Editors' ChoicePosttranslational Modifications

Longer life through cysteine sulfenylation

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Science Signaling  06 Sep 2016:
Vol. 9, Issue 444, pp. ec205
DOI: 10.1126/scisignal.aai9299

Reactive oxygen species (ROS) are constantly produced by normal cellular functions, as well as in response to oxidative stress, and ROS production can be stimulated by activation of the NOX (NADPH oxidase) enzymes. ROS-mediated disulfide bond formation regulates the activity of proteins and involves the formation of sulfenylated cysteines. Redox stress activates the antioxidant pathway mediated by the transcription factor NRF2, which in mammalian cells involves the ROS-mediated inhibition of the E3 ubiquitin ligase adaptor protein KEAP1. The nematode Caenorhabditis elegans has a homolog of NRF2 (SKN-1) but lacks a homolog of KEAP1; instead, SKN-1 is stimulated by activation of the stress-activated kinase cascade culminating in p38. Hourihan et al. identified a ROS-mediated pathway for the p38-dependent activation of SKN-1 and mammalian NRF2 through cysteine sulfenylation of the kinase and RNase IRE1, a protein that participates in the unfolded protein response of the endoplasmic reticulum (UPRER). Exposure of worms or cultured human cells to the ROS-generating toxin sodium arsenite stimulated the cysteine sulfenylation of IRE1 without changing its apparent molecular weight, indicating that the reaction did not result in disulfide bond formation. In worms, genetic or chemical conditions that enhanced ER or mitochondrial ROS production also triggered IRE1 cysteine sulfenylation, without triggering IRE1 phosphorylation, and promoted the activation of the upstream kinase NSY-1 in the p38 cascade. In worms lacking IRE-1 or expressing a C663S mutant, sodium arsenite did not stimulate p38 phosphorylation. Pretreating the worms with sodium arsenite prevented activation of the IRE1-dependent arm of the UPRER and pretreating the worms to a chemical that induces the UPRER blocked IRE1 cysteine sulfenylation and activation of NSY-1, indicating that IRE1’s function in these two processes is mutually exclusive. Knocking down the C. elegans homolog of the adaptor TRAF2, which recruits the upstream kinase of the p38 pathway to the activating receptor at the membrane in mammalian cells, blocked ROS stimulated p38 phosphorylation and SKN-1 nuclear translocation without reducing IRE1 cysteine sulfenylation, suggesting that like the mammalian pathway from receptor to p38 through TRAF2, TRAF2 recruited NSY-1 to IRE1 at the ER membrane. IRE-1 and TRAF2 were not required for ROS-mediated stimulation of NSY-1 phosphorylation at the activation loop but were required for cysteine sulfenylation of NSY-1 and p38 activation. Sodium arsenite-mediated activation of the SKN-1 pathway required the NOX BLI-3, which coimmunoprecipitated with IRE1. Worms in which this pathway was compromised—through a lack of TRAF2, a mutation of the cysteine in IRE1 that is sulfenylated, or a lack of SKN-1—had shortened lifespans. In contrast, knocking down the ER lumen protein ERO1 (an enzyme that drives disulfide formation and thus its absence increases ROS in and near the ER) promoted IRE1 cysteine sulfenylation, p38 activation, and SKN-1 activity. Worms lacking ERO1 had increased lifespan but only when the IRE1 to SKN-1 pathway was functional. Cysteine sulfenylation may be a common posttranslational modification because four other human kinases (ROCK1, PKC-δ, AKT1, and p70S6K) exhibited NOX-dependent cysteine sulfenylation at a conserved D-F-G-X-C motif that was present in a region predicted to be redox reactive. Commentary by Kim discusses the importance of integrating oxidative stress and ER stress through IRE1.

J. M. Hourihan, L. E. Moronetti Mazzeo, L. P. Fernández-Cárdenas, T. K. Blackwell, Cysteine sulfenylation directs IRE-1 to activate the SKN-1/Nrf2 antioxidant response. Mol. Cell 63, 553–566 (2016). [PubMed]

D. H. Kim, IRE1 sulfenylation by reactive oxygen species coordinates cellular stress signaling. Mol. Cell 63, 541–542 (2016). [PubMed]

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