Ribosome-inactivating proteins: A plant perspective

Ribosome-inactivating proteins (RIPs) are toxic N-glycosidases that depurin ate the universally conserved alpha -sarcin loop of large rRNAs. This depur ination inactivates the ribosome, thereby blocking its further participatio n in protein synthesis. RIPs are widely distributed among different plant g enera and within a variety of different tissues. Recent work has shown that enzymatic activity of at least some RIPs is not limited to site-specific a ction on the large rRNAs of ribosomes but extends to depurination and even nucleic acid scission of other targets. Characterization of the physiologic al effects of RIPs on mammalian cells has implicated apoptotic pathways. Fo r plants, RIPs have been linked to defense by antiviral, antifungal, and in secticidal properties demonstrated in vitro and in transgenic plants. How t hese effects are brought about, however, remains unresolved. At the least, these results, together with others summarized here, point to a complex bio logical role. With genetic, genomic, molecular, and structural tools now av ailable for integrating different experimental approaches, we should furthe r our understanding of these multifunctional proteins and their physiologic al functions in plants.