InterPro domain: IPR001938

Thaumatin [ 1 ] is an intensely sweet-tasting protein, 100 000 times sweeter than sucrose on a molar basis [ 2 ], found in berries from Thaumatococcus daniellii, a tropical flowering plant known as Katemfe. It is induced by attack by viroids, which are single-stranded unencapsulated RNA molecules that do not code for protein.

Thaumatin consists of about 200 residues and contains 8 disulphide bonds. Like other PR proteins, thaumatin is predicted to have a mainly beta structure, with a high content of beta-turns and little helix [ 2 ]. Several stress-induced proteins of plants have been found to be related to thaumatins:

• A maize alpha-amylase/trypsin inhibitor
• Two tobacco pathogenesis-related proteins: PR-R major and minor forms, which are induced after infection with viruses
• Salt-induced protein NP24 from tomato
• Osmotin, a salt-induced protein from tobacco [ 2 ]
• Osmotin-like proteins OSML13, OSML15 and OSML81 from potato [ 3 ]
• P21, a leaf protein from soybean
• PWIR2, a leaf protein from wheat [ 4 ]
• Zeamatin, a maize antifungal protein [ 5 ]

This family is also referred to as pathogenesis-related group 5 (PR5), as many thaumatin-like proteins accumulate in plants in response to infection by a pathogen and possess antifungal activity [ 6 ]. The proteins are involved in systemically acquired resistance and stress response in plants, although their precise role is unknown [ 7 ]. The PR5K receptor protein kinase from Arabidopsis comprises an extracellular domain related to the PR5 proteins, and an intracellular protein-serine/threonine kinase domain [ 7 ].

1. Cloning of cDNA encoding the sweet-tasting plant protein thaumatin and its expression in Escherichia coli. Gene 18, 1-12
2. Molecular Cloning of Osmotin and Regulation of Its Expression by ABA and Adaptation to Low Water Potential. Plant Physiol. 90, 1096-1101
3. Activation of two osmotin-like protein genes by abiotic stimuli and fungal pathogen in transgenic potato plants. Plant Physiol. 108, 929-37
4. A wheat glutathione-S-transferase gene with transposon-like sequences in the promoter region. Plant Mol. Biol. 16, 1089-91
5. Characterization and expression of an antifungal zeamatin-like protein (Zlp) gene from Zea mays. Plant Physiol. 106, 1471-81
6. Nucleotide sequence of an osmotin-like cDNA induced in tomato during viroid infection. Plant Mol. Biol. 20, 1199-202
7. The PR5K receptor protein kinase from Arabidopsis thaliana is structurally related to a family of plant defense proteins. Proc. Natl. Acad. Sci. U.S.A. 93, 2598-602