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J. Bacteriol., Sep 1997, 5458-5464, Vol 179, No. 17
AL Davidson and S Sharma
The maltose transport system of Escherichia coli, a member of the ABC
transport superfamily of proteins, consists of a periplasmic maltose
binding protein and a membrane-associated translocation complex that
contains two copies of the ATP-binding protein MalK. To examine the need
for two nucleotide-binding domains in this transport complex, one of the
two MalK subunits was inactivated by site-directed mutagenesis. Complexes
with mutations in a single subunit were obtained by attaching a
polyhistidine tag to the mutagenized version of MalK and by coexpressing
both wild-type MalK and mutant (His)6MalK in the same cell. Hybrid
complexes containing one mutant (His)6MalK subunit and one wild-type MalK
subunit were separated from those containing two mutant (His)6MalK proteins
based on differential affinities for a metal chelate column. Purified
transport complexes were reconstituted into proteoliposome vesicles and
assayed for maltose transport and ATPase activities. When a conserved
lysine residue at position 42 that is involved in ATP binding was replaced
with asparagine in both MalK subunits, maltose transport and ATPase
activities were reduced to 1% of those of the wild type. When the mutation
was present in only one of the two subunits, the complex had 6% of the
wild-type activities. Replacement of a conserved histidine residue at
position 192 in MalK with arginine generated similar results. It is clear
from these results that two functional MalK proteins are required for
transport activity and that the two nucleotide-binding domains do not
function independently to catalyze transport.
Copyright © 1997, American Society for Microbiology
Mutation of a single MalK subunit severely impairs maltose transport activity in Escherichia coli
Department of Microbiology and Immunology, Baylor College of Medicine, Houston, Texas 77030, USA. davidson@bcm.tmc.edu
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