thionin
{{Distinguish|Thionine|Gamma thionin}}
{{Infobox protein family
| Symbol = Thionin
| Name = Plant thionin
| image =Beta thionin.png
| width =
| caption = Wheat beta-purothionin. Alpha helices in red, beta sheets in blue, disulphide bridges in yellow. {{PDB|1BHP}}
| Pfam= PF00321
| InterPro= IPR001010
| SMART=
| PROSITE = PDOC00244
| SCOP = 1cnb
| TCDB = 1.C.44
| OPM family= 140
| OPM protein= 2plh
| PDB=1BHP
}}
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.
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.
Activity
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.{{cite journal |vauthors=Vernon LP, Evett GE, Zeikus RD, Gray WR |title=A toxic thionin from Pyrularia pubera: purification, properties, and amino acid sequence |journal=Arch. Biochem. Biophys. |volume=238 |issue=1 |pages=18–29 |year=1985 |pmid=3985614 |doi=10.1016/0003-9861(85)90136-5}} 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.{{cite journal |vauthors=Apel K, Andresen I, Becker W, Schluter K, Burges J, Parthier B |title=The identification of leaf thionin as one of the main jasmonate-induced proteins of barley (Hordeum vulgare) |journal=Plant Mol. Biol. |volume=19 |issue=2 |pages=193–204 |year=1992 |pmid=1377959 |doi=10.1007/BF00027341|bibcode=1992PMolB..19..193A |s2cid=31727379 }} The hydrophobic protein crambin from the Abyssinian kale (Crambe abyssinica) is also a member of the thionin family. Some thionins have cytotoxic activity and they are therefore interesting in the development of new drugs against cancer with novel action mechanisms.{{cite journal |doi=10.1007/BF00039517 |vauthors=Florack DE, Stiekema WJ |title=Thionins: properties, possible biological roles and mechanisms of action |journal=Plant Mol. Biol. |volume=26 |issue=1 |pages=25–37 |date=October 1994 |pmid=7948874 |bibcode=1994PMolB..26...25F |s2cid=5814475 }} No thionin has yet been developed into an anti-cancer drug. Thionin is also a minor protein found in mustard (Brassica napus L.) seeds.{{cite journal |author1=Bérot S |author2=Compoint JP |author3=Larré C |author4=Malabat C |author5=Guéguen J.|title=Large scale purification of rapeseed proteins (Brassica napus L.)|doi=10.1016/j.jchromb.2004.08.001 |volume=818 |year=2005 |journal=Journal of Chromatography B |issue=1 |pages=35–42|pmid=15722042 }}
Databases
A database for antimicrobial peptides, including thionins is available: PhytAMP.{{cite web | url = http://phytamp.hammamilab.org | title = PhytAMP Database }}; {{cite journal | vauthors = Hammami R, Ben Hamida J, Vergoten G, Fliss I | title = PhytAMP: a database dedicated to antimicrobial plant peptides | journal = Nucleic Acids Res. | volume = 37 | issue = Database issue | pages = D963–8 |date=January 2009 | pmid = 18836196 | pmc = 2686510 | doi = 10.1093/nar/gkn655 }}