Connectivity of membrane domains

Effects of glycine substitutions on the structure and function of gramicidin a channels

Tryptophan residues often are found at the lipid-aqueous interface region of membrane-spanning proteins, including ion channels, where they are thought to be important determinants of protein structure and function. To better understand how Trp residues modulate the function of membrane-spanning channels, we have examined the effects of Trp replacements on the structure and function of gramicidin A channels. Analogues of gramicidin A in which the Trp residues at positions 9, 11, 13, and 15 were sequentially replaced with Gly were synthesized, and the three-dimensional structure of each analogue was determined using a combination of two-dimensional NMR techniques and distance geometry-simulated annealing structure calculations. Though Trp --> Gly substitutions destabilize the beta6.3-helical gA channel structure, it is possible to determine the structure of analogues with Trp --> Gly substitutions at positions 11, 13, and 15, but not for the analogue with the Trp --> Gly substitution at position 9. The Gly11-, Gly13-, and Gly15-gA analogues form channels that adopt a backbone fold identical to that of native gramicidin A, with only small changes in the side chain conformations of the unsubstituted residues. Single-channel current measurements show that the channel function and lifetime of the analogues are significantly affected by the Trp --> Gly replacements. The conductance variations appear to be caused by sequential removal of the Trp dipoles, which alter the ion-dipole interactions that modulate ion movement. The lifetime variations did not appear to follow a clear pattern.

Electromagnetic-field-induced oscillations of the lipid domain structures in the mixed membranes

The effect of external electromagnetic field (EMF) on a percolation structure formed during phase separation in the mixed phospholipid membranes was studied by computer simulation. Decay of the percolation structure under electromagnetic radiation was detected. It was shown that oscillation regime can be realized in this system: periodic alternation of formation and decay of the percolation cluster was observed under 10 kHz EMF. The decay of the lipid domain structure in the EMF results from anomalous increase of the permittivity of the continuous fluid lipid phase in the percolation threshold region. It is proposed that detected EMF effect can influence the signal and transport processes associated with percolation properties of biomembranes.

Effect of the conformation of a peptide from gp41 on binding and domain formation in model membranes

Binding of the peptide fragment 828-848 (P828), amino acid sequence RVIEVVQGACRAIRHIPRRIR, from the carboxy-terminal region of the envelope glycoprotein gp41 of human immunodeficiency virus type 1 (HIV-1) to membranes composed of a mixture of neutral and negatively charged phospholipids results in domain or cluster formation of the charged lipid. The conformation and dynamics of the peptide are investigated in solution and in the presence of sodium dodecyl sulphate (SDS) micelles using high resolution nuclear magnetic resonance (NMR) spectroscopy and circular dichroism (CD) spectropolarimetry. The CD results demonstrate that addition of either SDS, negatively charged phospholipid liposomes, or trifluoroethanol (TFE) induces a conformational transition of the peptide from a random coil or an extended chain in water to a more ordered structure with an estimated helical content of up to 60%. The structure of the peptide in a membrane mimetic SDS solution was investigated in detail using two-dimensional NMR. The measurements demonstrate the existence of a helical component in the peptide conformation in the SDS-bound state. The peptide most likely exists as an ensemble of conformations with exchange times between them which are fast on the chemical shift NMR time scale (10(-3) s). Simple neutralization of the six arginine sidechain charges does not cause the peptide to adopt an ordered structure. Thus, there is an additional requirement for the structural transition such as that resulting from constraint of the peptide on a surface, or localization of the peptide at the lipid-water interface where the polarity is lower.(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.