Influence of transmembrane peptides on bilayers of phosphatidylcholines with different acyl chain lengths studied by...

The molecular orientation in a lipid membrane of the peptide fragment VEYAGIALFFVAAVLTLWSMLQYLSAAR (phosphatidylglycerophosphate synthase (Pgs) peptide E) of an integral membrane protein, Pgs, in Escherichia coli has been investigated by solid-state 15N nuclear magnetic resonance (NMR) on macroscopically aligned lipid bilayers. The secondary structure of the peptide in lipid vesicles was determined by circular dichroism spectroscopy. Furthermore, the phase behaviour of the Pgs peptide E/dierucoylphosphatidylcholine (DEruPC)/water system was determined by (2)H, (31)P and 15N solid-state NMR spectroscopy. The phase behaviour obtained was then compared to that of the Pgs peptide E solubilised in dioleoylphosphatidylcholine and water that was previously studied by Morein et al. [Biophys. J. 73 (1997) 3078-3088]. This was aimed to answer the question whether a difference in the length of the hydrophobic part of this peptide and the hydrophobic thickness of the lipid bilayer (hydrophobic mismatch) will affect the phase behaviour. The peptide mostly has a transmembrane orientation and is in an alpha-helical conformation. An isotropic phase is formed in DEruPC with high peptide content (peptide/lipid molar ratio (p/l) > or =1:15) and high water content (> or =50%, w/w) at 35 degrees C. At 55 and 65 degrees C an isotropic phase is induced at high water content (> or =50%, w/w) at all peptide contents studied (no isotropic phase forms in the lipid/water system under the conditions in this study). At high peptide contents (p/l> or =1:15) an isotropic phase forms at 20 and 40% (w/w) of water at 55 and 65 degrees C. A comparison of the phase behaviour of the two homologous lipid systems reveals striking similarities, although the thicknesses of the two lipid bilayers differ by 7 A. This suggests that the rationalisation of the phase behaviour in terms of the hydrophobic mismatch is not applicable to these systems. The C-terminus of Pgs peptide E is amphiphilic and a considerable part of the peptide is situated outside the hydrophobic part of the bilayer, a property of the peptide that to a large extent will affect the lipid/peptide phase behaviour.