Introduction

Many plant tissues produce ribosome-inactivating proteins (RIPs) which act as N-glycosidases removing a specific adenine residue from a highly conserved surface loop present in 23S, 26S and 28S ribosomal RNA (Lord, Hartley & Roberts, 1991). The adenine residue in question (adenine 4324 in rat liver 28S rRNA) (Endo et al., 1987), plays a necessary role in the binding of elongation factors, and ribosomes that have been depurinated by RIPs can no longer function in protein synthesis. Typically, a single molecule of RIP can depurinate 1500–2000 susceptible ribosomes per minute. Plant RIPs usually occur as monomeric proteins with molecular masses of around 30 kDa and are frequently but not always N-glycosylated. Although these RIPs can potently and irreversibly inactivate mammalian ribosomes they are not cytotoxic to mammalian cells since they are unable to enter such cells and reach the cytosol where their ribosome substrates are located. It has recently been found that these single-chain RIPs are also active against prokaryotic ribosomes (Hartley et al., 1991). In some instances, however, the RIP is joined via a disulphide bond to a second polypeptide which, in all cases described to date, is a galactose-binding lectin whose molecular mass is also around 30 kDa. These heterodimeric plant toxins are able to bind opportunistically to eukaryotic cells by interacting with galactose residues present on cellsurface glycoproteins and glycolipids. Such cytotoxic lectins are amongst the most potent cytotoxins in Nature.