Trametes versicolor belongs to white rot fungi which are the only organisms able to degrade lignin efficiently. Lignin degradation caused by these Basidiomycetes is accomplished by the action of secreted enzymes, the best characterized of which are laccases, lignin peroxidases, and manganese peroxidases. Lignin-modifying enzymes are mainly produced during secondary metabolism triggered in the white rot fungi by nutrient deprivation. Ligninolytic enzymes have been investigated mainly because of their ecological significance and industrial applications for pulping and bleaching as well as for bioremediation. Our previous studies have demonstrated that ubiquitin/proteasome-mediated proteolytic pathway is involved in the regulation of ligninolytic activities in the wood-decaying fungus T. versicolor upon nitrogen and carbon starvation [Staszczak M., 2002, Enzyme Microb. Technol. 30, 537-541]. This highly selective non-lysosomal pathway requires ATP and the 26S proteasome, a large ~2.5 MDa multisubunit complex which degrades protein targets marked by a covalently attached polyubiquitin chain. The 26S proteasome plays a major regulatory role in eukaryotic cells; it degrades many important proteins involved in cell cycle control, signaling pathway, and in general metabolism, including transcription factors and key metabolic enzymes. Proteasemes are implicated in stress response by removing damaged, denatured, or misfolded proteins. Our recent studies have indicated that proteasomal degradation of intracellular proteins is involved in the regulation of T. versicolor laccase in response to cadmium [Staszczak M., Jarosz-Wilkołazka A., 2005, Biochimie 87, 755-762]. Laccase (benzenediol:oxygen oxidoreductase; EC 1.10.3.2), a major ligninolytic enzyme of this fungus, is a glycosylated protein. Secretion of glycoproteins can be influenced by tunicamycin, an endoplasmic reticulum (ER) stress inducer. Tunicamycin, a nucleoside antibiotic produced by Streptomyces lysosuperficus inhibits N-linked glycosylation and blocks the formation of N-glycosidic protein-carbohydrate linkages. Secretory proteins that ultimately fail to fold properly are removed from the ER and degraded in the cytosol by the 26S proteasome. In the present study we tested whether the blocking of proteasome function would interfere with laccase production in tunicamycin-treated cultures of T. versicolor. These studies were performed using tunicamycin added (to a final concentration of 5 µg/ml) either alone or in combination with 40 µM carbobenzoxy-Leu-Leu-leucinal (MG 132), one of the most potent peptide aldehyde inhibitors of the proteasome, at the time of transfer of seven-day old mycelia to nutrient-deprived (carbon or nitrogen depleted) media or to nutrient-sufficient (trophophasic) media. Laccase activities (secreted as well as intracellular) and isozyme patterns were assessed in cultures treated with tunicamycin for 6 h or 24 h in the absence or presence of MG 132.

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