LIGAND CROSS-REACTIVITY WITHIN THE PROTEASE-ACTIVATED RECEPTOR FAMILY

Recently, a second member of the protease-activated receptor (PAR) fam ily, named PAR-2, has been identified, Similar to the thrombin recepto r, PAR-2 appears to be activated by proteolytic-mediated exposure of a ''tethered ligand'' sequence and can also be activated by the corresp onding synthetic peptides, Similarities in the amino acid sequence of the receptors' tethered ligand sequences suggest that their respective agonist peptides might not be absolutely specific for their particula r receptors, To test this, the receptor specificity of each agonist ha s been determined by measuring the responses of Xenopus oocytes expres sing the thrombin receptor or PAR-2 to agonist peptides or enzymes, Th rombin receptors responded to thrombin, the human thrombin receptor-ac tivating peptide SFLLRNP-NH2 (TRAP) (EC(50) = 0.1 mu m), and Xenopus T RAP, TFRIFD-NH2 (EC(50) = 1 mu M), but did not show any increase in ca lcium efflux over control levels with trypsin (50 mu M) or PAR-2 agoni st peptides (100 mu M). Human and murine PAR-2 receptors responded com parably to human and murine PAR-2 agonist peptides (SLIGKVD and SLIGRL , respectively) (EC(50) = 0.5-2.0 mu M) and trypsin, but not to thromb in, PAR-2 was also found to be responsive to TRAP (EC(50) = 1 mu M) bu t was unresponsive to Xenopus TRAP (50 mu M). Responses to additional peptide agonist analogs suggest that an amino-terminal serine is criti cal for PAR-2 agonist activity.