Molecular dissection of effector cell protease receptor-1 recognition of factor Xa. Assignment of critical residues involved in antibody reactivity and ligand binding.
Receptor-mediated assembly of blood proteases on vascular cells maintains the hemostatic balance and initiates intracellular signal transduction. Effector cell protease receptor-1 (EPR-1) is an approximately 62-kDa vascular cell membrane receptor for the clotting protease factor Xa, participating in thrombin formation and lymphocyte activation. Here, recombinant EPR-1 fragments were engineered in the frame of intercellular adhesion molecule-1, transfected in mammalian cells, and analyzed for antibody recognition and ligand binding. Chimeric transfectants containing the EPR-1 sequence Met1-Arg60 bound the immunosuppressive anti-EPR-1 monoclonal antibody (mAb) 2E1. In contrast, transfected cells expressing the EPR-1 sequence Pro120-Ala154 were recognized by the functionally inhibitory anti-EPR-1 mAbs 9D4 and B6, bound 125I-factor Xa in a reaction quantitatively indistinguishable from that of wild-type EPR-1 transfectants, and promoted factor Xa concentration-dependent prothrombin activation in the absence of exogenous factor V/Va. Chimeric transfectants expressing the COOH terminus end of the EPR-1 extracellular domain (Ala157-Glu221) did not bind anti-EPR-1 mAbs and did not associate with factor Xa. Mutagenesis of Asn131 or Lys133 in the EPR-1 ligand recognition domain abolished factor Xa binding by 80 +/- 5.5 and 96 +/- 4%, respectively, while mutation of Lys126, Gly128, Asn129, and Asn134 was without effect. A synthetic peptide duplicating the EPR-1 sequence S123PGKPGNQNSKNEPP137 dose dependently inhibited factor V/Va-independent thrombin generation of resting endothelium (IC50 approximately 1 microM), while the adjacent EPR-1 sequence P136PKK-RERERSSHCYP150 was ineffective. These findings demonstrate that EPR-1 contains two spatially distinct functional domains implicated in lymphocyte activation (Met1-Arg60) or factor Xa binding and prothrombin activation (Pro120-Ala154). These interacting sequences may provide a novel potential target for inhibition of factor Xa-dependent vascular cell responses.
Ambrosini
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Boyer Center for Molecular Medicine, Department of Pathology, Yale University School of Medicine, New Haven, Connecticut 06536, USA.
Altieri
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D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D016466:CHO Cells; D006224:Cricetinae; D015951:Factor Xa / Q000378:metabolism*; D051034:Inhibitor of Apoptosis Proteins; D008213:Lymphocyte Activation; D008214:Lymphocytes / Q000378:metabolism; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011956:Receptors, Cell Surface / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D000077022:Survivin
C107489:BIRC5 protein, human; D051034:Inhibitor of Apoptosis Proteins; D011956:Receptors, Cell Surface; D011993:Recombinant Fusion Proteins; D000077022:Survivin; D015951:Factor Xa