Membrane-bound conformation of a signal peptide: a transferred nuclear Overhauser effect analysis

We have determined the conformation of an analogue of the Escherichia coli LamB signal peptide inserted into a model membrane using the transferred nuclear Overhauser effect (trNOE) NMR technique. In order to make NMR analysis feasible, a water-soluble LamB signal peptide analogue was designed by inserting three basic residues (KRR) into the N-terminal region of the wild-type sequence (with a Val-->Trp mutation for fluorescence measurements), viz., MMITLRKRRKLPLAVAVAAGWMSAQAMA-NH2. For the purpose of the trNOE study, the binding affinity of the peptide for phospholipid vesicles was tuned by adjusting the proportion of acidic lipid in the vesicle. Circular dichroism and fluorescence measurements showed that the KRR-LamB signal peptide spontaneously inserted into the lipid bilayer with a conformational transition from a mostly random coil to a predominantly alpha-helical structure. The trNOE analysis revealed that the alpha-helix extended from approximately the beginning of the hydrophobic core (residue Leu8) to the C-terminus. The continuity of the helix was somewhat disrupted at the end of the hydrophobic core (around residue Gly17). Furthermore, the topological arrangement of the peptide within the lipid bilayer was explored by NMR line broadening induced by a paramagnetic nitroxide-labeled lipid. The line-broadening results demonstrated that the residues in the helical region are well integrated into the acyl chain region of the bilayer. The N-terminal part of the peptide showed many trNOEs, but without any indication of a helical conformation. The line-broadening analysis indicates that this part of the peptide primarily interacts with the membrane surface.