Substrate specificity of a maize ribosome-inactivating protein differs across diverse taxa.

The superfamily of ribosome-inactivating proteins (RIPs) consists of toxins that catalytically inactivate ribosomes at a universally conserved region of the large ribosomal RNA. RIPs carry out a single N-glycosidation 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 kernel-specific 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 species-dependent. 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), papain-activated 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 papain-activated 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 papain-activated rproRIP1 was much more active against rabbit ribosomes than homologous Zea mays ribosomes and had no detectable effect on Escherichia coli ribosomes.

Factors affecting activation of the 'cloaked' Zea maize ribosome inactivating protein (RIP) zymogen. 1. 'LID' peptide hydrophobicities and oxidative structural changes.

We want to understand how environmental factors influence zymogen activation of the "cloaked' active site of the 'Ribosome Inactivating Protein' (RIP) from corn kernels. In this study, we focus on how likely chemical effectors in the immediate environment of the 'lid' conspire to unleash the active site upon encountering target membranes of invading pests. Octanol-H2O partitioning free energies of peptides which (i) straddle the proteolysis site, and (ii) form the 'side' and 'bottom' of the proposed 'lid' were found to only slightly favor H2O, suggesting that the peptide is poised to detach from the less polar surface surrounding the RIP active site. Circular dichroism results obtained upon catalase/H2O2 oxidation of the 'lid' peptide suggest that the structure shifts from primarily alpha-helical to primarily beta-like. These results suggest that the active site is more easily 'uncloaked' as a result of the lowered solvent polarity conditions and higher oxidant concentrations in the presence of pest membranes encountered during crucial stages of seed germination.