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.

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), 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.

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.