Use of an oriented transmembrane protein to probe the assembly of a supported phospholipid bilayer.

Planar-supported phospholipid bilayers formed by the adsorption of vesicles are increasingly used in the investigation of lipid-dependent reactions. We have studied the way in which these bilayers are formed with phospholipid vesicles containing the transmembrane protein Tissue Factor (TF). TF complexed with the serine protease, factor VIIa, is the primary initiator of blood coagulation by way of activation of the zymogen factor X. TF has been shown to orient randomly on the inner and outer leaflets of vesicles. We used proteolytic digestion to produce vesicles in which the extracellular domain of TF is located on the inner leaflet. These vesicles show no cofactor activity for factor VIIa as a result of the inability of the extracellular domain of TF to bind VIIa. After freeze/thawing, 50% of the cofactor activity was regained, indicating reorientation of the sequestered, inner leaflet TF. Adsorption of these vesicles to the inner surface of glass microcapillaries results in a continuous phospholipid bilayer. The microcapillaries were perfused with a solution of factors VIIa and X, and the effluent was monitored for factor Xa production, a sensitive measure of the activity of the TF-VIIa complex. For coatings produced with the digested vesicles, minimal TF-VIIa activity was observed, showing that the supported bilayer preserves the orientation of the leaflets in the vesicles, i.e., the outer leaflet of the vesicles forms the outer leaflet of the supported bilayer.