Recombinant hepatitis B surface antigen production in Aspergillus niger: evaluating the strategy of gene fusion to native glucoamylase

Abstract

This study demonstrates the potential of Aspergillus niger as a candidate expression system for virus-like particle production using gene fusion. Hepatitis B surface antigen (HBsAg) production, targeted through the secretory pathway in A. niger, resulted in completely assembled and properly folded HBsAg. This was achieved by implementing a gene fusion strategy using the highly expressed catalytic domain of the native glucoamylase gene (GlaA ( G2 )) fused to the HBsAg S gene. The inducible glucoamylase promoter (GlaA ( p )) was used to control transcription in the A. niger D15 host. The gene fusion strategy was designed for cleavage of the fused product by the KEX2-like protease, resulting in intracellular accumulation of HBsAg and extracellular secretion of glucoamylase. Immunodetection using a monoclonal HBsAg antibody could not detect the fused GlaA ( G2 ) ::S product in intracellular and extracellular fractions, indicating that full assembly and maturation of HBsAg occurred after cleavage of the fused product in the Golgi complex. Several breakdown products showing an immunoreactive response to the glucoamylase polyclonal antibody indicated a level of intracellular degradation. The choice of carbon source used in cultivation significantly affected HBsAg production levels through induction of the glucoamylase promoter. The highest specific HBsAg production was observed during growth on inducing substrates of starch and its degradation products (maltodextrin and maltose), although residual glucose accumulation in the mid-exponential phase reduced HBsAg production. HBsAg production in starch-based cultures may be improved further by optimization of the rates of starch hydrolysis by glucoamylase and subsequent glucose consumption by the host.