In fission yeast, the transcription factor Pap1 undergoes H2O2-dependent oxidation that promotes its nuclear accumulation and the activation of an antioxidant gene program. However, the mechanisms that regulate the sensitivity and selectivity of Pap1 activation by peroxides are not fully understood. Here, we demonstrate that the peroxiredoxin Tpx1, the sensor of this signaling cascade, activates the otherwise unresponsive Pap1 protein once the main cytosolic reduced thioredoxin, Trx1, becomes transiently depleted. In other words, Pap1 works as an alternative electron donor for oxidized Tpx1. We have trapped the very transient Tpx1-Pap1 intermediate in cells depleted in Trx1, as we show here using mass spectrometry. Recycling of Tpx1 by Trx1 is required for the efficient signaling to Pap1, suggesting that the complete cycle of H2O2 scavenging by Tpx1 and further recycling of oxidized Tpx1 by Trx1 is required for full downstream activation of the redox cascade.

This work was supported by the Spanish Ministry of Science and Innovation (BFU2009-06933 and BFU2012-32045), PLAN E and FEDER, by the Spanish program Consolider-Ingenio 2010 grant CSD 2007-0020, and by SGR2009-195 from Generalitat de Catalunya (Spain) to E.H. E.H. and J.A. are recipients of ICREA Academia Awards (Generalitat de Catalunya)