Ribosomes plant toxins

Marine sponges contain a host of bioactive compounds, particularly small molecules, and also contain a range of peptides that are non-ribosomally synthesised, often containing non-native amino acids. However, there are examples of peptides of ribosomal origin, including, for example, asteropine A isolated from the sponge Asteropus simplex.133 This peptide comprises 36 residues and three disulphide bonds. It has potent sialidase inhibitory activity and thus has applications in the design of novel viral inhibitors. Structural analysis of asteropine A with NMR spectroscopy revealed a cystine-knot motif, similar to that already described for plant toxins. This observation emphasises the fact that the cystine-knot motif is extremely prevalent in disulphide-rich peptides.134 Asteropine A, discovered in 2006, was the first reported cystine-knot peptide isolated from marine invertebrates other than from cone snails, which are described in more detail below. 

Various plant toxins, mostly ribosome-inactivating proteins (RIPs), have been identified that bind to any mammalian cell surface expressing galactose units and are subsequently internalized by RME (67). Toxins such as nigrin b (68), a-sarcin (69), ricin and saporin (70), viscumin (71), and modeccin (72) are highly toxic upon oral administration (i.e., are rapidly internalized). The possibility exists, therefore, that modified and, most important, less toxic subunits of these compound can be used to facilitate the uptake of macro-molecular compounds or microparticulates. 

Ricin toxin, found in the bean of the castor plant, Ricinis communis, is one of the most toxic and easily produced plant toxins. It is a lectin consisting of two polypeptide chains, the A-chain and the B-chain, linked by a disulfide bond. It is one of a group of dichain ribosome-inactivating proteins, which are specific for the depurination of a single adenosine in ribosomal ribonucleic acid (RNA).1 The active chain (ie, the A-chain) has the ability to modify catalytically the 28S subunit of... 

Many molds and plants produce toxins, which are protective reagents, termed ribosome-inactivating proteins (RIPs), directed at particular cells and their ribosomes. These toxins are classified as either type I or type II RIPs according to the number of polypeptide chains. 

Ricin is a potent cellular protein toxin contained in the beans of the castor been plant (Ricinus communis), which is extensively cultivated for oil production and is also a common ornamental garden plant. Ricin is able to inhibit ribosomal protein synthesis eventually causing cell death, and owing to these properties it has been allegedly used in terrorist and criminal activities. After trypsin digestion of castor bean crude extracts, Ostin et al. [105] were able to unambiguously... 

RIPs are plant protein toxins that are able to inhibit enzymatically ribosomal activity and are therefore highly cytotoxic [98]. RIPs are taken up in the cells by means of endocytosis, and only a small fraction (5% or less) are translocated to the cytosol where the toxins inhibit the protein synthesis and eventually kill the cell. PCI may be used to increase both the efficacy and specificity of these toxins. RIPs are divided into two groups, type I and type II. Type II RIPs, like ricin, consists of two polypeptide chains, one cytotoxic A-chain with /V-glycosidase activity and one B-chain which binds to the cell surface. Type I RIPs, like gelonin, agrostin, and saporin, lack the B chain, which make them poorly transported over the cell- and intracellular membranes to the cell cytosol. Hence, the cytotoxic effect of these protein toxins is usually absent or very low. A considerable cytotoxic effect of type I RIPs has been shown in combination with PCI, both in vitro and in vivo [25, 99]. 

Shiga toxin produced by Shigella dysenteriae has similar structural features. The toxin binds to a glycolipid (Gb3), undergoes endocytosis, and the enzymatie Ai fragment, which is a specific N-glycosidase, removes adenine from one particular adenosine residue in the 28S RNA of the 60S ribosomal subunit. Removal of the adenine inactivates the 60S ribosome, blocking protein synthesis. Ricin, abrin, and a number of related plant proteins inhibit eukaryotic protein synthesis in a similar manner (Chapter 25). 

A group of plant lectins, such as abrin, ricin, and mod-eccin, are highly toxic to eukaryotic cells. Their mode of action consists of inhibition of protein synthesis by enzymatically inactivating the EF-2 binding region of the 60S ribosomal subunit, whereas the diphtheria toxin inactivates the EF-2 protein itself. Ricin is isolated from castor beans and has a molecular weight of 66,000. Like most plant and bacterial toxic proteins, ricin contains two... 

Toxin entry retrograde transport through the secretory pathway (Lord and Roberts, 1998) Ribosome-inactivating proteins a plant perspective (Nielsen and Boston, 2001)... 

The toxins used for the chemical construction of ITs are bRT, RTA in deglycosylated form (dgRTA), two ribosome-inactivatiing proteins (RIPs) (PAP and saporin), and PE. The preparation of some of these toxins is described in Subheading 3.3.1. It should be noted that presently almost all plant and bacterial toxins used for the preparation of ITs can also be expressed in recombinant form in E. coli. 

Likewise, the A chains of other plant [42-44] and bacterial [45-47] hetero-dimeric toxins are responsible for toxicity. These toxins contain a single A chain moiety which, in each case, has catalytic activity and efficiently inactivates its intracellular target [44]. The A chain is only toxic to intact cells when combined with B chain. The function of the B chain is to bind the toxin to cell-surface receptors, in the case of ricin to appropriate surface glycoproteins or glycolipids. This is the essential first step in the transfer of ricin A chain into the cytosol, where ribosome inactivation occurs [48]. Additionally, the B chain is believed to have a second function during the intoxication process in which it facilitates the transfer of the A chain across a membrane into the cytoplasm [49]. Separated A and B chains are essentially non-toxic, the toxic A chain lacking the ability to bind to and enter cells in the absence of the B chain. The toxicity of ricin therefore results from three sequential steps (1) binding of the whole molecule to the cell surface via the B chain (2) penetration of at least the A chain into the cytosol, and (3) inhibition of protein synthesis caused by the interaction of the A chain with the 60 S ribosomal subunit. 

A. Characteristics. Ricin is a glycoprotein toxin from the seed of the castor bean plant. Altering ribosomal RNA blocks protein synthesis, thereby killing infected cells. Ricin s significance as a potential biological warfare agent relates to its availability worldwide, ease of production, and extreme pulmonary toxicity when inhaled. 

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