The biological properties of Shiga-like toxin I
Shiga toxin and the Escherichia coli Shiga-like toxins (SLTs) are type 2 ribosome inactivating proteins (RIPs), exhibiting a specific RNA N-glycosidase activity comparable to that of the plant toxin ricin (Endo et al. 1988). Not only do they possess the same enzymatic activity as a plant toxin, but on examination at the molecular level, they may be seen to be classic bipartite toxins. The present study examines in detail the RNA N-glycosidase activity of Shiga-like toxin I (SLT I) ie, the ribosomes susceptibility to the toxin, of a range the exact of different eukaryotic site of SLT I-catalysed depurination of yeast ribosomal RNA and the kinetics of such depurination. The cytotoxic effect of SLT I on Vero cells is also examined. In addition to its characterisation and subsequent comparison to ricin, an attempt has been made to correlate certain structure - function aspects of SLT I. By structural comparison with other bacterial toxins it has been proposed that proteolytic cleavage with thin an arginine rich, trypsin sensitive, disulphide-bonded loop structure is essential for expression of cytotoxicity. Examination of the enzymatic activity and cytotoxicity of a 'protease insensitive' mutant SLT I A subunit (SLT lA) generated in the present study has led to the conclusion tha t proteolytic processing wi thin this disulphide-bonded loop is not essential, but that processing may occur at alternative sites within SLT lA to compensate for loss cleavage at the proposed target sites.