Enhanced heterologous protein production in Aspergillus niger through pH control of extracellular protease activity

Abstract

The extracellular protease activity of Aspergillus niger AB4.1[p gpdAGLAGFP]#11, a recombinant strain producing a glucoamylase (GLA)–green fluorescent protein (GFP) fusion protein, was investigated in a 15 l stirred tank reactor and accordingly a pH control strategy was designed to minimize protease activity and increase recombinant yield. By maintaining pH at 6 recombinant protein production was enhanced over 10-fold to 21.0 mg/l compared to growth at acidic pH or without pH control. Protease activity was found to increase after 2 days of culture corresponding to the point where glucose in the culture medium had been completely utilized. When grown at pH 6, A. niger protease activity in the culture was decreased 6-fold to 560 U/l, compared to 3600 U/l under normal, acidic culture conditions. Protease activity at fermentation pH 6 was consistently lower than that at fermentation pH 3 regardless of assay pH, and results indicate that this decrease in activity was a combination of sub-optimal enzyme activity and variation in the spectrum of proteases secreted under the different pH conditions. A comparison of the concentrations of recombinant GLA and GFP demonstrated that high protease activity was responsible for GFP losses. More GFP was secreted in the pH 3 run, but less GFP remained in the broth because of the high protease activity. Although controlling pH at 6 did not completely inhibit the proteases, the GFP concentration in the fermentation broth was increased greatly.