Environment- and Sequence-Dependent Modulation of the Double-Stranded to Single-Stranded Conformational Transition of Gramicidin A in Membranes

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Abstract
The role of the membrane lipid composition and the individual Trp residues in the conformational rearrangement of gramicidin A along the folding pathway to its channel conformation has been examined in phospholipid bilayers by means of previously described size-exclusion high-performance liquid chromatography HPLC-based strategy (Bañó et al. (1991) Biochemistry 30, 886). It has been demonstrated that the chemical composition of the membrane influences the transition rate of the peptide rearrangement from double-stranded dimers to β-helical monomers. The chemical modification of Trp residues, or its substitution by the more hydrophobic residues phenylalanine or naphthylalanine, stabilized the double-stranded dimer conformation in model membranes. This effect was more notable as the number of Trp-substituted residues increased (tetra > tri > di > mono), and it was also influenced by the specific position of the substituted amino acid residue in the sequence, in the order Trp-9 ≈ Trp-13 > Trp-11 > Trp-15. Moreover, it was verified that nearly a full contingent of indoles (Trp-13, -11, and -9) is necessary to induce a quantitative conversion from double-stranded dimers to single-stranded monomers, although Trp-9 and Trp-13 seemed to be key residues for the stabilization of the β-helical monomeric conformation of gramicidin A. The conformation adopted for monomeric Trp → Phe substitution analogues in lipid vesicles resulted in CD spectra similar to the typical single-stranded β6.3-helical conformation of gramicidin A. However, the Trp → Phe substitution analogues showed decreased antibiotic activity as the number of Trp decreased.

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