Influence of transmembrane peptides on bilayers of phosphatidylcholines with different acyl chain lengths studied by solid-state NMR

The molecular orientation in a lipid membrane of the peptide fragment VEYAG IALFFVAAVLTLWSMLQYLSAAR (phosphatidylglycerophosphate synthase (Pgs) peptid e E) of an integral membrane protein, Pgs, in Escherichia coli has been inv estigated by solid-state N-15 nuclear magnetic resonance (NMR) on macroscop ically aligned lipid bilayers. The secondary structure of the peptide in li pid vesicles was determined by circular dichroism spectroscopy. Furthermore , the phase behaviour of the Pgs peptide E/dierucoylphosphatidylcholine (DE ruPC)/water system was determined by H-2, P-31 and N-15 solid-state NMR spe ctroscopy. The phase behaviour obtained was then compared to that of the Pg s peptide E solubilised in dioleoylphosphatidylcholine and water that was p reviously 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 peptid e mostly has a transmembrane orientation and is in an cl-helical conformati on. An isotropic phase is formed in DEruPC with high peptide content (pepti de/lipid molar ratio (p/1) greater than or equal to 1:15) and high water co ntent (greater than or equal to 50%, w/w) at 35 degreesC. At 55 and 65 degr eesC an isotropic phase is induced at high water content (greater than or e qual to 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 pep tide contents (p/l greater than or equal to 1:15) an isotropic phase forms at 20 and 40% (w/w) Of water at 55 and 65 degreesC. A comparison of the pha se behaviour of the two homologous lipid systems reveals striking similarit ies, although the thicknesses of the two lipid bilayers differ by 7 Angstro m. This suggests that the rationalisation of the phase behaviour in terms o f the hydrophobic mismatch is not applicable to these systems. The C-termin us of Pgs peptide E is amphiphilic and a considerable part of the peptide i s situated outside the hydrophobic part of the bilayer, a property of the p eptide that to a large extent will affect the lipid/peptide phase behaviour . (C) 2000 Elsevier Science B.V. All rights reserved.