Chapter 20 - Protein Production: Quality Control and Secretion Stress Response

Abstract

Filamentous fungi are known to secrete high levels of proteins and metabolites in their culture medium which makes them industrially useful. Over a period of time this property of fungi has been improved and current industrial production is reported to be 100g/L homologous protein in the cultivation medium. Aspergillus is responsible for the production of more than 60% of the total industrial enzymes obtained from filamentous fungi. This high capacity of the secretion machinery of Aspergillus makes them a promising host. High-level protein production and secretion are two very important requirements for industrial production of enzymes. Aspergillus is known as one of the major workhorses in the industrial production of enzymes. Different fungal enzymes which are commercially available from Aspergillus are listed in Association of Manufacturers and Formulators of Enzyme Products. The world market for industrial enzymes has been estimated to be worth over $1.6 billion, where fungal fermentation contributes to 50% of the total. With the developments in the field of genetic engineering and molecular biology these filamentous fungi are explored more for the production of heterologous protein. However, there are a number of factors which have a negative effect on production level, which includes low level of transcription, instability of mRNA, inefficient translation and posttranscriptional bottlenecks, and sometimes degradation of the protein of interest. Unfolded protein response (UPR) is also one of the limitations which is a general reaction to protein folding stress in eukaryotes. The accumulation or presence of the aberrant proteins in the endoplasmic reticulum becomes fatal to the cell, and in order for the eukaryotic cell to react with these misfolded proteins it expresses several genes which are related to protein folding and its degradation, which is called the UPR. Another constraint is the high level of proteases which Aspergillus is produced in good amounts. The improvement of production of heterologous protein has gained significance in the field of fungal molecular biology and fungal fermentation.