Evidence for a Conformational Change in the Escherichia coli Maltose Receptor by Excited-State Fluorescence Lifetime Data

The initial signaling event during maltose che-moreception in Escherichia coli is identified with a delocalized ligand-induced conformational change in the maltose binding protein. Substantiation for the conformational change involves a new application of the “distant reporter group technique” [Zukin, R. S., Hartig, P. R., & Koshland, D. E., Jr. (1977a) Proc. Natl. Acad. Sci. U.S.A. 74, 1932-1936] utilizing excited-state fluorescence lifetime measurements. Binding of maltose to its receptor results in changes in the microenvi-ronment of the two tryptophan residues of the receptor protein and of an experimentally attached reporter group, 5-(iodo-acetamido)fluorescein. The minimum distance between the two tryptophans from efficiency of fluorescence energy transfer theory is 17 Å; the minimum distance from the farther tryptophan to the fluorescein is 50 Å. Thus, the maltose receptor is shown to undergo molecular rearrangements at distant sites upon ligand binding. The general feature of conformational change as the initial signaling event during chemoreception in the enteric bacteria is discussed.