Ribosome-inactivating proteins active site

Endo, Y., Tsurugi, K. and Lamberf, J.M. (1988a) The site of action of six different ribosome-inactivating proteins from plants on eukaryotic ribosomes the RNA A-glycosidase activity of the proteins. Biochem Biophys Res Commun, 150, 1032-1036. 

The inhibitory effect of ricin on protein synthesis in cell-free systems is greatly increased in the presence of / -mercaptoethanol. This is because free A chain is the enzymatically active toxin, whereas A chain linked to B chain in whole ricin is not active [ 121]. In the absence of reducing agents, even high concentrations of intact toxin do not inactivate ribosomes. Presumably the catalytic site on the A chain is formed or exposed only when the A chain is released from the B chain. Free A chain, however, is non-toxic to intact cells since, in the absence of B chain, it lacks the ability to bind to and enter cells. Ricin A-chain preparations frequently show some level of toxicity, however, because the complete removal of contaminating B chain can be difficult to achieve [ 122]. 

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. 

Poly(ADP-ribose) polymerase homology to other proteins. Sequence similarity comparison of the polymerase with the National Biomedical Center s protein data bases revealed no extensive identities with other proteins (> 4200 proteins searched). Although no extensive similarities were observed, the polymerase exhibits some short but interesting identities. The most statistictdly significant identity is with the catalytic site of the ricin A chain, a cytotoxic plant protein which inactivates the 60S ribosome. This may represent the active site of the enzyme, based upon the catalytic... 

Originally, type I and type II RIPs were identified based on biological activities. Type II RIPs were discovered more than a century ago, when Stillmark isolated the toxic protein ricin from castor beans. As mentioned, the toxicity of ricin was initially attributed to its agglutination activity for red blood cells and not by ribosome inactivation. Type II RIPs attribute their carbohydrate binding activity to their B-chain, which contains two or possibly three binding sites (Robertus, 1991). The multiple binding sites allow B-chains to aggregate with red blood cells and platelets. 

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... 

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