Thionins are a family of small proteins found solely in higher plants. Typically, a thionin consists of 45–48 amino acid residues. 6–8 of these are cysteine forming 3–4 disulfide bonds. They include phoratoxins and viscotoxins.
| Plant thionin | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| |||||||||||
| Identifiers | |||||||||||
| Symbol | Thionin | ||||||||||
| Pfam | PF00321 | ||||||||||
| InterPro | IPR001010 | ||||||||||
| PROSITE | PDOC00244 | ||||||||||
| SCOP2 | 1cnb / SCOPe / SUPFAM | ||||||||||
| TCDB | 1.C.44 | ||||||||||
| OPM superfamily | 140 | ||||||||||
| OPM protein | 2plh | ||||||||||
| |||||||||||
Alpha- and beta- thionins are related to each other. The gamma thionins have a superficially similar structure but are an unrelated class of protein, now called plant defensins.
The proteins are toxic to animal cells, presumably attacking the cell membrane and rendering it permeable: this results in the inhibition of sugar uptake and allows potassium and phosphate ions, proteins, and nucleotides to leak from cells.[1] Thionins are mainly found in seeds where they may act as a defence against consumption by animals. A barley (Hordeum vulgare) leaf thionin that is highly toxic to plant pathogens and is involved in the mechanism of plant defence against microbial infections has also been identified.[2] The hydrophobic protein crambin from the Abyssinian kale (Crambe abyssinica) is also a member of the thionin family.[1] Some thionins have cytotoxic activity and they are therefore interesting in the development of new drugs against cancer with novel action mechanisms.[3] No thionin has yet been developed into an anti-cancer drug. Thionin is also a minor protein found in mustard (Brassica napus L.) seeds.[4]
A database for antimicrobial peptides, including thionins is available: PhytAMP.[5]