Apolipoprotein H (ApoH), also known as β2-glycoprotein I, is a plasma glycoprotein with its in vivo physiological and pathogenic roles being closely related to its interaction with negatively charged membranes. Although the threedimensional crystal structure of ApoH has been recently solved, direct evidence about the spatial state of ApoH on the membrane is still lacking. In this work, the interactions of ApoH with the lipid layer are studied by a combination of lipid monolayer approach and surface concentration determination. The spatial state of the orientation of ApoH on the lipid layer is investigated by analyzing the process of membrane-attached ApoH molecules being extruded out from the phospholipid monolayer by compression. The results show that on neutral lipid layer ApoH has an upright orientation, which is not sensitive to the phase state of the lipid layer. However, on acidic lipid layer, ApoH may have two forms of orientation. One is an upright orientation in the liquid phase region, and the other is flat orientation on the condensed domain region. The variation of the spatial state of ApoH on the lipid layer may relate to a variety of its physiological functions.

SecA, an essential component of the general protein secretion pathwayof bacteria, is present in Escherichia coli as soluble andmembrane-integral forms. Hereweshow by electron microscopy thatSecA assumes two characteristic forms in the presence of phospholipidmonolayers: dumbbell-shaped elongated structures and ring-likepore structures. The ring-like pore structures with diameters of 8 nmand holes of 2nm are found only in the presence of anionic phospholipids.These ring-like pore structures with larger 3- to 6-nm holes(without staining) were also observed by atomic force microscopicexamination. They do not form in solution or in the presence ofuncharged phosphatidylcholine. These ring-like phospholipidinducedpore-structures may form the core of bacterial proteinconductingchannels through bacterial membranes.

Apolipoprotein H (ApoH) is a plasma glycoprotein with its in vivo physiological and pathogenic roles being closely related to its interaction with negatively charged membranes. In this paper, the interaction of ApoH with phospholipid vesicles was characterized by (i) detecting the wavelength shift of the fluorescence spectrum of ApoH and (ii) measuring the fluorescence quenching extent of ApoH by the membrane resident quencher 1-palmitoyl-2-stearoyl-(5-doxyl)-sn-glycero-3-phosphocholine (DPC). The observed blue shift upon addition of DMPG vesicles indicated that the tryptophan residues of ApoH moved from a polar to a nonpolar environment. The insertion ability of ApoH into PG-containing v esicles did not depend on the PG content in a stoichiometric way as did the blue shift, indicating that the negatively charged DMPG does not serve as a specific binding site but rather provides a suitable microenvironment for ApoH interaction. The finding that the detachment effect of cations on the blue shift is remarkably different from that on the quenching extent suggests that ApoH is capable of existing in two different conformations when membrane-bound.

Many proteins exhibit both a water-soluble and a membrane-bound state. The proteins in the membrane-bound state obtain a distinct structure from that in the bulk, which exists in many important biological processes. In the present paper we would stress that the variation of the physical chemistry properties of the microenvironment adjacent to the membranesurface region play an important role in the process of the membrane-induced conformational changes of the proteins.

The interaction of apolipoprotein H (Apo H) with lipid membrane has been considered to be a basic mechanism for the biological function of the protein. Previous reports have demonstratedthat Apo H can interact only with membranes containinganionic phospholipids. Here we study the membrane-inducedconformational change of Apo H by CD spectroscopy with twodifferent model systems: anionic-phospholipid-containing liposomes[such as 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol(DMPG) and cardiolipin], and the water}methanol mixtures atmoderately low pH, which mimic the micro-physicochemicalenvironment near the membrane surface. It is found that Apo Hundergoes a remarkable conformational change on interactionwith liposomes containing anionic phospholipid. To interact withliposomes containing DMPG, there is a 6.8%increase in a-helixin the secondary structures; in liposomes containing cardiolipin,however, there is a 12.6% increase in a-helix and a 9% decreasein b-sheet. The similar conformation change in Apo H can beinduced by treatment with an appropriate mixture of water}methanol. The results indicate that the association of Apo Hwith membrane is correlated with a certain conformationalchange in the secondary structure of the protein.