Microbiol. Rev., Dec 1996, 575-608, Vol 60, No. 4
IT Paulsen, MH Brown and RA Skurray
Multidrug efflux systems display the ability to transport a variety of
structurally unrelated drugs from a cell and consequently are capable of
conferring resistance to a diverse range of chemotherapeutic agents. This
review examines multidrug efflux systems which use the proton motive force
to drive drug transport. These proteins are likely to operate as
multidrug/proton antiporters and have been identified in both prokaryotes
and eukaryotes. Such proton-dependent multidrug efflux proteins belong to
three distinct families or superfamilies of transport proteins: the major
facilitator superfamily (MFS), the small multidrug resistance (SMR) family,
and the resistance/ nodulation/cell division (RND) family. The MFS consists
of symporters, antiporters, and uniporters with either 12 or 14
transmembrane-spanning segments (TMS), and we show that within the MFS,
three separate families include various multidrug/proton antiport proteins.
The SMR family consists of proteins with four TMS, and the multidrug efflux
proteins within this family are the smallest known secondary transporters.
The RND family consists of 12-TMS transport proteins and includes a number
of multidrug efflux proteins with particularly broad substrate specificity.
In gram-negative bacteria, some multidrug efflux systems require two
auxiliary constituents, which might enable drug transport to occur across
both membranes of the cell envelope. These auxiliary constituents belong to
the membrane fusion protein and the outer membrane factor families,
respectively. This review examines in detail each of the characterized
proton-linked multidrug efflux systems. The molecular basis of the broad
substrate specificity of these transporters is discussed. The surprisingly
wide distribution of multidrug efflux systems and their multiplicity in
single organisms, with Escherichia coli, for instance, possessing at least
nine proton- dependent multidrug efflux systems with overlapping
specificities, is examined. We also discuss whether the normal
physiological role of the multidrug efflux systems is to protect the cell
from toxic compounds or whether they fulfil primary functions unrelated to
drug resistance and only efflux multiple drugs fortuitously or
opportunistically.
Copyright © 1996, American Society for Microbiology
Proton-dependent multidrug efflux systems
School of Biological Sciences, University of Sydney, New South Wales, Australia.
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