Functional signal peptide reduces bilayer thickness of phosphatidylcholine liposomes.

To investigate the interaction between a signal peptide and the lipid bilayer, two kinds of peptides, L8-M5 (L8 = MRL8PLAALG, M5 = KVFER) and L14-M5 (L14 = MRL14PLAALG), were examined in membranes composed of dioleoylphosphatidylcholine (DOPC). Peptides L8 and L14 are artificially designed signal sequences, and M5 is the N-terminal five residues of human lysozyme; L8 mediated effective secretion of human lysozyme in yeast, while L14 did not [Yamamoto, Y., et al. (1987) Biochem. Biophys. Res. Commun. 149, 431-436]. DOPC liposomes incorporating L8-M5 or L14-M5 were observed by electron cryomicroscopy as pairs of concentric circles, and the separation of the bilayer was measured along the membrane. Peptide L8-M5 was found to reduce the bilayer thickness, but L14-M5 did not. CD measurements revealed that L8-M5 adopted an alpha-helical conformation with random coil in the liposome membranes and that L14-M5 adopted a more helical and less random conformation than L8-M5. Fluorescence spectroscopy using both aqueous and membranous probes revealed that L8-M5 destabilized the lipid bilayer more strongly than L14-M5. These results suggest that functional L8-M5 reduces the bilayer thickness and destabilizes the lipid bilayer and that these activities are important for signal peptide function.