Peptides in membranes: Lipid-induced secondary structure of substance P

Substance P (Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2), a neuromodulator involved in the transmission of pain information, exerts its biological effects by binding to membrane-embedded protein receptors. The influence of membrane lipids on neuropeptide conformation may be critical to these processes. We have characterized in detail the complexes formed between substance P and sodium dodecylsulfate (SDS), lysophosphatidylglycerol, and lysophosphatidylcholine micelles. CD spectra of substance P displayed significant induced secondary structure upon addition of these lipids. Potentiometric titration data demonstrated that the pKa of the peptide N-terminal amino group increased from ca. 7.0 to 9.0 in SDS-bound substance P, suggesting direct interaction of the substance P N-terminus with the lipid head-group region. Red shifts in uv spectra of the Phe rings in membrane-bound peptide suggested an increased hydrophobic environment for these substituents. High-resolution one- and two-dimensional correlated spectroscopy nmr spectra displayed differential chemical-shift movements of substance P Gln, Leu, and Met NH protons with added lipid, suggesting involvement of the C-terminal portion of the peptide in the induced secondary structure. The clear influence of the lipid environment on the substance P conformational ensemble suggests a role for the membrane in the events leading to receptor binding.