Plant and Bacterial Toxins as RNAN-Glycosidases

Abstract

Along with identification of the enzymatic function of ribosome-inactivating proteins (RIPs) came the recognition that certain bacterial toxins, called Shiga toxins (Stxs), fit into the category of plant proteins. We now know that these plant and bacterial poisons have the broader specificity of polynucleotide:adenosine glycosidases, at least in vitro. This chapter refers to the enzymes interchangeably as RIPs or RNA N-glycosidases. Recently, additional enzymatic activities have been attributed to the RIPs. However, when four different RIPs were highly purified, the only enzymatic activity they retained was the capacity to remove adenine from rRNA or DNA. This observation indicates that the RIPs, originally defined as RNA N-glycosidases, are actually polynucleotide:adenosine glycosidases, at least in vitro. One unusual feature of the RIPs is that, although they share the same enzymatic activity and their preferred target is intact mammalian ribosomes, they are not equally active on target ribosomes in vitro. There appear to be at least two explanations for this phenomenon. One reason that some RNA N-glycosidases do not appear to be as active as others in cell-free assays is that they require cofactors such as ATP. The second reason that some RIPs have varying specificities is that they appear to interact with ribosomal proteins. Consistent with a protective function for the plant RNA N-glycosidases is that the RIPs are both lethal and often stored in large quantities.