Bridge over Troubled Waters: Minireview Sensing Stress by Disulfide Bond Formation (1999)

Department of Microbiology andMolecular GeneticsHarvard Medical School200 Longwood AvenueBoston, Massachusetts 02115 ent in secreted proteins. All evidence points to a situa-tioninwhichcytosolicproteinshaveevolvedtomaintaintheir cysteines reduced in the native form, whereasThe regulation of protein activity is a major factor in many secreted proteins have evolved to be more stablethe cellular response to a changing environment. Well- when their cysteines are joined in disulfide bonds. Thus,established mechanisms for such regulation include changes in the reducing environment of the cytosol canprotein–protein interactions, allosteric changes gener- have profound effects on protein folding and activity.atedbyligandbinding,andchemicalmodificationssuch Perturbations of the cellular redox conditions can beas phosphorylation. It has long been postulated that achievedeitherbymutationsthateliminatecomponentsdisulfide bond formation represents a covalent modifi- of the thioredoxin and glutaredoxin systems or by envi-cation that can regulate protein activity. This proposal ronmental oxidative stress. Mutant analysis shows thathas come from studies in which enzymes or transcrip- a simultaneous block of both disulfide-reducing path-tion factors were shown to lose activity after oxidation ways is incompatible with growth under aerobic condi-of cysteine residues in vitro, but to regain activity when tions (Prinz et al., 1997). Functional overlap between theexposed to the disulfide reductant thioredoxin. How- two pathways is indicated by the finding that, in theever, such reports may often be based on the fact that absenceofonlyoneofthepathways,strainscansurvivethe proteins studied usually exist in the reducing envi- and grow reasonably well. Nevertheless, the oxidizingronment of the cytosol; the oxidative inactivation ob- conditions in the cytosol of such strains allows for theserved may simply reflect the unnatural oxidizing condi- formationofdisulfidebondsinsomeproteins.Thisdisul-tions of the in vitro system and not reflect the in vivo fidebond–formingactivitycanbemonitoredbyexpress-state of affairs. ing in the cytosol a normally exported protein, such asThus, while it is not clear that loss of function by alkalinephosphatase,whichrequiresdisulfidebondsfordisulfidebondformationhasbeenestablishedasaregu- its enzymatic activity. Although it has yet to be directlylatory mechanism, recent results suggest that the re- demonstrated, it seems likely that unwanted disulfideverse process, gain of function by disulfide bond forma- bondsarealsogeneratedinthenormalresidentproteinstion, may be a common way of responding to cellular of the cytosol during oxidative stress—a situation westress. The utilization of improved techniques for as- refer to as “disulfide stress.”sessing the disulfide-bonded states of proteins in vivo Bacteria encounter oxidative stresses in environ-hasallowedareexaminationoftheroleofthesebondsin ments with high levels of hydrogen peroxide or otherregulating protein activity. These studies provide strong reactive oxygen species. One example of such stressevidence that two bacterial proteins, the transcription occurs when pathogenic bacteria confront oxidativefactor OxyR and the chaperone heat shock protein 33 bursts upon invasion of eukaryotic host cells. These(Hsp33), are activated by the oxidation of cysteine resi- encounters, in addition to damaging other cellular mole-dues to disulfide bonds (Zheng et al., 1998; Jakob et al., cules, may also cause the introduction of deleterious1999). These findings and the approaches used should disulfide bonds into proteins. Furthermore, studies us-provide impetus to a search for what is likely to be ing cytosolic alkaline phosphatase suggest that