MAIZE RIBOSOME-INACTIVATING PROTEIN (B-32) - HOMOLOGS IN RELATED SPECIES, EFFECTS ON MAIZE RIBOSOMES, AND MODULATION OF ACTIVITY BY PRO-PEPTIDE DELETIONS

The ribosome-inactivating protein (RIP) from maize (Zea mays L.) is un usual in that it is produced in the endosperm as an inactive pro-form, also known as b-32, which can be converted by limited proteolysis to a two-chain active form, alpha beta RIP. Immunological analysis of see d extracts from a variety of species related to maize showed that pro/ alpha beta forms of RIP are not unique to maize but are also found in other members of the Panicoideae, including Tripsacum and sorghum. Rib osomes isolated from maize were quite resistant to both purified pro- and alpha beta maize RIPs, whereas they were highly susceptible to the RIP from pokeweed. This suggests that the production of an inactive p ro-RIP is not a mechanism to protect the plant's own ribosomes from de leterious action of the alpha beta RIP. RIP derivatives with various p ro-segments removed were expressed at high levels in Escherichia coli. Measurement of their activity before and after treatment with subtili sin Carlsberg clearly identified the 25-amino acid intradomain inserti on, rather than the N- or C-terminal extensions, as the major element responsible for suppression of enzymatic activity. A RIP with all thre e processed regions deleted had activity close to that of the native a lpha beta form.