Bulky high-mannose-type N-glycan blocks the taste-modifying activity of miraculin.

Background: Miraculin (MCL) is a taste-modifying protein that converts sourness into sweetness. The molecular mechanism underlying the taste-modifying action of MCL is unknown.

Methods: Here, a yeast expression system for MCL was constructed to accelerate analysis of its structure-function relationships. The Saccharomyces cerevisiae expression system has advantages as a high-throughput analysis system, but compared to other hosts it is characterized by a relatively low level of recombinant protein expression. To alleviate this weakness, in this study we optimized the codon usage and signal-sequence as the first step. Recombinant MCL (rMCL) was expressed and purified, and the sensory taste was analyzed.

Results: As a result, a 2 mg/l yield of rMCL was successfully obtained. Although sensory taste evaluation showed that rMCL was flat in taste under all the pH conditions employed, taste-modifying activity similar to that of native MCL was recovered after deglycosylation. Mutagenetic analysis revealed that the N-glycan attached to Asn42 was bulky in rMCL.

Conclusions: The high-mannose-type N-glycan attached in yeast blocks the taste-modifying activity of rMCL.

General Significance: The bulky N-glycan attached to Asn42 may cause steric hindrance in the interaction between active residues and the sweet taste receptor hT1R2/hT1R3.