Neoculin, a taste-modifying sweet protein, accumulates in ripening fruits of cultivated Curculigo latifolia

Neoculin is a sweet protein with a taste-modifying activity of converting sourness to sweetness. It occurs in the fruits of Curculigo latifolia, a wild plant found in tropical Asia. We successfully cultivated the plant and evaluated the production of neoculin. The neoculin content of the fruit was high for 10 weeks after flowering, following which the yield decreased gradually. The optimal period for harvesting the fruits with sensory activity coincided with this 10-week peak period during which the amount of neoculin was 1–3 mg in the whole fruit and 1.3 mg/g of pulp. Immunohistochemical staining showed that neoculin occurred in the whole fruit, especially at the basal portion. Although it is known that neoculin comprises an acidic subunit (NAS) with an N-glycosylated moiety and a basic subunit (NBS), protein gel blot analysis revealed the presence of a non-glycosylated NAS species. This suggests the presence of multiple NAS–NBS heterodimers in our cultivar.