A New Family of Membrane Electron Transporters and Its Substrates, Including a New Cell Envelope Peroxiredoxin, Reveal a Broadened Reductive Capacity of the Oxidative Bacterial Cell Envelope

Abstract

<jats:title>ABSTRACT</jats:title><jats:p>The<jats:named-content content-type="genus-species">Escherichia coli</jats:named-content>membrane protein DsbD functions as an electron hub that dispatches electrons received from the cytoplasmic thioredoxin system to periplasmic oxidoreductases involved in protein disulfide isomerization, cytochrome<jats:italic>c</jats:italic>biogenesis, and sulfenic acid reduction. Here, we describe a new class of DsbD proteins, named ScsB, whose members are found in proteobacteria and<jats:italic>Chlamydia</jats:italic>. ScsB has a domain organization similar to that of DsbD, but its amino-terminal domain differs significantly. In DsbD, this domain directly interacts with substrates to reduce them, which suggests that ScsB acts on a different array of substrates. Using<jats:named-content content-type="genus-species">Caulobacter crescentus</jats:named-content>as a model organism, we searched for the substrates of ScsB. We discovered that ScsB provides electrons to the first peroxide reduction pathway identified in the bacterial cell envelope. The reduction pathway comprises a thioredoxin-like protein, TlpA, and a peroxiredoxin, PprX. We show that PprX is a thiol-dependent peroxidase that efficiently reduces both hydrogen peroxide and organic peroxides. Moreover, we identified two additional proteins that depend on ScsB for reduction, a peroxiredoxin-like protein, PrxL, and a novel protein disulfide isomerase, ScsC. Altogether, our results reveal that the array of proteins involved in reductive pathways in the oxidative cell envelope is significantly broader than was previously thought. Moreover, the identification of a new periplasmic peroxiredoxin indicates that in some bacteria, it is important to directly scavenge peroxides in the cell envelope even before they reach the cytoplasm.</jats:p><jats:p><jats:bold>IMPORTANCE</jats:bold>Peroxides are reactive oxygen species (ROS) that damage cellular components such as lipids, proteins, and nucleic acids. The presence of protection mechanisms against ROS is essential for cell survival. Bacteria express cytoplasmic catalases and thiol-dependent peroxidases to directly scavenge harmful peroxides. We report the identification of a peroxide reduction pathway active in the periplasm of<jats:named-content content-type="genus-species">Caulobacter crescentus</jats:named-content>, which reveals that, in some bacteria, it is important to directly scavenge peroxides in the cell envelope even before they reach the cytoplasm. The electrons required for peroxide reduction are delivered to this pathway by ScsB, a new type of membrane electron transporter. We also identified two additional likely ScsB substrates, including a novel protein disulfide isomerase. Our results reveal that the array of proteins involved in reductive pathways in the oxidative environment of the cell envelope is significantly broader than was previously thought.</jats:p>

FipB, an Essential Virulence Factor of Francisella tularensis subsp. tularensis, Has Dual Roles in Disulfide Bond Formation

“…Bioinformatic searches of F. tularensis have not revealed any DsbD ortholog. There are other related families of proteins that can also provide reducing equivalents, such as CcdA and ScsB, but there are no orthologs of these protein families either (20). Interestingly, there is a strain of a Francisella sp., Tx077308, which contains a DsbD ortholog (F7308_1227) that is lacking in other sequenced strains and species, suggesting that this gene may have been a later addition to the strain's genome.…”

“…DsbC is returned to a reduced state by interaction with the inner membrane protein DsbD, which transfers reducing equivalents from the cytoplasm. The F. tularensis species lacks a recognizable DsbD or orthologs of other proteins that play similar roles (20). Since FipA was required for FipB isomerase activity in E. coli, we predicted that FipA would influence the oxidation state of FipB.…”

Genomic island genes in a coastal marine Synechococcus strain confer enhanced tolerance to copper and oxidative stress

“…As noted in the introduction sync_1495 has a domain similar to DsbD in E.coli, which is involved in disulfide bond isomerization. Additionally, the DsbD superfamily is present in many bacterial groups and the DsbD homolog in Caulobacter crescentus, ScsB, acts on a different set of substrates from E.coli, including a novel cell envelope reductive pathway providing electrons to a thioredoxinlike protein, which then reduces a novel peroxiredoxin that acts in cell envelope to directly scavenge peroxides (Cho et al, 2012). This suggests that this superfamily may be involved in several mechanisms of reduction in the cell envelope including disulfide isomerization as in E.coli, but also potentially directly scavenging of reactive oxygen species produced by copper or methyl viologen in the periplasm.…”

Disulfide-Bond-Forming Pathways in Gram-Positive Bacteria

“…In addition to maintaining DsbC in a reduced state, DsbD and DsbD homologs also shuttle electrons to DsbG, DsbE, and peroxiredoxins in the periplasm (54)(55)(56). Although DsbG has no known substrates, in vitro data suggest that it is a disulfide isomerase (54).…”

“…DsbE, also known as CcmG, is required for the synthesis of cytochrome c, a component of the electron transport chain (57). The reduction of peroxiredoxin is important, as this redox protein scavenges peroxides in the periplasm to combat oxidative stress (56).…”