The glycoprotein Ib-IX-V complex mediates localization of factor XI to lipid rafts on the platelet membrane

Factor XI binds to activated platelets where it is efficiently activated by thrombin. The factor XI receptor is the platelet membrane glycoprotein (GP) Ib-IX-V complex (Baglia, F. A., Badellino, K. O., Li, C. Q., Lopez, J. A., and Walsh, P. N. (2002) J. Biol. Chem. 277, 1662-1668), a significant fraction of which exists within lipid rafts on stimulated platelets (Shrimpton, C. N., Borthakur, G., Larrucea, S., Cruz, M. A., Dong, J. F., and Lopez, J. A. (2002) J. Exp. Med. 196, 1057-1066). Lipid rafts are membrane microdomains enriched in cholesterol and sphingolipids implicated in localizing membrane ligands and in cellular signaling. We now show that factor XI was localized to lipid rafts in activated platelets ( approximately 8% of total bound) but not in resting platelets. Optimal binding of factor XI to membrane rafts required prothrombin (and Ca2+) or high molecular weight kininogen (and Zn2+), which are required for factor XI binding to platelets. An antibody to GPIb (SZ-2) that disrupts factor XI binding to the GPIb-IX-V complex also disrupted factor XI-raft association. The isolated recombinant Apple 3 domain of factor XI, which mediates factor XI binding to platelets, also completely displaces factor XI from membrane rafts. To investigate the physiological relevance of the factor XI-raft association, the structural integrity of lipid rafts was disrupted by cholesterol depletion utilizing methyl-beta-cyclodextrin. Cholesterol depletion completely prevented FXI binding to lipid rafts, and initial rates of factor XI activation by thrombin on activated platelets were inhibited >85%. We conclude that factor XI is localized to GPIb in membrane rafts and that this association is important for promoting the activation of factor XI by thrombin on the platelet surface.