Electrostatics and reduction of dimensionality produce apparent cooperativity when basic peptides bind to acidic lipids in membranes

Adsorption of Ca2+ and La3+ to bilayer membranes: measurement of the adsorption enthalpy and binding constant with titration calorimetry

The adsorption of Ca2+ and La3+ ions to the surface of lipid bilayer membranes was studied with high sensitivity titration calorimetry. Ca2+ adsorbs to mixed phosphatidylcholine/phosphatidylglycerol membranes with a reaction enthalpy of delta H approximately 0.1-0.2 kcal/mol. La3+ binds to sonified phosphatidylcholine vesicles with a reaction enthalpy of delta H approximately +1.8 kcal/mol. Adsorption of La3+ to phosphatidylcholine bilayers imparts a net positive charge to the membrane surface which makes the binding of further La3+ increasingly more difficult. From the decreasing amplitudes in the calorimetric titration experiment a La3+ adsorption constant of K approximately (4.1 +/- 1.1) x 10(3) M-1 was evaluated. Electrostatic effects were corrected for by means of the Gouy-Chapman theory. The adsorption constant of Ca2+ was determined previously as K approximately 10-20 M-1 using the same binding model. Since the reaction enthalpies of Ca2+ and La3+ adsorption are endothermic, the adsorption of both metal ions to the membrane surface is driven by a distinct change in entropy.

Interaction of phospholipids with proteins and peptides. New advances IV

1. The review deals with the newest achievements in the field of the various interactions between phospholipids and proteins and peptides. 2. Interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecular assemblies is also discussed.

Calcium-dependent annexin V binding to phospholipids: stoichiometry, specificity, and the role of negative charge

Annexin V is a Ca(2+)-dependent, phospholipid-binding protein that may have one or more membrane-related functions. The binding of annexin V to phospholipids in a detergent micelle matrix was studied to attempt to determine directly the stoichiometry of specific phospholipid-binding sites and the importance of negative charge. When annexin V binds to phospholipids, a large increase (severalfold) of the emission intensity of tryptophan 187 is observed. This intensity change was used to monitor the binding to phosphatidylcholine (PC) or phosphatidylserine (PS) at varying ratios with the detergent, octaethylene glycol monododecyl ether (C12E8). No binding to PC alone in these micelles could be observed, while approximately 10 PS molecules per micelle were required to observe binding. However, inclusion of negatively charged amphiphiles in the micelles, such as oleic acid or dodecyl sulfate, allowed the observation of binding to PC and decreased the number of phospholipids per micelle necessary for binding to both PS and PC. By including increasing proportions of dodecyl sulfate in the C12E8 micelles, a minimum average number of PS or PC per micelle of approximately 3-4 was required for complete binding. Labeling with photoreactive phospholipids under similar conditions led to an average of approximately 4-5 phospholipids covalently bound per annexin V monomer. Since annexin V has four similar domains, it is reasonable to suggest that one phospholipid binding site is associated with each domain, although as few as three functional domains may be sufficient for binding. Efficient binding required certain structural features of the phospholipid, including a phosphate group, an sn-2 acyl chain, and at least a few carbons on the sn-2 chain. Phospholipid headgroups were almost irrelevant, except for important surface charge effects on the interfacial ionic double layer. A negative surface charge on the micellar aggregate nonspecifically increases the Ca2+ concentration near the micelle surface and may also directly enhance the affinity of annexin V for phospholipids, as shown by the decreased two-dimensional phospholipid concentration necessary for binding. The ability to bind to zwitterionic phospholipids in the presence of a nonspecific negative surface charge may be relevant to the extracellular functions of this protein. Relatively weak individual phospholipid-binding sites that easily exchange were observed, suggesting rapid exchange of phospholipids between the sites on membrane-bound annexin V. These data suggest a working hypothesis that includes approximately four binding sites specific for phospholipid phosphate groups and sn-2 acyl chains.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Interaction of peptide fragment 828-848 of the envelope glycoprotein of human immunodeficiency virus type I with lipid bilayers

The interaction of the peptide fragment 828-848, called P828, from the carboxy-terminal region of the envelope glycoprotein gp41 of HIV-I with model membranes composed of phosphatidylcholine (PC) and phosphatidylglycerol (PG) was investigated using microelectrophoretic mobility of liposomes, fluorescence polarization of labeled lipids, NMR, and differential scanning calorimetry. The peptide binds to negatively charged lipid surfaces. No interaction between P828 and neutral PC surfaces is observed. The interaction between the peptide and the lipid is exclusively electrostatic with the six positively charged arginines of P828 acting as binding sites for PG. Circular dichroism measurements of P828 indicate that the peptide undergoes a transition from a random coil to an ordered conformation upon binding to negatively charged PG bilayers or SDS micelles, but not in the presence of neutral PC bilayers. The ordered structure has an apparent helical content of 60%. IN DOPG/DOPC mixtures containing 20 mol % DOPG, the peptide causes the formation of lipid domains enriched in DOPG, as assessed by measurement of fluorescence energy transfer between labeled PG and PC. The formation of these domains requires energy and therefore reduces the strength of peptide binding to the lipid matrix. Our data support and quantitate the results from antibody binding studies [Haffar, O.K., Dowbenko, D. J., & Berman, P. W. (1988) J. Cell Biol. 107, 1677-1687] that the carboxy-terminal segment of the envelope glycoprotein gp41 interacts with microsomal membranes.

Thermodynamic characterization of the association of small basic peptides with membranes containing acidic lipids

We measured the binding of the peptide acetyl-Trp-Lys7-amide to membranes formed from mixtures of the zwitterionic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC) and the acidic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (PG). Direct filtration and equilibrium dialysis measurements demonstrate that binding increases sigmoidally with the mole fraction of PG as predicted from a simple Gouy-Chapman/mass action theoretical model. We used these measurements to calibrate two binding assays, one based on the increase in Trp fluorescence that occurs when the peptide binds to the membrane, the other on the quenching of Trp fluorescence that occurs when the peptide binds to membranes containing fluorescent lipids. Both fluorescence assays demonstrate that binding does not depend strongly on temperature, which suggests the enthalpy change, delta H, is small. Calorimetric measurements demonstrate this directly for the analogous basic peptide Lys5: delta H congruent to +1 kcal/mol for the binding of Lys5 to sonicated phospholipid vesicles and delta H congruent to 0 kcal/mol for its binding to large unilamellar vesicles. Thus, the decrease in the free energy that occurs when these peptides bind to the membrane is due to a positive change in the entropy of the system. Fluorescence measurements demonstrate the binding of the Trp-containing peptide to 4:1 PC/PG membranes is independent of pressure up to 2 kbar, which suggests that binding occurs without a significant change in volume.

Phosphoinositide-specific phospholipase C-delta 1 binds with high affinity to phospholipid vesicles containing phosphatidylinositol 4,5-bisphosphate

We studied the binding of phosphoinositide-specific phospholipase C-delta 1 (PLC-delta) to vesicles containing the negatively charged phospholipids phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylserine (PS). PLC-delta did not bind significantly to large unilamellar vesicles formed from the zwitterionic lipid phosphatidylcholine (PC) but bound strongly to vesicles formed from mixtures of PC and PIP2. The apparent association constant for the putative 1:1 complex formed between PLC-delta and PIP2 was Ka congruent to 10(5) M-1. The binding strength increased further (Ka congruent to 10(6) M-1) when the vesicles also contained 30% PS. High-affinity binding of PLC-delta to PIP2 did not require Ca2+. PLC-delta bound only weakly to vesicles formed from mixtures of PC and either PS or phosphatidylinositol (PI); binding increased as the mole fraction of acidic lipid in the vesicles increased. We also studied the membrane binding of a small basic peptide that corresponds to a conserved region of PLC. Like PLC-delta, the peptide bound weakly to vesicles containing monovalent negatively charged lipids; unlike PLC-delta, it did not bind strongly to vesicles containing PIP2. Our data suggest that a significant fraction of the PLC-delta in a cell could be bound to PIP2 on the cytoplasmic surface of the plasma membrane.