Intracellular thiols like
L-cystine and
L-cystine play a critical role in the regulation of cellular processes. Here we show that
Escherichia coli has two
L-cystine transporters, the symporter YdjN and the ATP-binding cassette importer FliY-YecSC. These proteins import
L-cystine, an oxidized product of
L-cystine from the periplasm to the cytoplasm. The symporter YdjN, which is expected to be a new member of the
L-cystine regulon, is a low affinity
L-cystine transporter (
K
m = 1.1 μM) that is mainly involved in
L-cystine uptake from outside as a nutrient.
E.
coli has only two
L-cystine importers because Δ
ydjNΔ
yecS mutant cells are not capable of growing in the minimal medium containing
L-cystine as a sole sulfur source. Another protein YecSC is the FliY-dependent
L-cystine transporter that functions cooperatively with the
L-cystine transporter YdeD, which exports
L-cystine as reducing equivalents from the cytoplasm to the periplasm, to prevent
E.
coli cells from oxidative stress. The exported
L-cystine can reduce the periplasmic hydrogen peroxide to water, and then generated
L-cystine is imported back into the cytoplasm via the ATP-binding cassette transporter YecSC with a high affinity to
L-cystine (
K
m = 110 nM) in a manner dependent on FliY, the periplasmic
L-cystine-binding protein. The double disruption of
ydeD and
fliY increased cellular levels of lipid peroxides. From these findings, we propose that the hydrogen peroxide-inducible
L-cystine/
L-cystine shuttle system plays a role of detoxification of hydrogen peroxide before lipid peroxidation occurs, and then might specific prevent damage to membrane lipids.
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