Dynamic Fluorescence Spectroscopy on Single Tryptophan Mutants of EIImtl in Detergent Micelles. Effects of Substrate Binding and Phosphorylation on the Fluorescence and Anisotropy Decay

Abstract

The effects of substrate and substrate analogue binding and phosphorylation on the conformational dynamics of the mannitol permease of Escherichia coli were investigated, using time-resolved fluorescence spectroscopy on mutants containing five single tryptophans situated in the membrane-embedded C domain of the enzyme [Swaving Dijkstra et al. (1996) Biochemistry 35, 6628-6634]. Since no fluorescent impurities are present in these mutants, the changes in fluorescence and anisotropy could be related with changes in the tryptophan microenvironment. Tryptophans at positions 30 and 42 showed changes in fluorescence intensity decay upon binding mannitol, which were reflected in the changes in lifetime distribution patterns. The disappearance of the shortest-lived decay component in these mutants, as well as in the mutant with a single tryptophan at position 109, indicates a change in the local environment such that quenching via neighboring side chains or solvent is reduced. Phosphorylation at histidine 554 and cysteine 384, located in the cytoplasmatic A and B domains of EII(mtl), respectively, induced an increase in the average fluorescence lifetimes of all of the tryptophans. The effect was most pronounced for tryptophans 30 and 109 which show large increases in the average fluorescence lifetime mainly due to loss of short-lived decay components. A correlation time distribution of the individual tryptophans deduced from an analysis of the anisotropy decay showed that they differed in their rotational mobility with tryptophan 30 showing the least local flexibility. Phosphorylation resulted in immobilization of W109 which, together with changes in the average fluorescence lifetime, is evidence for a conformational coupling between the phosphorylated B domain and the C domain. The influence of mannitol binding on the rotational behavior of the tryptophans is limited; it induces more internal flexibility at all tryptophan positions. A rotational correlation time of 30 ns was resolved for tryptophan 30, which probably represents a rotational mode of the micelle-embedded C-domain of EII(mtl) or a portion thereof. Upon phosphorylation, this rotational correlation time increases to 50 ns, probably reflecting a changed spatial orientation of W30 with respect to the C domain. Although kinetic experiments have shown that none of the tryptophans is essential for the catalytic activity of EII(mtl), it is significant that the residues most sensitive to mannitol binding, W30 and W42, are both located in the first membrane-spanning alpha-helix, a portion of which is highly conserved among mannitol-specific EII's of different bacteria.