Self-assembly of actin scaffolds at ponticulin-containing supported phospholipid bilayers

Phospholipid vesicles containing ponticulin have been used to form solid supported and tethered bilayer lipid membranes. The ponticulin serves as both a nucleation site for actin polymerization as well as a binding site for F-actin. Studies of F-actin binding to such bilayers have demonstrated the formation of an in vitro actin scaffold. The dissociation constant for the binding of F-actin filaments to a ponticulin-containing tethered bilayer was found to be 11 +/- 5 nM, indicative of high affinity binding.

A mild photoactivated hydrophilic/hydrophobic switch

Surface modification using light is one of the most powerful methods for controlling the physical and chemical properties offunctionalized surfaces. In this paper, we report on systems where soft UV irradiation (lambda = 365 nm) converts a "low" activity fluorocarbon to a "high" activity amine-functionalized surface. An amine-functionalized SAM (self-assembled monolayer) is first masked using a tertiary amine catalyzed reaction with an N-hydroxysuccinimidyl carbonyl reagent. This mild, room-temperature reaction introduces a hydrophobic photocleavable nitrobenzyl "protecting group" terminated with a fluorocarbon end-chain. UV irradiation (lambda = 365 nm) of this hydrophobic/fluorocarbon surface cleaves the nitrobenzyl residue, returning the surface to the original hydrophilic/amine-functionalized state. This provides a mild, generic method of producing surfaces with hydrophilic/hydrophobic patterns or patterned with amine functional residues. Two different protecting groups, one terminated with a single and the other with three fluorocarbon end chains, are compared. In the case of the more bulky protecting group, only a small proportion of the amine residues react, but the surface is equally hydrophobic and the amine residues equally well shielded from further reaction. Surfaces are characterized by X-ray photoelectron spectroscopy, ellipsometry, surface potential, and contact angle measurements. Images of the photopatterned SAMs were obtained using scanning electron microscopy.

Direct electrochemical interaction between a modified gold electrode and a bacterial membrane extract

A novel electrochemical approach is described for redox-active membrane proteins. A total membrane extract (in the form of vesicles) of Bacillus subtilis is tethered onto gold surfaces modified with cholesterol based thiols. The membrane vesicles remain intact on the surface and do not rupture or fuse to form a planar bilayer. Oxidation/reduction signals are obtained of the natural co-enzyme, menaquinone-7, located in the membrane. The membrane protein, succinate menaquinone oxidoreductase (SQR), remains in the vesicles and is able to reduce fumarate using menaquinone as mediator. The catalysis of the reverse reaction (oxidation of succinate), which is the natural catalytic function of SQR, is almost absent with menaquinone. However, adding the co-enzyme ubiquinone, which has a reduction potential that is about 0.2 V higher, restores the succinate oxidation activity.

Discrete membrane arrays

This review describes various methods for the attachment of phospholipid bilayers to solid supports. The simplest approach involves vesicle unrolling onto a surface that has been previously modified with a continuous self-assembled monolayer (SAM). The choice of a suitable SAM can lead to the formation of attached bilayers that have the desired biomimetic properties and are suitable for studying transmembrane proteins. However, there are intrinsic problems associated with this approach if one is interested in studying ion transport phenomena. In particular, the relatively low resistance values found for such bilayers do not permit studies of single ion channels. For such studies to be carried out the background leakage through the lipid film must be greatly reduced. In an attempt to reduce the problems of leakage we have formed patterned SAMs in which a blocking, hydrophobic, layer covers 90% of the electrode surface. The remaining portion of the surface, which is hydrophilic, supports the formation of a bilayer. This approach has led to an improvement in the quality of the bilayers formed but has still not provided bilayers with sufficiently high specific resistances to study single ion channels. Finally, we describe new approaches based on the formation of bilayers suspended over small apertures. These 'suspended' bilayers are similar in structure to those used in black lipid membrane experiments and give rise to highly blocking bilayer membranes. Unfortunately, this approach requires the use of solvents to create the suspended bilayer and they are relatively fragile.

Orientation of the headgroup of phosphatidylinositol in a model biomembrane as determined by neutron diffraction

Derivatives of the sodium salt of dimyristoylphosphatidylinositol (DMPI) have been synthesized specifically deuterated in the headgroup. A 50:50 (molar) mixture of DMPI with dimyristoylphosphatidylcholine (DMPC) hydrated to the level of 16 waters/lipid gives a biomembrane-like Lalpha phase at 50 degrees C. Comparison of the neutron diffraction scattering profiles for deuterated and undeuterated membranes allowed the depth of each deuterium (hydrogen) within the bilayer to be determined to +/-0.5 A. This gave the orientation of the inositol ring which lies more-or-less along the bilayer normal projecting directly out into the water. This orientation is similar to that of the sugar residue in glycolipids and confirms previous models for PI. On the assumption that the (P)O-DAG bond is more-or-less parallel to the bilayer normal, it is consistent with a roughly trans, trans, trans, gauche- conformation for the glyceryl-phosphate-inositol link. In the case of DMPI, it is the C4-hydroxy group which is most fully extended into the water layer, but when this is phosphorylated, the inositol ring turns over and tilts so that the C5-hydroxy group is now the one furthest extended into the water layer. Hence, at each stage in the pathway PI --> PI-4P --> PI-4,5-P2, it is the hydroxy position most exposed to the water which undergoes phosphorylation. Whereas the orientation of the inositol ring in DMPI can be seen simply as maximizing its hydration, the tilt of the ring in DMPI-4P cannot be explained in this way. It is suggested that it is due to an electrostatic interaction.

The headgroup orientation of dimyristoylphosphatidylinositol-4-phosphate in mixed lipid bilayers: a neutron diffraction study

The trisodium salt of dimyristoylphosphatidylinositol-4-phosphate (DMPI-4P) has been synthesised specifically deuterated at particular sites in the headgroup. These materials have been used in neutron diffraction experiments, which successfully located the position (depth) of each of these deuterated sites to within +/- 0.5 A in a mixed model membrane (a 1:1 molar mixture of DMPI-4P with dimyristoyl-phosphatidylcholine, DMPC, in the L alpha phase, hydrated to the level of 28 water molecules per lipid molecule). The diffracted intensities were measured at four different D2O/H2O ratios and six orders of diffraction were obtained. These data sets, in conjunction with computer modelling, have been used to determine the orientation of the inositol ring of DMPI-4P, localising each vertical H-H distance to within approximately +/- 0.03 A. The orientation of the inositol ring is found to be one in which the C5 hydroxyl is extended out into the aqueous medium. This is, therefore, the most accessible site for water-borne reagents. This may be significant for the important pathway leading from PI-4P to PI-4,5P2. On the assumption that the P/ODAG bond is orientated parallel to the bilayer normal, these results are consistent with two possible conformations for the portion of the headgroup connecting the diacylglycerol to the inositol ring. Distinction between these two is difficult, but one may be favoured since the other involves close atom-atom contacts.

Neutron diffraction reveals the orientation of the headgroup of inositol lipids in model membranes

Neutron diffraction studies show that the inositol ring in the headgroup of phosphatidylinositol extends perpendicular to the membrane surface but that phosphorylation of the 4-position causes the ring to tilt over.

The conformational behaviour of phosphatidylinositol in model membranes: 2H-NMR studies

Dimyristoylphosphatidylinositol (DMPI) has been synthesized with the appropriate natural stereochemistry and labelled with deuterium at specific sites in the D-myo-inositol headgroup. 2H-NMR spectroscopy of DMPI in its lamellar phase at a molar ratio of water-to-lipid RW/L of 129 and at 70 degrees C reveals quadrupolar splittings delta v of 3.83 and 2.17 kHz, respectively, for the five axially oriented C-D bonds and the single equatorially oriented C-D bond of the D-myo-inositol headgroup. Between RW/L ratios of 129 and 210 and between 30 degrees C and 80 degrees C the value of the ratio of these splittings delta nu ax/delta nu eq varies significantly (between 1.17 and 4.38). If it is assumed that, at a particular temperature, there is a single preferred orientation of the inositol headgroup, and that motion of the DPMI molecule establishes axial symmetry with respect to the bilayer normal then the ratio of these quadrupolar splittings can be used to impose constraints on that orientation. For example, the data are inconsistent with a situation in which the inositol ring lies parallel to the membrane surface and are difficult to reconcile with an arrangement where the inositol ring lies perpendicular to the surface. Computational modelling identifies four possible 'tilted' orientations, all of which are consistent with the data, and two of these allow good intramolecular hydrogen bonds to be formed. In one there is hydrogen bonding between the inositol C2-OH and the phosphate pro-R oxygen. This is close to the conformation previously identified as being dominant in DMSO solution (Bushby, R.J., Byard, S.J., Hansbro, P.M. and Reid, D.G. (1990) Biochim. Biophys. Acta 1044, 231-236).

The conformational behaviour of phosphatidylinositol

The temperature dependence of the 1H-NMR spectrum of phosphatidylinositol (PI) in d6-dimethylsulphoxide (DMSO) shows that the hydroxy groups at C2 and at C6 of the inositol ring are internally hydrogen-bonded. This probably implies a trans/gauche conformation for the phosphate/inositol linkage. The presence of a trans phosphate-alkyl-oxygen bond is confirmed by 31P-NMR studies. If the conformation of PI in membranes is the same as that in DMSO solution, this implies that the inositol ring points out into the aqueous phase with its C1/C4 axis almost perpendicular to the membrane surface. Progress is also reported in attempts to characterise headgroup orientation and dynamics by 2H-NMR using deuterated synthetic PI, prepared by the route devised by Ward, J.G. and Young, R.C. (Tetrahedron Lett. 29 (1988) 6013-6016).