Increased Heterologous Protein Production in Aspergillus niger Fermentation through Extracellular Proteases Inhibition by Pelleted Growth

The dependence of filamentous fungal protease secretion on morphology was investigated by employing the recombinant Aspergillus niger strain AB4.1[pgpdAGLAGFP] which contains a gene for the glucoamylase-GFP (green fluorescence protein) fusion protein. Different inoculum levels were used to obtain different sizes of pellet or free mycelia. The extracellular protease activity of the cultures varied with the pellet size and decreased dramatically when the morphology was changed from free mycelia to pellets. The culture with an optimal pellet size of 1.6 mm was obtained from an inoculum of 4 x 10^6 spores/mL. It resulted in a specific protease activity of 158 units/ L, only one-third of that in free mycelial growth, and a maximum specific GFP yield of 0.98 mg/g (cell mass) compared to 0.29 mg/g for free mycelial growth with an inoculum of 10^7 spores/mL. The results indicate that this bioprocessing strategy can be effectively used to inhibit protease activity in filamentous fungal fermentation and thereby to enhance heterologous protein production.