Osmotic pressure studies of some phospholipid sols

Conformations of proline residues in membrane environments

Although noted as hydrophilic residues with helix-breaking potential, proline residues are observed in putatively alpha-helical transmembrane (TM) segments of many channel-forming integral membrane proteins. In addition to the recognized property of X-Pro peptide bonds (where X = any amino acid) to occur in cis as well as trans isomeric states, the tertiary amide character of the X-Pro bond confers increased propensity for involvement of its carbonyl group in specific H-bonded structures (e.g., beta- and gamma-turns) and/or liganding interactions with positively charged species. To examine this latter situation in further detail, we identified Leu-Pro-Phe as a consensus sequence triad based on actual occurrences of intramembranous Pro residues in transport protein TM segments. Accordingly, we have undertaken the synthesis of hydrophobic peptides with potential membrane affinity, of which t-butyloxycarbonyl-L-Ala-L-Ala-L-Ala-L-Leu-L-Pro-L-Phe-OH (t-Boc-AAALPF-OH) is an initial compound. Partitioning of this peptide into model membrane environments composed of lipid micelles induces specific conformation(s) for the membrane-bound hexapeptide, as monitored by 75-MHz 13C-nmr spectral behavior of 13C-enriched Leu and Pro carbonyl carbons, and by 300-MHz 1H-nmr spectra of peptide alpha, beta, and aromatic protons. Data are interpreted in terms of an intramolecularly H-bonded inverse gamma-turn conformation in the membrane environment involving the Leu-Pro-Phe triad. The inherent structural instability of a Pro-containing segment in a TM helix due to the multiplicity of possible local conformations is discussed as a functional aspect of membrane-buried prolines in transport proteins.

Dependence on lipid structure of the coil-to-helix transition of bovine myelin basic protein

In aqueous solution bovine myelin basic protein has a close-to-random coil structure that is partially transformed to helix on interaction with lipids. Circular dichroism spectra have been used to follow this conformational transition which, with phospholipids, decreases in the order phosphatidylglycerol, phosphatidic acid approximately equal to phospholipids, decreases in the order phosphatidylethanolamine. There appears to be a strong correlation between the extent of alpha-helix formation and the degree of penetration of the hydrophobic region of the bilayer, as assessed by other methods. Cholesterol mixed in bilayers with phosphatidylserine has little effect on the protein secondary structure. Although basic protein binds strongly to cerebroside and to cerebroside sulphate, two of the other major myelin lipids, the intrinsic chirality of these lipids precludes assessment of their effect on the protein conformation. No significant changes in the circular dichroism spectra accompany the protein association with either of the zwitterionic bilayer-forming lipids, phosphatidylethanolamine and phosphatidylcholine. This seems to exclude extensive penetration into bilayers of these lipids and hence to exclude appreciable hydrophobic interactions; on the other hand, it is argued that little evidence exists for ionic attractions to these lipids. The optical activity of peptides derived from the basic protein by cleavage at the 42-43 and 88-89 peptides bonds (with cathepsin D) and at the 115-116 bond (with a skatole derivative) has also been measured in an attempt to locate the helix-forming regions within the primary structure.

Association of myelin basic protein with detergent micelles

Equilibrium measurements of the binding of central nervous system myelin basic protein to sodium dodecyl sulphate, sodium deoxycholate and lysophosphatidylcholine have been obtained by gel permeation chromatography and dialysis. This protein associates with large amounts of each of these surfactants: the apparent saturation weight ratios (surfactant/protein) being 3.58 +/- 0.12 and 2.30 +/- 0.15 for dodecyl sulphate at ionic strengths 0.30 and 0.10, respectively 1.34 +/- 0.10 for deoxycholate (at 0.12 ionic strength) and 4.0 +/- 0.5 for lysophosphatidylcholine. Binding to the ionic surfactants increases markedly close to their critical micelle concentrations. Sedimentation analysis shows that at 0.30 ionic strenght in excess dodecyl sulphate the protein is monomeric. It becomes dimeric when the binding ratio falls below 1 at a free detergent concentration of approximately 0.25 mM: below this concentration much of the protein and deterent forms an insoluble complex. The amount of dodecyl sulphate bound at high concentrations and at both above-mentioned ionic strengths corresponds closely to that expected for interaction of a single poly-peptide with two micelles. Variability of deoxycholate micelle size on interaction with other molecules precludes a similar analysis for this surfactant. Association was observed only with single micelles of lysophosphatidylcholine. The results provide strong evidence for dual lipid-binding sites on basic protein and indicate that lipid bilayer cross-linking by this protein may be effected by single molecules.

The solubilization of progesterone by mixed bile salt-phospholipid sols

The solubility of progesterone was determined in several different bile salt-phospholipid mixtures, and it is concluded that: (1) The solubility in unconjugated bile salts is greater than in the conjugated analogues, and the solubility in deoxycholate solutions is twice that in cholate solutions. (2) Substitution of hydroxyl groups in the 11 and 21 positions of progesterone increases solubility, whilst substitution in the 17-position decreases solubility in bile salt solutions. (3) Progesterone solubility in mixed bile salt solutions is proportional to the mole ratio of the surfactant mixture. (4) Sodium deoxycholate (SDC)-phospholipid sols show no such linear solubilizing properties; a minimum occurring at a mole ratio of SDC to phospholipid of 1 : 4. (5) There is a break in the solubility curve of progesterone in lysophosphatidycholine (LPC)/phosphatidylcholine (PC) mixtures at a mole ratio of 65 : 35 coincident with maximum viscosity. (6) Introduction of SDC into LPC/PC mixtures results in decreased progesterone solubility.

Detection of lipid phase transitions by surface tensiometry

A technique for the detection of lipid phase transitions is described, which involves measurement of the surface tension as a function of temperature. In the case of insoluble lipids, such as dipalmitoylphosphatidylcholine (DPPC) the lipid is spread as a multibilayer film on an aqueous substrate, while in the case of water-soluble lipids such as lysophosphatidylcholine (LPC) the surface tension of aqueous sols is measured. Surface tension at the interface, is monitored using a Wilhelmy plate while the temperature is continuously varied. Discontinuities or changes in slope in the surface tension-temperature (gamma-T) curve reflect phase transitions in the lipid. In the case of DPPC, the technique has been used to demonstrate the well-known gel-liquid crystalline thermal transition. This occurs at 36-38 degrees C in the multibilayer films; in bulk DPPC-water dispersions the transition is at 41 degrees. Cholesterol has the effect of lowering the thermal transition and broadening the temperature range. In films containing DPPC-cholesterol at a molar ratio of 2:1 or less, the transition is not present. These results are in agreement with a large number of previous studies of this system. In the case of LPC sols, a phase transition at about 70 degrees was detected when the concentration of SPC was close to the critical micelle concentration (CMC) at 70 degrees. This transition appears to reflect an increase in the equilibrium constant for micelle formation at this temperature. At higher concentrations of LPC a transition at 30 degrees, corresponding to a gel-liquid crystalline transition, was also detected. A complete description of gamma as a function of concentration and temperature in the range 10(-7) to 10(-3) g cm-3 and 20 degrees to 80 degrees has been obtained for LPC sols. The CMC varies from 6 X 10(-6) g cm-3 at 20 degrees to 10(-5) g cm-3 at 80 degrees.