Ribosome-inactivating proteins from plants: more than RNA N-glycosidases?

Many plants contain proteins that are capable of inactivating ribosomes and accordingly are called ribosome-inactivating proteins or RIPs. These typic al plant proteins receive a lot of attention in biological and biomedical r esearch because of their unique biological activities toward animal and hum an cells. In addition, evidence is accumulating that some RIPs play a role in plant defense and hence can be exploited in plant protection. To underst and the mode of action of RIPs and to optimize their medical and therapeuti cal applications and their use as antiviral compounds in plant protection, intensive efforts have been made to unravel the enzymatic activities of RIP s and provide a structural basis for these activities. Though marked progre ss has been made during the last decade, the enzymatic activity of RIPs has become a controversial issue because of the concept that RIPs possess, in addition to their classical RNA N-glycosidase and polynucleotide: adenosine glycosidase activity, other unrelated enzymatic activities. Moreover, the presumed novel enzymatic activities, especially those related to diverse nu clease activities, are believed to play an important role in various biolog ical activities of RIPs. However, both the novel enzymatic activities and t heir presumed involvement in the biological activities of RIPs have been qu estioned because there is evidence that the activities observed are due to contaminating enzymes. We offer a critical review of the pros and cons of t he putative novel enzymatic activities of RIPs. Based on the available data , it is suggested that there is little conclusive evidence in support of th e presumed activities and that in the past too little attention has been gi ven to the purity of the RIP preparation. The antiviral activity and mode o f action of RIPs in plants are discussed in view of their classical and pre sumed novel enzymatic activities.