Membrane protein-ligand interactions in Escherichia coli vesicles and living cells monitored via a biosynthetically incorporated tryptophan analogue

This paper presents a deceptively straightforward experimental approach to monitoring membrane protein-ligand interactions in vesicles and in living Escherichia coli cells. This is achieved via the biosynthetic incorporation of 7-azatryptophan, a tryptophan analogue with a red-shifted absorption spectrum, allowing collection of the emission signal of the target protein in a high tryptophan background via red-edge excitation. The approach is demonstrated for the mannitol permease of E. coli (EIImtl), an integral membrane protein of 637 amino acids, including four tryptophans, and single-tryptophan mutants of EIImtl. By using a tryptophan auxotroph, a high level of 7-azatryptophan incorporation in EIImtl was achieved. The change in emission signal of the purified enzyme upon mannitol binding (-28%) was 4-fold larger than with EIImtl containing tryptophan, demonstrating the known higher sensitivity of this analogue for changes in the microenvironment [Schlesinger, R. (1968) J. Biol. Chem. 243, 3877-3883]. Changes in emission signal could also be monitored (-5%) when the enzyme was situated in vesicles, although it constituted only 10-15% of the total cytoplasmic membrane fraction. Of the five single-tryptophan mutants, the emission signal of the mutant with 7-azatryptophan at position 198 was the most sensitive for mannitol binding. Changes in emission signal not only were observed in vesicles (-18%) but also could be monitored in viable cells (-5%). The fact that only modest expression levels and no protein purification are needed makes this a useful approach for the characterization of numerous protein systems under in vitro and in vivo conditions.