Investigation of Gramicidin Channel Function by Single Amino Acid Replacement Using Non-Genetic Code Amino Acids

Abstract

Oligonucleotide-directed mutagenesis is limited to amino acids which can be specified by the genetic code. By contrast, chemical synthesis of “mutant” polypeptides offers the possibility of introducing non-genetic code amino acids for particular investigations, e.g., comparisons of the functional effects of essentially isosteric side chains of different polarities (such a Val vs. trifluoro-Val). We have used a semisynthetic approach, consisting of degradation from the formyl-N-terminal followed by re-synthesis, to introduce variant amino acid side chains into the selective channel-forming pentadecapeptide, gramicidin A. Even though the side chains are not in direct contact with the permeating ions, the single- channel conductances for Na⁺ and Cs⁺ through gramicidin channels are markedly affected by changes in the physico-chemical characteristics of the side chains. The maximal single-channel conductance for Na⁺ is decreased up to ten-fold when a polar side chain is present at position #1 in gramicidin. Furthermore, the selectivity for Cs⁺ over Na⁺ is increased when a polar side chain is at position #1. The transmembrane channels are dimers of gramicidin, and it is possible to observe hybrid channels formed between the natural and modified gramicidins. These hybrid channels suggest that the observed conductance changes are not due to gross changes in channel structure, but rather to subtle ion-side chain interactions which occur over distances of about 5-10 A.