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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.
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