Functional analysis of thioredoxin from the desert lichen-forming fungus, Endocarpon pusillum Hedwig, reveals its role in stress tolerance.

Endocarpon pusillum is a lichen-forming fungus with an outstanding stress resistance property closely related to its antioxidant system. In this study, thioredoxin (Trx), one of the main components of antioxidant defense systems in E. pusillum (EpTrx), was characterized and analyzed both in transgenic yeasts and in vitro. Our analyses identified that the heterologous expression of EpTrx in the yeast Pichia pastoris significantly enhanced its resistance to osmotic and oxidative stresses. Assays in vitro showed EpTrx acted as a disulfide reductase as well as a molecular chaperone by assembling into various polymeric structures. Upon exposure to heat-shock stress, EpTrx exhibited weaker disulfide reductase activity but stronger chaperone activity, which coincided with the switching of the protein complexes from low molecular weight forms to high molecular weight complexes. Specifically, we found that Cys31 near but not at the active site was crucial in promoting the structural and functional transitions, most likely by accelerating the formation of intermolecular disulfide bond. Transgenic Saccharomyces cerevisiae harboring the native EpTrx exhibited stronger tolerance to oxidative, osmotic and high temperature stresses than the corresponding yeast strain containing the mutant EpTrx (C31S). Our results provide the first molecular evidence on how Trx influences stress response in lichen-forming fungi.