Effect of endoplasmic reticulum stress on laccase production and the 26S proteasome activity in the white rot fungus Trametes versicolor

The ubiquitin-proteasome pathway is a major system for degrading intracellular proteins in eukaryotes. It degrades many important proteins involved in signal transduction, cell cycle progression, and in general metabolism, including key metabolic enzymes and transcription factors. In addition, the ubiquitin-proteasome system is responsible for endoplasmic reticulum-associated degradation. Our previous studies have demonstrated the presence of proteasomes in white rot Basidiomycetes and indicated that ubiquitinproteasome-mediated pathway in Trametes versicolor is involved in the regulation of laccase, a main ligninolytic enzyme of this fungus, upon nutrient starvation as well as in response to cadmium exposure. Fungal laccases are useful biocatalysts in a wide range of biotechnological applications. Here, we studied the effects of tunicamycin-induced endoplasmic reticulum stress on laccase activity and the 26S proteasomemediated proteolysis in nitrogen-sufficient and nitrogen-deprived cultures of T. versicolor. We found that short-term ER stress (6 h) leads to inhibition of both laccase activity and the 26S proteasome activity. This effect was more pronounced in the nitrogen-deprived cultures. However, after prolonged ER stress (24 h) laccase activity in tunicamycin-treated cultures recovered to levels comparable to those of untreated cultures. In contrast to what was observed after short-term stress, a 24-h tunicamycin treatment resulted in a significant increase in the 26S proteasome activity detected in mycelia from nitrogen-sufficient and nitrogen-deprived cultures (approximately 1.5-fold and 2-fold increase, respectively). Moreover, we found that blocking of proteasome function in T. versicolor subjected to prolonged ER stress resulted in a decrease of laccase activity. These findings suggest a potential role of the proteasome-mediated degradation as an important mechanism by which laccase is regulated under prolonged ER stress.