Orientation of Functional and Nonfunctional PTS Permease Signal Sequences in Lipid Bilayers. A Polarized Attenuated Total Reflection Infrared Study

Synthetic peptides corresponding to the N-terminal 23 and 22 residues, respectively, of two integral plasma membrane proteins of Escherichia coli, namely the mannitol- and glucitol-specific permeases of the bacterial sugar phosphotransferase system, were incorporated into single planar phospholipid bilayers supported on germanium plates. Polarized attenuated total reflection infrared spectra were recorded, and order parameters were derived from the measured dichroic ratios. The order parameters of the two wild-type peptides which form amphiphilic alpha-helices in membranes were -0.4 to -0.5, indicating a preferential alignment of the alpha-helix long axis parallel to the membrane surface. Nonfunctional mutant peptides of the mannitol permease sequence in which serine-3 or aspartate-4 were substituted with prolines (S3P and D4P) or lysine (D4K), but which were still largely alpha-helical, exhibited peptide order parameters close to zero, indicating a high degree of disorder of these peptides in the lipid bilayers. The lipid was well ordered at low concentrations of peptides in the membranes but became disordered at high peptide concentrations. This effect of lipid disordering was more pronounced for the D4K mutant than for the wild-type mannitol peptide.