Penetration and fusion of phospholipid vesicles by lysozyme

The cytosolic domain of Fis1 binds and reversibly clusters lipid vesicles

Every lipid membrane fission event involves the association of two apposing bilayers, mediated by proteins that can promote membrane curvature, fusion and fission. We tested the hypothesis that Fis1, a tail-anchored protein involved in mitochondrial and peroxisomal fission, promotes changes in membrane structure. We found that the cytosolic domain of Fis1 alone binds lipid vesicles, which is enhanced upon protonation and increasing concentrations of anionic phospholipids. Fluorescence and circular dichroism data indicate that the cytosolic domain undergoes a membrane-induced conformational change that buries two tryptophan side chains upon membrane binding. Light scattering and electron microscopy data show that membrane binding promotes lipid vesicle clustering. Remarkably, this vesicle clustering is reversible and vesicles largely retain their original shape and size. This raises the possibility that the Fis1 cytosolic domain might act in membrane fission by promoting a reversible membrane association, a necessary step in membrane fission.

Molten-globule structure and membrane binding of the N-terminal protease-resistant domain (63-193) of the steroidogenic acute regulatory protein (StAR)

The first step in steroidogenesis is the movement of cholesterol from the outer to inner mitochondrial membrane; this movement is facilitated by the steroidogenic acute regulatory protein (StAR). StAR has molten-globule properties at low pH and a protease-resistant N-terminal domain at pH 4 and pH 8 comprising residues 63-193. To explore the mechanism of action of StAR we investigated the structural properties of the bacterially expressed N-terminal domain (63-193 StAR) using CD, limited proteolysis and NMR. Far- and near-UV CD showed that the amount of secondary structure was greater at acidic than at neutral pH, but there was little tertiary structure at any pH. Unlike 63-193 StAR liberated from N-62 StAR by proteolysis, biosynthetic 63-193 StAR was no longer resistant to trypsin or proteinase K at pH 7, or to pepsin at pH 4. Addition of trifluoroethanol and SDS increased secondary structure at pH 7, and dodecylphosphocholine and CHAPS increased secondary structure at pH 2, pH 4 and pH 7. However, none of these conditions induced tertiary structure, as monitored by near-UV CD or NMR. Liposomes of phosphatidylcholine, phosphatidylserine and their mixture increased secondary structure of 63-193 StAR at pH 7, as monitored by far-UV CD, and stable protein-liposome complexes were identified by gel-permeation chromatography. These results provide further evidence that the N-terminal domain of StAR is a molten globule, and provide evidence that this conformation facilitates the interaction of the N-terminal domain of StAR with membranes. We suggest that this interaction is the key to understanding the mechanism of StAR's action.

Surface potential regulation of phospholipid composition and in-out translocation in yeast

In yeast cells the anionic phospholipids, phosphatidylinositol and phosphatidylserine, determine to a large extent the magnitude of the negative surface charge density (sigma) [Cerbón, J. & Calderón, V. (1990) Biochim. Biophys. Acta 1028, 261-267]. We now report further findings. (a) When the yeast phi out was reduced by increasing the concentration of monovalent (C+) or divalent (C2+) cations in the culture medium, the relative amount of anionic phospholipids increased (45-52%). (b) For each such increment, a corresponding increase in the external surface charge density (sigma) was found, due to the translocation from the cytoplasmic side to the exoplasmic side of the plasma membrane. (c) These changes were reversed when the phi out was increased by reducing the concentration of cations in the culture medium. (d) When the phi out was reduced and phosphatidylserine decarboxylation or phosphatidylinositol degradation were inhibited, to measure synthesis of anionic phospholipids, a 1.4 times further increase in the anionic/zwitterionic phospholipid ratio occurred. As a consequence, a similar increase in the external surface charge (sigma) was found. (e) Under all the conditions studied, the percentage of anionic phospholipid at the external surface of the plasma membrane calculated from the sigma values was 2.3-3.0 times less than that in the cells, indicating that the asymmetric composition (more inside) was maintained. A model for the regulation of the anionic phospholipid composition of the yeast membranes is proposed.

The role of protein charge in protein-lipid interactions. pH-dependent changes of the electrophoretic mobility of liposomes through adsorption of water-soluble, globular proteins

The role of electrostatics in the adsorption process of proteins to preformed negatively-charged (phosphatidylcholine/phosphatidylglycerol) and neutral (phosphatidylcholine) liposomes was studied. The interaction was monitored at low ionic strength for a set of model proteins as a function of pH. The adsorption behavior of trypsin inhibitor (pI = 4.6), myoglobin (pI = 7.4), ribonuclease (pI = 9.6), and lysozyme (pI = 10.7) with preformed liposomes was investigated, along with changes in the electrophoretic mobility of liposomes through the adsorption of charged proteins. Mean protein charge was determined by acid/base titration. Significant adsorption of the proteins to negatively-charged liposomes was only found at pH values where the number of positive charge moieties exceeds the number of negative charge moieties on the protein by at least three charge units. Negligible adsorption to liposomes composed of zwitterionic lipids was observed in the pH range tested (4-9). The absolute value of the electrophoretic mobilities of negatively-charged, empty liposomes decreased after adsorption of positively-charged proteins. With increasing protein to phospholipid ratio, the drop in the electrophoretic mobility leveled off and reached a plateau; protein adsorption profiles showed a similar shape. Analysis of the data demonstrated that neutralization of the liposome charge due to the adsorption of the positively-charged proteins is the controlling factor in their adsorption. The plateau level reached depended on the type of protein and the pH of the incubation medium. This pH dependency could be ascribed to the mean positive charge of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Association of lysozyme to phospholipid surfaces and vesicle fusion

Lysozyme-induced fusion of phosphatidylserine (PS) vesicles was studied as a function of pH. Fusion, monitored by lipid-mixing, was measured by following the dilution of pyrene-labelled phosphatidylcholine, incorporated in PS vesicles, into unlabelled bilayers. It is demonstrated that lysozyme-induced fusion is pH-dependent and significant fusion is triggered at pH 5 or below. The interaction of lysozyme with the vesicle bilayer was characterized by measuring resonance energy transfer from tryptophane, present in the protein, to pyrene. It is shown that concomitant with fusion, a strong resonance energy transfer signal appears at pH 5 or below. Furthermore, in monolayer experiments it was found that addition of lysozyme to the subphase caused an increase in surface pressure, when the pH was kept below 5.5. Very low concentrations of lysozyme sufficed to bring about the observed effects. The results are taken to indicate that lysozyme-induced fusion results from penetration of protein into the hydrophobic core of the bilayer, occurring at acidic pH.

Characterization of complex formation between lipopolysaccharide and lysozyme

The binding of lysozyme (LZM) to bacterial lipopolysaccharide (LPS) inhibited the biological activities of LPS as well as the enzymic activity of LZM. The mode of binding has been characterized by using dansylated LZM and enzyme inhibition. The binding of LPS to LZM significantly increased the fluorescence intensity (Fl-intensity) of the danyl group and was found to be time-dependent; the complex was produced gradually and became stabilized within 20 min at 37 degrees, 10 min at 50 degrees, and 1 min at 70 degrees. The maximum level of binding was also dependent on the reaction temperature, and more complex was formed at higher temperatures. Complexation was strongly dependent on the salt concentration and was not observed at greater than 0.5M NaCl. From collected evidence of the Fl-intensities of various dansyl derivatives and amphiphiles, it is concluded that LZM interacts with LPS by multiple binding-modes, the first being strongly related to the enzyme inhibition, the second being close to the Fl-intensity, and the third being dependent on the inhibition of immunopharmacological activities. For the amphiphiles used in this study, sodium dodecyl sulfate (SDS), 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-propanesulfonate (CHAPSO), decansulfonic acid, and cardiolipin have binding modes similar to that of LPS.

Interaction of phospholipids with proteins, peptides and amino acids. New advances 1987-1989

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins, peptides and amino acids. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. The effect of the interaction on the structure and biological activity of the interacting biomolecules is discussed.