Neoculin as a New Sweet Protein with Taste-Modifying Activity: Purification, Characterization, and X-ray Crystallography

The majority of sweet compounds are of low-molecular-weight, but several proteins elicit sweet taste responses in humans. The fruit of Curculigo latifolia has been known to contain a protein that has both sweetness and a taste-modifying activity to convert sourness to sweetness. Recently, we have purified and re-identified the active component to reveal that it is a heterodimeric protein named "neoculin". The result of Xray crystallographic analysis has indicated that the overall structure of neoculin is similar to those of monocot mannose-binding lectins, while there is little structural similarity between neoculin and structure-solved sweet proteins. Direct interaction between neoculin and human sweet taste receptor hT1R2-hT1R3 has been indicated by response of HEK293T cells expressing this receptor, and by the inhibition of neoculin activity with lactisole, a hT1R2-hT1R3 blocker. Combining the results of molecular dynamics simulations and docking model generation between neoculin and hT1R2-hT1R3, we propose a hypothesis that neoculin is in dynamic equilibrium between "open" and "closed" states, and that the addition of an acid shifts the equilibrium to the "open" state for easier fitting to the receptor.