RNA N-glycosidases

Endo, Y. and Tsurugi, K. (1987) RNA N-glycosidase activity of ricin A-chain. Mechanism of action of the toxic lectin ricin on eukaryotic ribosomes. J. Biol. Chem. 262, 8128-8130. 

Endo Y and Tsurugi K (1988). The RNA N-glycosidase activity of ricin A-chain. The characteristics of the enzymatic activity of ricin A-chain with ribosomes and with rRNA. J Biol Chem, 263, 8735-8739. 

Endo Y, Tsurugi K, Franz H (1988) The site of action of the A-chain of mistletoe lectin 1 on eukaryotic ribosomes the RNA N-glycosidase activity of the protein. FEBS Lett 231 378-380... 

Only recently, a very interesting protein family, namely RIPs, has been isolated from S. nigra [57-60]. RIPs are RNA N-glycosidases inactivating ribosomes through a site-specific deadenylation of the large ribosomal RNA [61]. In addition to this, some RIPs have been reported to have superoxide dismutase [62,63] and phospholipase [64] type of activities. It is supposed that RIPs are defense-related... 

Peumans, W.J., Hao, Q. and Van Damme, E.J., Ribosome-inactivating proteins from plants more than RNA N-glycosidases FASEB J., 15, 1493-1506 (2001). 

Endo, Y, Tsurugi, K., Yutsudo, T., et al., 1988. Site of action of a Vero toxin (VT2) from Escherichia coli 0157 H7 and of Shiga toxin on eukaryotic ribosomes. RNA N-glycosidase activity of the toxins. Eur. J. Biochem. 171, 45-50. 

Xu, L., Wang, Y, Wang, L., et al., 2008. TYchi, a novel chitinase with RNA N-glycosidase and anti-tumor activities. Front. Biosci. 13, 3127-3135. 

Endo Y. and Tsurugi K. 1998. The RNA N-glycosidase acivity of ricin-A chain. /. Biol. Chem. 263 8735-8739. 

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). 

RT is the major protein of the Ricinus communis seed. It is composed of two polypeptide chains, RTA and RTB, of approx the same molecular mass (30-32 kDa) linked to each other with a disulfide bond (Fig. 2). RTA is an N glycosidase, which removes a specific adenine residue from the 28S ribosomal RNA, thereby inhibiting protein synthesis. The RTB chain is a galactose-specific lectin that allows the RT to bind to the cell-surface glycoproteins and glycolipids on virtually all mammalian cells. Both chains also contain carbohydrate moieties, which are responsible, at least in part, for their interaction with the carbobydrate-binding lectins of liver cells. The procedure used in our laboratory for isolation and purification of RT and its RTA chain is as follows (Fig. 6). 

The A-chain is effectively an enz)une which interferes with protein synthesis by inactivating the 60S ribosomal subunit. The A-chain depurinates a specific adenine residue of the ribosomal RNA the adenine ring is hydrolysed, by the N-glycosidase action of the A-chain, when it becomes situated between two tyrosine rings in the enzyme s active site. As the ribosome is modified it can no longer act as a site of protein synthesis and this leads to the eventual death of the cell. The A-chain enzyme then moves on to deactivate another ribosome. Although only a very small proportion of the ricin molecules that enter the cell are actually moved into the cytosol, one A-chain is sufficient to destroy it. A single ricin molecule is able to deactivate more than 1500 ribosomes per minute. 

Figure 20.9 illustrates a graphical representation of ricin s protein structure found by crystallography [76]. This protein inhibits the function of the ribosome and stops protein synthesis by N-glycosidase enzymatic activity in ribosomal RNA [77]. 

---