High-resolution NMR study of a GdAGA tetranucleotide loop that is an improved substrate for ricin, a cytotoxic plant protein.

Ricin is a cytotoxic plant protein that inactivates ribosomes by hydrolyzing the N-glycosidic bond at position A4324 in eukaryotic 28S rRNA. Recent studies showed that a four-nucleotide loop, GAGA, can function as a minimum substrate for ricin (the first adenosine corresponds to the site of depurination). We previously clarified the solution structure of this loop by NMR spectroscopy [Orita et al. (1993) Nucleic Acids Res. 21, 5670-5678]. To elucidate further details of the structural basis for recognition of its substrate by ricin, we studied the properties of a synthetic dodecanucleotide, r1C2U3C4A5G6dA7G8A9U10G11A12G (6dA12mer), which forms an RNA hairpin structure with a GdAGA loop and in which the site of depurination is changed from adenosine to 2'-deoxyadenosine. The N-glycosidase activity against the GdAGA loop of the A-chain of ricin was 26 times higher than that against the GAGA loop. NMR studies indicated that the overall structure of the GdAGA loop was similar to that of the GAGA loop with the exception of the sugar puckers of 6dA and 7G. Therefore, it appears that the 2'-hydroxyl group of adenosine at the depurination site (6A) does not participate in the recognition by ricin of the substrate. Since the 2'-hydroxyl group can potentially destabilize the developing positive charge of the putative transition state intermediate, an oxycarbonium ion, the electronic effect may explain, at least in part, the faster rate of depurination of the GdAGA loop compared to that of GAGA loop. We also show that the amino group of 7G is essential for substrate recognition the ricin A-chain.