Due to its natural properties, Trichoderma reesei is commonly used in industry-scale production of secretory proteins. Since almost all secreted proteins are O-glycosylated, modulation of the activity of enzymes of the O-glycosylation pathway are likely to affect protein production and secretion or change the glycosylation pattern of the secreted proteins, altering their stability and biological activity. Understanding how the activation of different components of the O-glycosylation pathway influences the glycosylation pattern of proteins and their production and secretion could help in elucidating the mechanism of the regulation of these processes and should facilitate creation of engineered microorganisms producing high amounts of useful proteins. In this review we focus on data concerning Trichoderma, but also present some background information allowing comparison with other fungal species.

In Trichoderma reesei, dolichyl phosphate mannose (dpm) synthase, a key enzyme in the O-glycosylation process, requires three proteins for full activity. In this study, the dpm2 and dpm3 genes coding for the DPMII and DPMIII subunits of T. reesei DPM synthase were cloned and functionally analyzed after expression in the Saccharomyces cerevisiae dpm1Δ [genotype (BY4743; his3Δ1; /leu2Δ0; lys2Δ0; /ura3Δ0; YPR183w::kanMX4] mutant. It was found that apart from the catalytic subunit DPMI, the DPMIII subunit is also essential to form an active DPM synthase in yeast. Additional expression of the DPMII protein, considered to be a regulatory subunit of DPM synthase, decreased the enzymatic activity. We also characterized S. cerevisiae strains expressing the dpm1, 2, 3 or dpm1, 3 genes and analyzed the consequences of dpm expression on protein O-glycosylation in vivo and on the cell wall composition.

In fungi, transfer of the first mannosyl residue to proteins during their O-glycosylation is catalyzed by protein O-mannosyltransferases. Integration of additional copies of the pmt1 gene into Trichoderma reesei genome unexpectedly resulted in the silencing of pmt1 expression. Strains carrying the additional copies of pmt1 gene exhibited lower total activity of protein O-mannosyltransferases, lower O- and N-glycosylation of secreted proteins and showed defects in their cell wall composition. Moreover, the strains grew slowly on solid medium and were hypersensitive to an antifungal reagent, Calcofluor white. These results indicate that protein O-mannosyltransferases are required for proper cell wall formation, and their decreased activity influences not only O- but also N-glycosylation.

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The mevalonate pathway is the most diverse metabolic route resulting in the biosynthesis of at least 30,000 isoprenoid compounds, many of which, such as sterols or dolichols, are indispensable for living cells. In the filamentous fungus Trichoderma of major biotechnological interest isoprenoid metabolites are also involved in the biocontrol processes giving the mevalonate pathway an additional significance. On the other hand, little is known about genes coding for enzymes of the mevalonate pathway in Trichoderma. Here, we present cloning and functional analysis of the erg20 gene from Trichoderma reesei coding for farnesyl pyrophosphate (FPP) synthase (EC 2.5.1.10), an enzyme located at the branching point of the mevalonate pathway. Expression of the gene in a thermosensitive erg20-2 mutant of Saccharomyces cerevisiae impaired in the FPP synthase activity suppressed the thermosensitive phenotype. The same gene overexpressed in T. reesei significantly enhanced the FPP synthase activity and also stimulated the activity of cis-prenyltransferase, an enzyme of the dolichyl branch of the mevalonate pathway. Unexpectedly, the activity of squalene synthase from the other, sterol branch, was significantly decreased without, however, affecting ergosterol level.