The RNA N-glycosidase activity of ricin A-chain.
The RNA N-glycosidase activity of ricin A-chain has been characterized. When rat liver ribosomes were used as substrates, the A-chain cleaved the N-glycosidic bond at A-4324 in 28S rRNA. An apparent Michaelis constant (Km) for the reaction was determined to be 2.6 microM and the turnover number (Kcat) was 1777 min-1. When naked rRNA was the substrate, the A-chain cleaved the same bond in 28S rRNA but at a greatly reduced rate. The Km value was 5.8 microM. The results suggest that the A-chain has a similar affinity for 28S rRNA in both ribosomes and the naked states. When the deproteinized Escherichia coli rRNA was the substrates, ricin A-chain cleaved a N-glycosidic bond at A-2600 in 23S rRNA which corresponds to the ricin-site in 28S rRNA of rat liver ribosomes, while the A-chain has little activity on 23S rRNA in the ribosomes. The results suggest that ricin A-chain acts directly on RNA by recognizing a certain structure in the molecules. Using the secondary structure models for each species of rRNA, we have deduced a loop and stem structure having GAGA in the loop to be a minimum requirement for the substrate of ricin A-chain.
Endo
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Department of Biochemistry, Yamanashi Medical College, Japan.
Tsurugi
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D000818:Animals; D008099:Liver / Q000378:metabolism; D009699:N-Glycosyl Hydrolases; D012335:RNA, Ribosomal / Q000378:metabolism*; D051381:Rats; D054788:Ribosome Inactivating Proteins; D012270:Ribosomes / Q000378:metabolism; D012276:Ricin / Q000378:metabolism*; D013379:Substrate Specificity
D012335:RNA, Ribosomal; D012276:Ricin; D009699:N-Glycosyl Hydrolases; D054788:Ribosome Inactivating Proteins