A number of bacterial and plant toxins act by inhibiting protein synthesis in eukaryotic cells. The toxins of the Shiga and ricin family inactivate 60S ribosomal subunits by an N-glycosidic cleavage which releases a specific adenine base from the sugar-phosphate backbone of 28S rRNA [1,2,3]. The toxins which are known to function in this manner are:

• Shiga toxin from Shigella dysenteriae [4]. This toxin is composed of one copy of an enzymatically active A subunit and five copies of a B subunit responsible for binding the toxin complex to specific receptors on the target cell surface.
• Shiga-like toxins (SLT) are a group of Escherichia coli toxins very similar in their structure and properties to Shiga toxin. The sequence of two types of these toxins, SLT-1 [5] and SLT-2 [6], is known.
• Ricin, a potent toxin from castor bean seeds. Ricin consists of two glycosylated chains linked by a disulfide bond. The A chain is enzymatically active. The B chain is a lectin with a binding preference for galactosides. Both chains are encoded by a single polypeptidic precursor. Ricin is classified as a type-II ribosome-inactivating protein (RIP); other members of this family are agglutinin, also from castor bean, and abrin from the seeds of the bean Abrus precatorius [7].
• Single chain ribosome-inactivating proteins (type-I RIP) from plants. Examples of such proteins are: barley protein synthesis inhibitors I and II, mongolian snake-gourd trichosanthin, sponge gourd luffin-A and -B, garden four-o'clock MAP, common pokeberry PAP-S and soapwort saporin-6 [7].

All these toxins are structurally related. A conserved glutamic residue has been implicated [8] in the catalytic mechanism; it is located near a conserved arginine which also plays a role in catalysis [9]. The signature we developed for these proteins includes these catalytic residues.