Transition state analogues in structures of ricin and saporin ribosome-inactivating proteins

Small-molecule inhibitors of ricin and Shiga toxins

This review summarizes the successes and continuing challenges associated with the identification of small-molecule inhibitors of ricin and Shiga toxins, members of the RNA N-glycosidase family of toxins that irreversibly inactivate eukaryotic ribosomes through the depurination of a conserved adenosine residue within the sarcin-ricin loop (SRL) of 28S rRNA. Virtual screening of chemical libraries has led to the identification of at least three broad classes of small molecules that bind in or near the toxin's active sites and thereby interfere with RNA N-glycosidase activity. Rational design is being used to improve the specific activity and solubility of a number of these compounds. High-throughput cell-based assays have also led to the identification of small molecules that partially, or in some cases, completely protect cells from ricin- and Shiga-toxin-induced death. A number of these recently identified compounds act on cellular proteins associated with intracellular trafficking or pro-inflammatory/cell death pathways, and one was reported to be sufficient to protect mice in a ricin challenge model.

Differential expression of saporin genes upon wounding, ABA treatment and leaf development

Saporins are type 1 ribosome-inactivating proteins (RIPs: EC 3.2.2.22) produced in various organs of Saponaria officinalis L. Two distinct saporin types, saporin-L and saporin-S isoforms, were respectively purified from the intra- and extra-cellular fractions of soapwort leaves. The saporin-L isoform was lowly identical, differed for toxicity, molecular mass and amino acid composition from saporin-S proteins forming a new monophyletic group. Genes encoding both L- and S-type isoforms were cloned from leaf-specific cDNA library; the encoded products included the N-terminal diversity observed by protein sequencing and showed compatible weights with those from mass spectra. These genes were intron-less belonging to small gene families. Reverse transcription polymerase chain reaction/quantitative reverse transcription polymerase chain reaction experiments evidenced their differential expression during leaf development, wounding and abscisic acid treatment. These results suggest that the saporin-L and -S proteins may play diversified roles during stress responses.