Substrate specificity of a maize ribosomeâinactivating protein differs across diverse taxa
2000
Krawetz, Julie E. | Boston, Rebecca S
The superfamily of ribosomeâinactivating proteins (RIPs) consists of toxins that catalytically inactivate ribosomes at a universally conserved region of the large ribosomal RNA. RIPs carry out a single Nâglycosidation event that alters the binding site of the translational elongational factor eEF1A and causes a cessation of protein synthesis that leads to subsequent cell death. Maize RIP1 is a kernelâspecific RIP with the unusual property of being produced as a zymogen, proRIP1. ProRIP1 accumulates during seed development and becomes active during germination when cellular proteases remove acidic residues from a central domain and both termini. These deletions also result in RIP activation in vitro. However, the effectiveness of RIP1 activity against target ribosomes remains speciesâdependent. To determine the potential efficiency of maize RIP1 as a plant defense protein, we used quantitative RNA gel blots to detect products of RIP activity against intact ribosomal substrates from various species. We determined the enzyme specificity of recombinant maize proRIP1 (rproRIP1), papainâactivated rproRIP1 and MOD1 (an active deletion mutant of rproRIP1) against ribosomal substrates with differing levels of RIP sensitivity. The rproRIP1 had no detectable enzymatic activity against ribosomes from any of the species assayed. The papainâactivated rproRIP1 was more active than MOD1 against ribosomes from either rabbit or the corn pathogen, Aspergillus flavus, but the difference was much more marked when rabbit ribosomes were used as a substrate. The papainâactivated rproRIP1 was much more active against rabbit ribosomes than homologous Zea mays ribosomes and had no detectable effect on Escherichiaâcoli ribosomes.
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