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Glycoprotein VI (GPVI) mediates collagen-induced platelet activation after vascular damage and is an important contributor to the onset of thrombosis, heart attack, and stroke. Animal models of thrombosis have identified GPVI as a promising target for antithrombotic therapy. Although for many years the crystal structure of GPVI has been known, the essential details of its interaction with collagen have remained elusive. Here, we present crystal structures of the GPVI ectodomain bound to triple-helical collagen peptides, which reveal a collagen-binding site across the β-sheet of the D1 domain. Mutagenesis and binding studies confirm the observed binding site and identify Trp76, Arg38, and Glu40 as essential residues for binding to fibrillar collagens and collagen-related peptides (CRPs). GPVI binds a site on collagen comprising two collagen chains with the core formed by the sequence motif OGPOGP. Potent GPVI-binding peptides from Toolkit-III all contain OGPOGP; weaker binding peptides frequently contain a partial motif varying at either terminus. Alanine-scanning of peptide III-30 also identified two AGPOGP motifs that contribute to GPVI binding, but steric hindrance between GPVI molecules restricts the maximum binding capacity. We further show that no cooperative interactions could occur between two GPVI monomers binding to a stretch of (GPO)5 and that binding of ≥2 GPVI molecules to a fibril-embedded helix requires non-overlapping OGPOGP motifs. Our structure confirms the previously suggested similarity in collagen binding between GPVI and leukocyte-associated immunoglobulin-like receptor 1 (LAIR-1) but also indicates significant differences that may be exploited for the development of receptor-specific therapeutics.

The osteoclast-associated receptor (OSCAR) is a collagen-binding immune receptor with important roles in dendritic cell maturation and activation of inflammatory monocytes as well as in osteoclastogenesis. The crystal structure of the OSCAR ectodomain is presented, both free and in complex with a consensus triple-helical peptide (THP). The structures revealed a collagen-binding site in each immunoglobulin-like domain (D1 and D2). The THP binds near a predicted collagen-binding groove in D1, but a more extensive interaction with D2 is facilitated by the unusually wide D1-D2 interdomain angle in OSCAR. Direct binding assays, combined with site-directed mutagenesis, confirm that the primary collagen-binding site in OSCAR resides in D2, in marked contrast to the related collagen receptors, glycoprotein VI (GPVI) and leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1). Monomeric OSCAR D1D2 binds to the consensus THP with a KD of 28 µM measured in solution, but shows a higher affinity (KD 1.5 μM) when binding to a solid-phase THP, most likely due to an avidity effect. These data suggest a 2-stage model for the interaction of OSCAR with a collagen fibril, with transient, low-affinity interactions initiated by the membrane-distal D1, followed by firm adhesion to the primary binding site in D2.

Abstract 2207 Multimerin 1 (MMRN1) is a large, homopolymeric adhesive protein stored in platelets and endothelium that binds to activated platelets, endothelial cells and the extracellular matrix after agonist stimulation. MMRN1 supports platelet adhesion by von Willebrand factor (VWF) dependent and independent mechanisms, and also increases platelet adhesion to Horm collagen. Mice deficient in Mmrn1/Snca (α-synuclein) showed defective platelet adhesion to collagen in vitro and in vivo which was corrected by MMRN1. The ability of MMRN1 to support platelet adhesion in vivo and enhance platelet adhesion to collagen ex vivo, led us to explore the molecular basis of MMRN1 interactions with collagen. Solid phase binding assays were used to test MMRN1 binding to human fibril forming collagen (types I, II, III and V) and to types IV and VI collagen. Binding assays were also used to map the MMRN1 binding sites on collagen using collagen peptide toolkits III and II (which has significant similarity to type I collagen). Static adhesion assays were used to test selected collagen peptides for platelet adhesion. Platelet adhesion assays at high shear rates 1500s−1, were used to test the adhesion of washed platelets from normal and VWF-deficient subjects to type I collagen pre-treated or in fluid phase with: BSA (negative control), MMRN1, VWF or their combination. Human collagens types I, II, III and VI (p-values Disclosures: No relevant conflicts of interest to declare.