Shape and lipid-binding site of the nonspecific lipid-transfer protein (sterol carrier protein 2): a steady-state and time-resolved fluorescence study

The nonspecific lipid-transfer protein (nsL-TP) from bovine liver was studied with time-resolved and steady-state fluorescence techniques. From the decay of the intrinsic tryptophanyl fluorescence, it was estimated that the rotational correlation time of nsL-TP is 15 ns. This parameter increased only slightly upon addition of an excess of negatively charged vesicles, indicating that the basic nsL-TP is not immobilized at the membrane surface under these conditions. Binding studies using fluorescent lipid analogues revealed that nsL-TP is able to extract sn-2-(pyrenehexanoyl) phosphatidylcholine and 1-palmitoyl-2-[3-(diphenylhexatrienyl) propionyl]-sn-3-phosphocholine (DPHp-PC) from a quenched donor vesicle. The fluorescence increase resulting from this binding was poorly quenched by either acrylamide or iodide. This indicates that nsL-TP shields the bound PC molecules from the aqueous environment. Time-resolved analysis of DPH fluorescence originating from DPHp-PC bound to nsL-TP yielded a rotational correlation time of 7.4 ns. This correlation time strongly suggests that the DPH moiety of the bound molecule is immobilized and that the nsL-TP/DPHp-PC complex is not attached to the donor vesicle. In view of the longer rotational correlation time obtained for the intrinsic tryptophanyl fluorescence, we conclude that nsL-TP is highly asymmetric. The data are consistent with a model in which the shape of nsL-TP is ellipsoidal with an axis ratio of 2.8. The implications for the mode of action of nsL-TP are discussed.