Extracellular production of neoculin, a sweet-tasting heterodimeric protein with taste-modifying activity, by Aspergillus oryzae.

Neoculin (NCL), a protein with sweetness approximately 500-fold that of sugar, can be utilized as a nonglycemic sweetener. It also has taste-modifying activity to convert sourness to sweetness. NCL is a heterodimer composed of an N-glycosylated acidic subunit (NAS) and a basic subunit (NBS), which are conjugated by disulfide bonds. For the production of recombinant NCL (rNCL) by Aspergillus oryzae, alpha-amylase with a KEX2 cleavage site, -K-R-, was fused upstream of each of NAS and NBS and the resulting fusion proteins were simultaneously expressed. For accurate and efficient cleavage of the fusion construct by KEX2-like protease, a triglycine motif was inserted after the KEX2 cleavage site. As NBS showed lower production efficiency than did NAS, a larger amount of the NBS expression plasmid than of NAS expression plasmid was introduced during cotransformation, resulting in successful production of rNCL in the culture medium. Moreover, to obtain a higher production yield of rNCL, the active form of hacA cDNA encoding a transcription factor that induces an unfolded protein response was cloned and expressed constitutively. This resulted in a 1.5-fold increase in the level of rNCL production (2.0 mg/liter). rNCL was purified by chromatography, and its NAS was found to be N-glycosylated as expected. The original sweetness and taste-modifying activity of rNCL were comparable to those of native NCL when confirmed by calcium imaging with human embryonic kidney cells expressing the human sweet taste receptor and by sensory tests.