High-yield Secretion of the Recombinant Sweet-Tasting Protein Thaumatin I

Thaumatin, an intensely sweet-tasting protein used as a sweetener, was secreted by the methylotrophic yeast Pichia pastoris. Approximately 100 mg L-1 of recombinant thaumatin I was obtained using an expression vector which possesses three copies of the thaumatin gene containing the 22-amino acid pre-sequence. Expression yield was about three-fold higher than when the a-factor secretion signal from Saccharomyces cerevisiae was used. The circular dichroism and tryptophan fluorescence spectra for recombinant thaumatin I were almost the same as those for plant thaumatin. Large amounts of homogeneous recombinant thaumatin allowed for preparation of high-quality crystals in the presence of cryoprotective glycerol used in high-resolution x-ray structural analysis to help further understand the perception of the sweet taste of thaumatin.