Escherichia coli K12 arabinose-binding protein mutants with altered transport properties

The arabinose-binding protein (ABP) of Escherichia coli binds L-arabinose in the periplasm and delivers it to a cytoplasmic membrane complex consisting of the AraG and AraH proteins, for uptake into the cell. To study the interaction between the soluble and membrane components of this periplasmic transport system, regions of the ABP surface containing the opening of the arabinose-binding cleft were subjected to site-directed mutagenesis. Thirty-eight ABP variants containing one to three amino acid substitutions were recovered. ABP variants were expressed with wild-type AraG and AraH from a plasmid, in a strain lacking the chromosomal araFGH operon, and the whole cell uptake parameters, Ven (maximum initial velocity of arabinose entry) and K(en) (concentration of arabinose yielding half-maximal entry) were determined. Twenty-four mutants had normal Ven values, 3 mutants had Ven and K(en) values twice wild type, and 11 mutants had Ven and K(en) values 20-50% of wild type. Binding proteins that had altered uptake properties were each expressed, processed, and localized to the periplasm at levels equivalent to wild type. The mutant binding proteins behaved the same as wild type during purification, and each had a Kd (dissociation constant for bound arabinose) comparable to that of wild-type ABP. Mutations that resulted in altered uptake identified nine amino acids surrounding the arabinose-binding cleft, all of which are charged in the wild-type protein, and all of whose side chains project outward from the cleft. The evidence suggests that this surface of the binding protein and these nine charged loci play a major role in ABP interactions with the membrane complex.