Chaperone-like properties of two tobacco plastid thioredoxins

Abstract

Thioredoxins (Trxs) are small (around 12 kDa) ubiquitous disulfide reductases originally discovered in plants in the context of photosynthesis. The major role of Trxs is to exert posttranslational redox modifications of target proteins implicated in nearly all cell processes, shifting them to an active state upon disulfide reduction. However, several observations show that Trxs may have a second redox-independent regulatory mechanism that depends on their ability to promote the folding of proteins as molecular chaperones (reviewed in {Berndt, 2008 #41}. In vivo, we have recently shown that co-expression of either tobacco Trx f or Trx m allows us to avoid inclusion body formation of the recombinant human serum albumin within the chloroplast {Sanz-Barrio, 2011 #85}, suggesting that both plastid Trxs may act as molecular chaperones. The present study aims to investigate the putative chaperone activity of the well-known plastid Trxs f and m. For that purpose, the cDNA of both Trxs was isolated from Nicotiana tabacum plants. We found that bacterially expressed tobacco Trx f and Trx m, in addition to their disulfide reductase activity possessed chaperone-like properties. In vitro, Trx f and Trx m could both facilitate the reactivation of the cysteine-free form of chemically denatured glucose-6 phosphate dehydrogenase (foldase chaperone), and prevent heat-induced malate dehydrogenase aggregation (holdase chaperone). Our results lead us to infer that the disulfide reductase and foldase chaperone functions occur mainly as monomers, being the well-conserved non-active cysteine present in the Trx f critical for both functions. However, the holdase chaperone activity of both Trxs depended on their oligomeric status, being functional only when they were associated into high molecular mass protein complexes. Because the oligomeric status of both Trxs was induced by salt and temperature, our data suggest that plastid Trxs could operate as a molecular holdase chaperone upon oxidative stress, acting as a type of small stress protein.