ENVIRONMENT-DEPENDENT AND SEQUENCE-DEPENDENT MODULATION OF THE DOUBLE-STRANDED TO SINGLE-STRANDED CONFORMATIONAL TRANSITION OF GRAMICIDIN-AIN MEMBRANES

The role of the membrane lipid composition and the individual Trp resi dues in the conformational rearrangement of gramicidin A along the fol ding pathway to its channel conformation has been examined in phosphol ipid bilayers by means of previously described size-exclusion high-per formance liquid chromatography HPLC-based strategy (Bano et al. (1991) Biochemistry 30, 886). It has been demonstrated that the chemical com position of the membrane influences the transition rate of the peptide rearrangement from double-stranded dimers to beta-helical monomers. T he chemical modification of Trp residues, or its substitution by the m ore 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 approximate to 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-strand ed dimers to single-stranded monomers, although Trp-9 and Trp-13 seeme d to be key residues for the stabilization of the beta-helical monomer ic conformation of gramicidin A. The conformation adopted for monomeri c Trp --> Phe substitution analogues in lipid vesicles resulted in CD spectra similar to the typical single-stranded beta(6.3)-helical confo rmation of gramicidin A, However, the Trp --> Phe substitution analogu es showed decreased antibiotic activity as the number of Trp decreased .