Expression, purification and characterization of a novel double-sites mutant of the single-chain sweet-tasting protein monellin (MNEI) with both improved sweetness and stability

The sweet protein monellin has high sweet potency with limited stability. In this study, 3 double-sites mutants (E2N/E23A, E2N/Y65R and E23A/Y65R) of the single-chain monellin (MNEI) were constructed. The proteins were expressed in E. coli BL21 and purified to homogeneity by nickel affinity chromatography with yields above 10 mg/L cell culture. Introduction of a sweeter mutant E2N into E23A or Y65R (E2N/E23A and E2N/Y65R) led to about 3-fold increase of sweetness, while addition of a more stable mutant E23A into E2N or Y65R (E2N/E23A and E23A/Y65R) resulted in improved thermal stability (about 10 °C). The results indicate that residues E2 and E23 mediate the sweetness and thermal stability of the protein, respectively. Multiple mutations of different residues (E2N/E23A) led to an additive performance with both improved sweetness and stability, suggesting that the sweetness and stability could be modulated by the independent molecular mechanism. The sweeter and thermal stable variant has a potential in further industrial applications.