Ribosome-inactivating protein explained

Symbol:	RIP
Ribosome-inactivating protein
Pfam:	PF00161
Interpro:	IPR001574
Prosite:	PDOC00248
Scop:	1paf

A ribosome-inactivating protein (RIP) is a protein synthesis inhibitor that acts at the eukaryotic ribosome. This protein family describes a large family of such proteins that work by acting as rRNA N-glycosylase (EC 3.2.2.22). They inactivate 60S ribosomal subunits by an N-glycosidic cleavage, which releases a specific adenine base from the sugar-phosphate backbone of 28S rRNA.^{[1] [2] [3]} RIPs exist in bacteria and plants.^[4]

Members of the family include shiga toxins, and type I (e.g. trichosanthin and luffin) and type II (e.g. ricin, agglutinin, and abrin) ribosome inactivating proteins (RIPs). All these toxins are structurally related. RIPs have been of considerable interest because of their potential use, conjugated with monoclonal antibodies, as immunotoxins to treat cancers. Further, trichosanthin has been shown to have potent activity against HIV-1-infected T cells and macrophages.^[5] Elucidation of the structure-function relationships of RIPs has therefore become a major research effort. It is now known that RIPs are structurally related. A conserved glutamic residue has been implicated in the catalytic mechanism;^[6] this lies near a conserved arginine residue, which also plays a role in catalysis.^[7]

Only a minority of RIPs are toxic to humans when consumed, and proteins of this family are found in the vast majority of plants used for human consumption, such as Rice, Maize and Barley. In plants, they are thought to defend against pathogens and insects.^[8]

Classification

Ribosome-inactivating proteins (RIPs) are separated into the following types based on protein domain composition:

• Type I (A): RIPs-I are polypeptides composed of an A domain. This is the site of N-glycosidase activity.
• Type II (AB): RIPs-II are composed of an A domain with similar catalytic activity to Type I RIPs, and a B domain with carbohydrate-binding (lectin) properties. The B domain is able to bind galactosyl moieties on the cell surface which facilitates entry into the cell, thus making Type II particularly cytotoxic. The A and B domains are fused together by disulfide bonds.^[9] This group excludes bacterial AB5 toxins such as Shiga toxin, as the carbohydrate-binding ability evolved separately and these toxins are more similar to type I RIPs.
• Type III: RIPs-III are separated into two subgroups. One subgroup (AC) contains the same original RIP domain (A), and a C-terminal with unknown functionality. The other subgroup (AD) is similar to Type I, but contains a site for inactivation.^[10]

Examples

Examples include:

• Abrin
• Beetin
• Ricin
• Saporin
• Shiga toxin
• A Spiroplasma toxin^[11]
• Trichosanthin
• Viscumin (European mistletoe)
• Pokeweed antiviral protein (Phytolacca americana)^[12]

Notes and References

1. Endo Y, Tsurugi K, Yutsudo T, Takeda Y, Ogasawara T, Igarashi K . Site of action of a Vero toxin (VT2) from Escherichia coli O157:H7 and of Shiga toxin on eukaryotic ribosomes. RNA N-glycosidase activity of the toxins . European Journal of Biochemistry . 171 . 1–2 . 45–50 . January 1988 . 3276522 . 10.1111/j.1432-1033.1988.tb13756.x . free .
2. May MJ, Hartley MR, Roberts LM, Krieg PA, Osborn RW, Lord JM . Ribosome inactivation by ricin A chain: a sensitive method to assess the activity of wild-type and mutant polypeptides . The EMBO Journal . 8 . 1 . 301–8 . January 1989 . 2714255 . 400803 . 10.1002/j.1460-2075.1989.tb03377.x .
3. Funatsu G, Islam MR, Minami Y, Sung-Sil K, Kimura M . Conserved amino acid residues in ribosome-inactivating proteins from plants . Biochimie . 73 . 7–8 . 1157–61 . 1991 . 1742358 . 10.1016/0300-9084(91)90160-3 .
4. Mak AN, Wong YT, An YJ, Cha SS, Sze KH, Au SW, Wong KB, Shaw PC . 6 . Structure-function study of maize ribosome-inactivating protein: implications for the internal inactivation region and the sole glutamate in the active site . Nucleic Acids Research . 35 . 18 . 6259–67 . 2007 . 17855394 . 2094058 . 10.1093/nar/gkm687 .
5. Zhou K, Fu Z, Chen M, Lin Y, Pan K . Structure of trichosanthin at 1.88 A resolution . Proteins . 19 . 1 . 4–13 . May 1994 . 8066085 . 10.1002/prot.340190103 . 21524411 .
6. Hovde CJ, Calderwood SB, Mekalanos JJ, Collier RJ . Evidence that glutamic acid 167 is an active-site residue of Shiga-like toxin I . Proceedings of the National Academy of Sciences of the United States of America . 85 . 8 . 2568–72 . April 1988 . 3357883 . 280038 . 10.1073/pnas.85.8.2568 . 1988PNAS...85.2568H . free .
7. Monzingo AF, Collins EJ, Ernst SR, Irvin JD, Robertus JD . The 2.5 A structure of pokeweed antiviral protein . Journal of Molecular Biology . 233 . 4 . 705–15 . October 1993 . 8411176 . 10.1006/jmbi.1993.1547 .
8. Zhu . Feng . Zhou . Yang-Kai . Ji . Zhao-Lin . Chen . Xiao-Ren . The Plant Ribosome-Inactivating Proteins Play Important Roles in Defense against Pathogens and Insect Pest Attacks . Frontiers in Plant Science . 9 February 2018 . 9 . 146 . 10.3389/fpls.2018.00146. 29479367 . 5811460 . free .
9. Fredriksson. Sten-Åke. Artursson. Elisabet. Bergström. Tomas. Östin. Anders. Nilsson. Calle. Åstot. Crister. December 2014. Identification of RIP-II Toxins by Affinity Enrichment, Enzymatic Digestion and LC-MS. Analytical Chemistry. 87. 2. 967–974. 10.1021/ac5032918. 25496503. 0003-2700.
10. Lapadula WJ, Ayub MJ . Ribosome Inactivating Proteins from an evolutionary perspective . Toxicon . 136 . 6–14 . September 2017 . 28651991 . 10.1016/j.toxicon.2017.06.012 . 9814488 .
11. Hamilton PT, Peng F, Boulanger MJ, Perlman SJ . A ribosome-inactivating protein in a Drosophila defensive symbiont . Proceedings of the National Academy of Sciences of the United States of America . 113 . 2 . 350–5 . January 2016 . 26712000 . 4720295 . 10.1073/pnas.1518648113 . 2016PNAS..113..350H . free .
12. Domashevskiy AV, Goss DJ . Pokeweed antiviral protein, a ribosome inactivating protein: activity, inhibition and prospects . Toxins . 7 . 2 . 274–98 . January 2015 . 25635465 . 4344624 . 10.3390/toxins7020274 . free .