Miraculin
Miraculin is a glycoprotein extracted from the miracle fruit plant, a shrub native to west Africa (Richardella dulcifica). Local names for the plant include taami, asaa, and ledidi. Miraculin itself is not sweet, but the human tongue, once exposed to miraculin, perceives ordinarily sour foods, such as citrus, as sweet for up to an hour afterwards.
Miraculin was sequenced in 1989. While attempts to express it in E. coli bacteria have failed, Japanese researchers have succeeded in preparing genetically modified plants, such as lettuce, that express miraculin.