INTERACTION BETWEEN MALTOSE-BINDING PROTEIN AND THE MEMBRANE-ASSOCIATED MALTOSE TRANSPORTER COMPLEX IN ESCHERICHIA-COLI

Active transport of maltose in Escherichia coli requires the presence of both maltose-binding protein (MBP) in the periplasm and a complex of MalF, MalG, and MalK proteins (FGK2) located in the cytoplasmic membrane. Earlier, mutants in malF or malG were isolated that are able to grow on maltose in the complete absence of MBP. When the wild-type malE+ allele, coding for MBP, was introduced into these MBP-independent mutants, they frequently lost their ability to grow on maltose. Furthermore, starting from these Mal- strains, Mal+ secondary mutants that contained suppressor mutations in malE were isolated. In this study, we examined the interaction of wild-type and mutant MBPs with wild-type and mutant FGK2 complexes by using right-side-out membrane vesicles. The vesicles from a MBP-independent mutant (malG511) transported maltose in the absence of MBP, with K(m) and V(max) values similar to those found in intact cells. However, addition of wild-type MBP to these mutant vesicles produced unexpected responses. Although malE+ malG511 cells could not utilize maltose, wild-type MBP at low concentrations stimulated the maltose uptake by malG511 vesicles. At higher concentrations of the wild-type MBP and maltose, however, maltose transport into malG511 vesicles became severely inhibited. This behaviour of the vesicles was also reflected in the phenotype of malE+ malG511 cells, which were found to be capable of transporting maltose from a low external concentration (1-mu-M), but apparently not from millimolar concentrations present in maltose minimal medium. We found that the mutant FGK2 Complex, containing MalG511, had a much higher apparent affinity towards the wild-type MBP than did the wild-type FGK2 complex. We propose that the wild-type FGK2 complex exists in at least two conformations, active and inactive, and that the binding of the liganded MBP converts the latter into the former. The mutant complex presumably exists predominantly in the active form that has a higher affinity toward liganded MBP, and the inhibition of the mutant complex by an excess of maltose and wild-type MBP may be explained as a form of inhibition by excess substrate.