text). One receptor, PAR2, is activated by trypsin and tryptase but not by thrombin. Its roles in vivo remain to be explored. The four PAR genes share a common two-exon structure. In essence, the first exon encodes a signal peptide and the second the mature receptor protein. The genes encoding PARs 1, 2, and 3 are adjacent in the mouse and human genomes, whereas the PAR4 gene resides at a separate location (32, 65, 66).