The Factor Xi Receptor On Collagen Activated Platelets Is Gc1qr/P33.

Factor XI (FXI) binds to high affinity sites on the surface of thrombin-stimulated platelets where it is efficiently activated by thrombin. We have previously shown that FXI binding to thrombin-activated platelets is mediated by residues in the FXI Apple 3 (A3) domain interacting with leucine-rich repeat sequences within the NH 2 -terminal globular domain of glycoprotein (GP) Ibα (J. Biol. Chem.277:1662–1668, 2002; J. Biol. Chem.279:45470–45476, 2004; J. Biol. Chem.279:49323–49329, 2004). The cell surface receptor protein gC1qR/P33, expressed on collagen-stimulated platelets, binds FXII and high molecular weight kininogen (HK). We now provide evidence that the FXI binding site on collagen-activated platelets is gC1qR/P33: 1) two anti-gC1qR/P33 mAbs, 60.11 and 74.5.2, inhibited FXI binding to collagen-stimulated platelets whereas SZ-2 (a monoclonal antibody directed against GPIbα) does not; 2) by surface plasmon resonance, FXI was bound specifically to gC1qR/P33 in a Zn 2+ -dependent fashion (K d app ~5 nM); 3) soluble gC1qR/P33 inhibited the binding of FXI to collagen-stimulated platelets with an IC 50 ~10 nM. Optimal binding of FXI to collagen-activated platelets (~1,500 binding sites per platelet; K d ~10 nM) was achieved in the absence of HK. A synthetic peptide comprising residues Asn 235-Arg 266 within the FXI A3 domain inhibited FXI binding to collagen-activated platelets with a K i ~10 nM which is identical to the K d for FXI binding to platelets, whereas A1, A2 and A4 peptides had no effect. We then investigated whether gC1qR/P33 could promote activation of FXI by thrombin or FXIIa. In the presence of Zn 2+ and Ca 2+ ions, and the absence of HK, FXI was activated by thrombin in the presence of gC1qR/P33 at rates comparable to those seen in the presence of thrombin-activated platelets, whereas rates of FXI activation by FXIIa in the presence of gC1qR/P33 were 8-fold lower. Thus, thrombin is the preferred activator of FXI bound to gC1qR/P33. We conclude that whereas FXI binds to GPIbα on thrombin-activated platelets, it binds to gC1qR/P33 on collagen-activated platelets. Finally, we show here that both collagen and vonWillebrand factor can inhibit FXI binding to GPIbα, suggesting that on collagen-activated platelets gC1qR serves as an alternative FXI receptor that promotes FXI activation by thrombin.