Identification and modulation of the key amino acid residue responsible for the ph sensitivity of neoculin, a taste-modifying protein

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Abstract

Neoculin occurring in the tropical fruit of Curculigo latifolia is currently the only protein that possesses both a sweet taste and a taste-modifying activity of converting sourness into sweetness. Structurally, this protein is a heterodimer consisting of a neoculin acidic subunit (NAS) and a neoculin basic subunit (NBS). Recently, we found that a neoculin variant in which all five histidine residues are replaced with alanine elicits intense sweetness at both neutral and acidic pH but has no taste-modifying activity. To identify the critical histidine residue(s) responsible for this activity, we produced a series of His-to-Ala neoculin variants and evaluated their sweetness levels using cell-based calcium imaging and a human sensory test. Our results suggest that NBS His11 functions as a primary pH sensor for neoculin to elicit taste modification. Neoculin variants with substitutions other than His-to-Ala were further analyzed to clarify the role of the NBS position 11 in the taste-modifying activity. We found that the aromatic character of the amino acid side chain is necessary to elicit the pH-dependent sweetness. Interestingly, since the His-to-Tyr variant is a novel taste-modifying protein with alternative pH sensitivity, the position 11 in NBS can be critical to modulate the pH-dependent activity of neoculin. These findings are important for understanding the pH-sensitive functional changes in proteinaceous ligands in general and the interaction of taste receptor-taste substance in particular. © 2011 Nakajima et al.

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Nakajima, K. ichiro, Yokoyama, K., Koizumi, T., Koizumi, A., Asakura, T., Terada, T., … Abe, K. (2011). Identification and modulation of the key amino acid residue responsible for the ph sensitivity of neoculin, a taste-modifying protein. PLoS ONE, 6(4). https://doi.org/10.1371/journal.pone.0019448

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