Ribosome-Inactivating Proteins (RIPs) are enzymes that trigger the catalytic inactivation of ribosomes and other substrates. They are present in a large number of plants and have been found also in fungi, algae and bacteria. RIPs are currently classified as type 1, those formed by a single polypeptide chain with the enzymatic activity, and type 2, those formed by 2 types of chains, i.e. A chains equivalent to a type 1 RIPs and B chains with lectin activity. Type 2 RIPs usually contain the formulae A-B, (A-B)2 and less frequent (A-B)4 and polymeric forms of type 2 RIPs lectins. RIPs are broadly distributed in plants, and are present also in fungi, bacteria, at least in one alga; recently RIP-type activity has been described in mammalian tissues. The highest number of RIPs has been found in Caryophyllaceae, Sambucaceae, Cucurbitaceae, Euphorbiaceae, Phytolaccaceae and Poaceae. However there are no systematic screening studies to allow generalisations about occurrence. The most known activity of RIPs is the translational inhibitory activity, which seems a consequence of a N-glycosidase on the 28 S rRNA of the eukaryotic ribosome that triggers the split of the A4324 (or an equivalent base in other ribosomes), which is key for translation. This activity seems to be part of a general adenine polynucleotide glycosylase able to act on several substrates other than ribosomes, such as tRNA, mRNA, viral RNA and DNA. Other enzymatic activities found in RIPs are lipase, chitinase and superoxide dismutase. RIPs are phylogenetically related. In general RIPs from close families share good amino acid homologies. Type 1 RIPs and the A chains of type 2 RIPs from Magnoliopsida (dicotyledons) are closely related. RIPs from Liliopsida (monocotyledons) are at the same time closely related and distant from Magnoliopsida. Concerning the biological roles played by RIPs there are several hypotheses, but the current belief is that they could play significant roles in the antipathogenic (viruses and fungi), stress and senescence responses. In addition, roles as antifeedant and storage proteins have been also proposed. Future research will approach the potential biological roles played by RIPs and their use as toxic effectors in the construction of immunotoxins and conjugates for target therapy.

Sugar beet (Beta vulgaris L.) leaves contain virus-inducible type 1 (single chain) ribosome-inactivating proteins that have been named beetins. The structural and functional characterization, the cellular location, and the potential role of beetins as antiviral agents are reported here. Beetins are formed of a single polypeptide chain with a varying degree of glycosylation and strongly inhibited in vitro protein synthesis in rabbit reticulocyte lysates (IC₅₀=1.15 ng ml⁻¹) and a Vicia sativa L. cell-free system (IC₅₀=68 ng ml⁻¹) through the single depurination of the large rRNA. Beetins trigger the multidepurination of tobacco mosaic virus (TMV) genomic RNA which underwent extensive degradation upon treatment with acid aniline. Beetins are extracellular proteins that were recovered from the apoplastic fluid. Induction of sugar beet RIPs with either H₂O₂ or artichoke mottled crinkle virus (AMCV) was observed in leaves distant from the site of application of such elicitors. The external application of purified beetin to sugar leaves prevented infection by AMCV which supports the preliminary hypothesis that beetins could be involved in plant systemic acquired resistance subjected to induction by phytopathogens.