Ribosome-inactivating proteins: A family of plant proteins that do more than inactivate ribosomes

Many plants contain proteins that are commonly designated as ribosome-inact ivating proteins (RIPs). Based on the structure of the genes and the mature proteins a novel system is proposed to unambiguously classify all RIPs in type-1, type-2, and type-3 RIPs. In addition, the concept of one- and two-c hain type-1 RIPs is introduced. After an overview of the occurrence, molecu lar structure, and amino acid sequences of RIPs, the formation of the matur e proteins from the primary translation products of the corresponding mRNAs is elaborated in detail in a section dealing with the biosynthesis, posttr anslational modifications, topogenesis, and subcellular location of the dif ferent types of RIPs. Details about the three-dimensional structure of type -1 RIPs and the A and B chains of type-2 RIPs are discussed in a separate s ection. Based on the data given in the previous sections, the phylogenic an d molecular evolution of RIPs is critically assessed and a novel model is p roposed for the molecular evolution of RIPs. Subsequently, the enzymatic ac tivities of RIPs are critically discussed whereby special attention is give n to some presumed novel activities, and a brief overview is given of the b iological activities of the different types of RIPs on cells and whole orga nisms. By combining the data on the enzymatic activities and biological act ivities of RIPs, and the current knowledge of different plant physiological aspects of these proteins, the role of RIPs in plants is revisited. Thereb y the attention is focussed on the role of RIPs in plant defense with the e mphasis on protection against plant-eating organisms and viruses. Finally, there is a short discussion on the discovery of a novel class of enzymes ca lled RALyases that use ribosomes damaged by RIPs as a substrate and may act cooperatively with RIPs. There is discussion regarding why the identificat ion of this novel enzyme gives valuable clues to the origin and original fu nction of RIPs and may be helpful to unravel the physiological role of mode m RIPs.