title: Phospholipid-independent and -dependent interactions required for tissue factor receptor and cofactor function.
abstract: Membrane anchoring of tissue factor (TF), the cell receptor for coagulation factor VIIa (VIIa), exemplifies an effective mechanism to localize proteolysis at the cell surface. A recombinant TF mutant (TF1-219), deleted of membrane spanning and intracellular domains, was used to evaluate the role of phospholipid interactions for assembly of substrate with the catalytic TF.VIIa complex. TF1-219 was secreted by cells rather than expressed as a cell membrane protein. Unlike free VIIa, TF1-219 as well as the TF1-219.VIIa complex demonstrated no stable association with phospholipid. In the absence of lipid, kinetic evaluation of substrate factor X cleavage by free VIIa, TF.VIIa, and TF1-219.VIIa suggests that the catalytic function of VIIa rather than substrate recognition is enhanced by complex formation. Furthermore, compared with free factor X, factor X on phospholipid was preferentially cleaved as a substrate by TF1-219.VIIa. TF-dependent initiation of the coagulation protease cascades thus involves an enhancement of the activation of factor X on the cell surface by a crucial role of the TF transmembrane domain to membrane anchor the reaction, by the TF extracellular domain to provide protein-protein interactions with VIIa to enhance the activity of the catalytic domain of VIIa, and the preferential presentation of factor X as a substrate when associated with phospholipid surfaces.
authors.affiliation: Department of Immunology, Research Institute of Scripps Clinic, La Jolla, California 92037.
authors.forename: W
authors.initials: W
authors.lastname: Ruf
authors.forename: A
authors.initials: A
authors.lastname: Rehemtulla
authors.forename: J H
authors.initials: JH
authors.lastname: Morrissey
authors.forename: T S
authors.initials: TS
authors.lastname: Edgington
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D015942:Factor VIIa / Q000378:metabolism*; D005170:Factor X / Q000378:metabolism; D007700:Kinetics; D008969:Molecular Sequence Data; D010743:Phospholipids / Q000378:metabolism*; D013379:Substrate Specificity; D013925:Thromboplastin / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*
chemical_list: D010743:Phospholipids; D005170:Factor X; D013925:Thromboplastin; D015942:Factor VIIa