Heterologous expression of cellobiohydrolases in filamentous fungi – An update on the current challenges, achievements and perspectives

• Heterologous expression of CBHs in single hosts may produce multi-enzyme cocktails.
• Activity of heterologously expressed CBHs is (often) lower than of the native enzyme.
• Native inducible promoters are most successful for heterologous expression of CBHs.
• Over-glycosylation of heterologously produced CBHs decreases enzyme activity.
• Modulating protein quality control mechanisms in the host could improve CBH secretion.

Cellobiohydrolases are among the most important enzymes functioning in the hydrolysis of crystalline cellulose, significantly contributing to the efficient biorefining of recalcitrant lignocellulosic biomass into biofuels and bio-based products. Filamentous fungi are recognized as both well-known producers of commercial preparations of cellulolytic enzymes and efficient hosts for heterologous protein secretion. Thus, Aspergillus and Trichoderma species have been chosen as hosts for the heterologous expression of native or engineered enzymes aiming at the overproduction of single enzymes or as hosts for the secretion of multi-enzyme cocktails for on-site production in biorefineries, which is important for reducing the costs of biomass conversion. An even more interesting aspect is consolidated bioprocessing, in which a single fungus both hydrolyzes lignocellulose polymers and ferments the resulting sugars into valuable products. However, due to low cellobiohydrolase activities, certain fungi might be deficient with regard to enzymes of value for cellulose conversion, and improving cellobiohydrolase expression in filamentous fungi has proven to be challenging. In this review, we examine the effects of altering promoters, signal peptides, culture conditions and host post-translational modifications. For heterologous cellobiohydrolase production in filamentous fungi to become an industrially feasible process, the construction of site-integrating plasmids, development of protease-deficient strains and glycosylation engineering are obvious targets for constructing efficient enzyme producers.