Design and characterization of gramicidin channels with side chain or backbone mutations
Mutations and chemical substitutions of amino acid side chains and backbone atoms have proved vital for understanding the folding, structure and function of gramicidin channels in phospholipid membranes. The channel's pore is lined by peptide backbone groups; their importance for channel structure and function is shown by a single amide-to-ester replacement within the backbone, which greatly reduces the resulting channel conductance and lifetime. The four tryptophans and the intervening leucines together govern the formation and dissociation of conducting channels from single-stranded subunits. Conducting double-stranded gramicidin conformations (channels) occur rarely in membranes--except when the sequence has been altered to permit special arrangements of tryptophans or (infrequently) in unusually thick membranes. The tryptophans anchor the single-stranded channels to the membrane/solution interface, and the indole dipoles promote cation transport through the channels. Removal of any indole dipole reduces ion conductance; whereas 5-fluorination of an indole, which increases its dipole moment, enhances ion conductance. Some sequence changes at the formyl-NH-terminus (in the membrane interior, away from the tryptophans), including fluorination of the formyl-NH-terminal valine, introduce voltage-dependent channel gating. Gramicidin channels are not just static conductors, but also dynamic entities whose structure and function can be manipulated by backbone and side chain modifications.