Human Platelet Glycoprotein VI: Identification of Residues Involved in the Binding to Collagen.

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Glycoprotein VI (GPVI), an essential platelet collagen receptor, belongs to the immunoglobulin (Ig) superfamily and presents two Ig-like loops in its extracellular domain (EC1, EC2). Little is known about the structures involved in GPVI binding to collagen and to its specific ligands, convulxin and collagen related peptides (CRP). Our aim was to characterize these sites using different approaches: competition assays, monoclonal antibodies (MoAb) directed against the GPVI extracellular par, phage display, molecular modelling and site-directed mutagenesis. In binding assays using soluble or cell-expressed human GPVI, we observed that: (i) collagen, CRP and convulxin, competed with one another for the binding to GPVI; (ii) GPVI interaction with either ligand is inhibited by at least two MoAb directed to different independent epitopes, and (iii) MoAb displayed selective inhibitory properties on GPVI interaction with each ligand. These observations indicate that the binding sites on GPVI for the diverse ligands seem to be distinct, exhibiting specific features but at the same time sharing common elements. MoAb 9O12 is a potent inhibitor of GPVI binding to collagen. Its epitope was mapped by screening a 15 mer linear peptide library. This allowed the identification of a motif of 7 residues of which 5 are conserved and that presented a relative homology with the GPVI 30–40 sequence. In a computer model, these residues appeared in close vicinity on a surface loop exposed at the surface of the molecule and accessible to ligands. Site-directed mutagenesis revealed that Val34 and Leu36 are critical for GPVI interaction with collagen and CRP. This study allowed the identification of residues within a loop on GPVI EC1 that might be part of a collagen/CRP-binding site.