Enzyme Production by Recombinant Aspergillus

The use of molds, principally A. niger and T. reesei, in the production of heterologous proteins is becoming more successful. The high levels expected for commercial success have already been attained with a limited number of proteins, and other examples are bound to follow. It must be remembered that, although yields will always need to be maximized, the yield required for a bulk enzyme will be much higher than that for a high-value therapeutic protein. A commercial application is not the only driving force for the use of molds as hosts for heterologous gene expression and protein production. Amenable hosts are necessary for production of sufficient protein for many structural investigations, especially for engineered proteins, and deficiencies are often encountered in alternative hosts. For the expression and production of some proteins, there is no doubt that filamentous fungi will offer the system of choice and be serious contenders for the production of other proteins. Severe problems have been encountered in the development of molds as hosts for heterologous protein production, but I am greatly encouraged in the way these obstacles have been overcome. Problems remain, and it is these that will shape the direction of future research. Optimization of yield still needs to tackled for each protein, and it is true, for example, that fungal proteins tend to be secreted at higher yield than nonfungal proteins. In some cases there is evidence that gene expression is a limiting factor in protein production, but in the main, and especially with A. niger, secretion is the major limitation. Unraveling the secretion process in A. niger and understanding the importance of protein motifs in folding in vivo will be major areas of research activity. Authenticity of the target protein is equally important, with the major limitations being proteolytic modification and glycosylation. Although the degradation of secreted proteins in cultures is a problem, it has already been tackled successfully with the development of proteinase-deficient strains. Proteolytic degradation of proteins during the secretion process and the control of glycosylation during secretion are areas for detailed study. There will undoubtedly be major advances made in the coming years.