β-Amyloid peptide (Aβ), a normal constituent of neuronal and non-neuronal cells, has been proven to be the major component of extracellular plaque of Alzheimer's disease. Interactions between Aβand neuronal membranes have been postulated to play an important role in the neuropathology of Alzheimer’s disease. Here we show that Aβ is able to insert into lipid bilayer. The membrane insertion ability of Aβ is critically controlled by the ratio of cholesterol to phospholipids. In a low concentration of cholesterol Aβ prefers to stay in membrane surface region mainly in aβ-sheet structure. In contrast, as the ratio of cholesterol to phospholipids rises above 30mol%, Aβ can insert spontaneously into lipid bilayer by its C terminus. During membrane insertion Aβ generates about 60%β-helix and removes almost all β-sheet structure. Fibril formation experiments show that such membrane insertion can reduce fibril formation. Our findings reveal a possible pathway by which Aβ prevents itself from aggregation and fibril formation by membrane insertion.

Neuronal exocytosis is mediated by Ca21-triggered rearrangementsbetween proteins and lipids that result in the opening anddilation of fusion pores. Synaptotagmin I (syt I) is a Ca21-sensingprotein proposed to regulate fusion pore dynamics via Ca21-promoted binding of its cytoplasmic domain (C2A-C2B) to effectormolecules, including anionic phospholipids and other copies of syt.Functional studies indicate that Ca21-triggered oligomerization ofsyt is a critical step in excitation–secretion coupling; however, thisactivity has recently been called into question. Here, we show thatCa21 does not drive the oligomerization of C2A-C2B in solution.However, analysis of Ca21zC2A-C2B bound to lipid monolayers,using electron microscopy, revealed the formation of ring-likeheptameric oligomers that are '11 nm long and '11 nm indiameter. In some cases, C2A-C2B also assembled into long filaments.Oligomerization, but not membrane binding, was disruptedby neutralization of two lysine residues (K326,327) within the C2Bdomain of syt. These data indicate that Ca21 first drives C2AC2Bzmembraneinteractions, resulting in conformational changesthat trigger a subsequent C2B-mediated oligomerization step.Ca21-mediated rearrangements between syt subunits may regulatethe opening or dilation kinetics of fusion pores or may play arole in endocytosis after fusion.

The interaction of rabbit C-reactive protein (rCRP) with a supported monolayer containing a phosphorylcholine moiety was studied. Three types of phospholipids were synthesized, each containing a insertion spacer of eight, six, or three atoms between the phosphorylcholine group and hydrophobic tail. By varying the length of the insertion spacer, we can vary the extension of the phosphorylcholine group from the membrane surface. By varying the monolayer composition, we can control the lateral distance between the exposed phosphorylcholine groups. Using the surface plasmon resonance technique (SPR), we demonstrated that the calcium-dependent binding of rCRP to the model membrane is governed not only by the ability of the ligand to access the binding pocket fully (spacer length), but also by lateral hindrance within the two-dimensional plane of the membrane. The value of the apparent binding constant was estimated by theoretical analysis, which is obviously dependent on the composition of the lipid mixture, and a maximum of (9.9 6 1.5) 3 106 M21 was obtained.

Trichosanthin (TCS) is the active component extracted from Tianhuafen, a traditional herbal medicine that has been used for abortion in China for centuries. It belongs to the type-I ribosomeinactivating protein (RIP) family and can inactivate the eukaryotic ribosome through its RNA N-glycosidase activity. Recentstudies have shown TCS to be multifunctional, its pharmacologicalproperties including immunomodulatory, anti-tumourand anti-HIV activities. The membrane-insertion property ofTCS is thought to be essential for its physiological effect, for itmust get across the membrane before it can enter the cytoplasmand exert its RIP function. In this paper, the membrane-insertionmechanism of TCS was studied. The monolayer experimentrevealed that TCS's membrane-insertion ability was dependenton low pH. Fluorescence spectroscopy using 1-anilinonaph-thalene-8-sulphonic acid as a probe showed that low pH mayinduce the conformational change of TCS that leads to thehydrophobic-site exposure, and the CD result showed that thisconformational change did not alter its secondary structure.Such conformational change leads to an intermediate state,called the molten globular state' by previous investigators. ThepH-dependent membrane insertion and conformational changewere related by the fact that the optimal membrane-surface pHneeded was the same for the two events. From these and otherresults, a membrane-insertion model was proposed.

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.