Genetic Analysis of Periplasmic Binding Protein Dependent Transport in Escherichia Coli. Each Lobe of Maltose-binding Protein Interacts with a Different Subunit of the MalFGK2 Membrane Transport Complex

Escherichia coli is able to accumulate maltose and maltodextrins by an ATP-binding cassette transporter known as the maltose transport system. This transport system is comprised of five proteins: the LamB protein in the outer membrane; the periplasmic maltose-binding protein (MBP); two integral inner membrane proteins, MalF and MalG; and MalK, which is associated with the cytoplasmic face of the inner membrane. It has been previously suggested that MBP interacts with MalF and MalG during sugar transport across the inner membrane. In two independent genetic studies, reported here, residue 210 of MBP has been identified as an important site for its interaction with MalF. In one study, allele-specific suppressors of a malF mutation, malF506, were isolated and yielded mutations which altered residue tyrosine 210 of MBP to aspartic acid. In the other study, dominant mutations in malE (the structural gene of MBP) were isolated; one of these altered the same tyrosine residue (210) to cysteine. It was shown that the Y210C MBP mutant is also an allele-specific suppressor malF506, and that of the suppressor MBP alleles also exhibited dominant-negative phenotypes. Previously it was shown that alterations at residues glycine 13 and aspartate 14 of MBP can result in suppression of a malG mutant. From these results and those described, it is possible to propose a simple model in which the amino-terminal lobe of MBP interacts with MalG and the carboxy-terminal lobe of MBP interacts with MalF. The locations of residues 13, 14 and 210 on the three-dimensional structure of MBP are in keeping with this model.