Human thrombin receptor

A THEORETICAL MODEL OF THE HUMAN THROMBIN RECEPTOR (PAR-1), THE FIRST KNOWN PROTEASE-ACTIVATED G-PROTEIN-COUPLED RECEPTOR... 

Figure 1. Diagram of the human thrombin receptor, viewed edge-on, showing the sequence around the proteolytic cleavage site (LDPR-SFLLRN) in the long extracellular amino-terminus and the seven transmembrane domains (TM1-TM7).

Initially, we attempted to build a 3-D homology model of the transmembrane regions of the human thrombin receptor from the structure of IBRD, which is available from the Brookhaven Protein Database. Unfortunately, the suboptimal placement of several amino acid side chains resulted in severe deviations from structural standards for membrane-bound receptors with a seven-helix bundle topology (Figure 2). For example, carboxylate and ammonium groups on amino acid side chains at a mid-helix location were directed into the membrane, rather than toward the inside of the helix bundle, and the hydrophobic packing between some helices was either poor or nonexistent. 

Figure 3. Key hydrogen-bonding network between side chains of the GPCR-con-served amino acids Asn-120 (TM1), Asp-148 (TM2), and Asp-367 (TM7) for human thrombin receptor.

SFLLRN.15 The cleaved human thrombin receptor has an extracellular N-terminus of approximately 55 amino acids (before the beginning of TM1 and up to the SFLLRN motif). Given also the key role of the extracellular epitopes in other peptide-ligand GPCRs, we were interested in examining the role of the N-terminus of the thrombin receptor, in addition to its extracellular loops. 

At this point, we had assembled a preliminary model of the human thrombin receptor that contained all three extracellular loops linked to the seven TM domains, as well as the N-terminus out to residue 75. Docking studies with agonist peptide SFLLRN, and additional mutagenesis experiments, were carried out in parallel. Measurement of the effects of site-specific mutations in human PAR-1 identified several key amino acids of the receptor as being important for activation (Table 1),... 

Figure 9. Hydrophobic cluster involving Phe-182 (magenta), Leu-340 (orange), Tyr-337 (red), and Phe-339 (magenta) of the human thrombin receptor complexed with the Phe residue (white) of the agonist peptide.

Overall, our results provide a working hypothesis for the molecular recognition that occurs between the human thrombin receptor (PAR-1) and its agonist peptide SFLLRN. Similar features may contribute to the interactions of the X. laevis homologue of PAR-1 with TFRIFD and of human PAR-2 with SLIGKV, although... 

Invention Significance Activation of the human thrombin receptor,... 

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