Lactose carrier protein of Escherichia coli. Transport and binding of 2'-(N-dansyl)aminoethyl .beta.-D-thiogalactopyranoside and p-nitrophenyl .alpha.-D-galactopyranoside

Abstract

The elevated level of lactose carrier protein present in cytoplasmic membranes derived from E. coli strain T31RT, which carries the Y gene of the lac operon on a plasmid vector, allowed the detection of a complex between the carrier and the fluorescent substrate 2''-(N-dansyl)-aminoethyl .beta.-D-thiogalactopyranoside (Dns2-S-Gal). Binding is accompanied by a 50-nm blue shift in the emission maximum of the dansyl residue. The complex (dissociation constant, KD = 30 .mu.M) rapidly dissociates upon addition of competing substrates such as .beta.-D-galactopyranosyl 1-thio.beta.-D-galactopyranoside or upon reaction with the thiol reagent p-chloromercuribenzenesulfonate. Binding of both Dns2-S-Gal and p-nitrophenyl .alpha.-D-galactopyranoside (.alpha.-NPG) occurs spontaneously in the absence of an electrochemical potential gradient across the membrane. Comparison of equilibrium binding experiments using Dns2-S-Gal or .alpha.-NPG and differential labeling of the carrier with radioactive amino acids shows that the carrier binds 1 mol of substrate/mol of polypeptide (MW 30,000). Besides specific binding to the lactose carrier, Dns2-S-Gal binds unspecifically to lipid vesicles or membranes, as described by a partition coefficient .kappa. = 60, resulting in a 25 nm blue shift in the emission maximum of the dansyl group. Both Dns2-S-Gal and .alpha.-NPG are not only bound by the lactose carrier, but also transported across the membrane by this transport protein in cells and membrane vesicles. The fluorescence changes observed with dansylated galactosides in membrane vesicles in the presence of an electrochemical gradient are interpreted as an increase in unspecific binding after translocation.