Fluorescence studies on the interaction of a synthetic signal peptide and its analog with liposomes

Aqueous solubility and membrane interactions of hydrophobic peptides with peptoid tags

Lysine tagging of hydrophobic peptides of parent sequence KKAAALAAAAALAAWAALAAAKKKK-NH(2) has been shown to facilitate their synthesis and purification through water solubilization, yet not impact on the intrinsic properties of the hydrophobic core sequence with respect to its insertion into membranes in an alpha-helical conformation. However, due to their positively charged character, such peptides often become bound to phospholipid head groups in membrane surfaces, which inhibits their transbilayer insertion and/or prevents their transport across cellular bilayers. We sought to develop more neutral peptides of membrane-permeable character by replacing most Lys residues with uncharged peptoid [N-(R)glycyl] residues, which might similarly confer water solubility while retaining membrane-interactive properties of the hydrophobic core. Several "peptoid-tagged" derivatives of the parent peptide were prepared with varying peptoid content, with five of the six Lys residues replaced with peptoids Nala and/or Nval. Conformations of these peptides measured by circular dichroism spectroscopy demonstrated that these water-soluble peptides retain the alpha-helix structure in micelles (lysophosphatidylcholine and sodium dodecyl sulfate) notwithstanding the known helix-breaking capacity of the peptoid tags. Blue shifts in Trp fluorescence spectra and quenching experiments with acrylamide confirmed that peptoid-tagged peptides insert spontaneously into micellar membranes. Results suggest that upon introduction of uncharged tags, the interaction between the membrane and the peptides is dominated by the hydrophobicity of the peptide core rather than the electrostatic interactions between the Lys and the head groups of the lipids. The overall findings indicate that peptoid residues are effective surrogates for Lys as uncharged water-solubilizing tags and, as such, provide a potentially valuable feature of design of membrane-interactive peptides.

The F–G loop region of cytochrome P450scc (CYP11A1) interacts with the phospholipid membrane

Cytochrome P450scc (CYP11A1) is a protein attached to the inner surface of the inner mitochondrial membrane that uses cholesterol from the membrane phase as its substrate for the first step in steroid hormone synthesis. We investigated the mechanism by which CYP11A1 interacts with the membrane. Hydrophobicity profiles of CYP11A1 and two other mitochondrial cytochromes P450, plus a model structure of CYP11A1 using CYP2C5 as template, suggest that CYP11A1 has a monotopic association with the membrane which may involve the A' helix and the F-G loop. Deletion of the A' helix reduced the proportion of expressed CYP11A1 associated with the bacterial membrane fraction, indicating a role for the A' helix in membrane binding. However, introduction of a cysteine residue in this helix at position 24 (L24C) and subsequent labelling with the fluorescent probe N'-(7-nitrobenz-2-oxal,3-diazol-4-yl)ethylenediamine (NBD) failed to show a membrane localisation. Cysteine mutagenesis and fluorescent labelling of other residues appearing on the distal surface of the CYP11A1 model revealed that V212C and L219C have enhanced fluorescence and a blue shift following association of the mutant CYP11A1 with phospholipid vesicles. This indicates that these residues, which are located in the F-G loop, become localised to a more hydrophobic environment following membrane binding. Analysis of the quenching of tryptophan residues in CYP11A1 by acrylamide indicates that at least one and probably two tryptophans are involved in membrane binding. We conclude that CYP11A1 has a monotopic association with the membrane that is mediated, at least in part, by the F-G loop region.