A new class of enzyme acting on damaged ribosomes: ribosomal RNA apurinic site specific lyase found in wheat germ

A new enzyme, which we named ribosomal RNA apurinic site specific lyase (RALyase), is described. The protein was found in wheat embryos and has a molecular weight of 50 625 Da, The enzyme specifically cleaves the phosphodiester bond at the 3' side of the apurinic site introduced by ribosome-inactivating proteins into the sarcin/ricin domain of 28S rRNA, The 3' and 5' ends of wheat 28S rRNA at the cleavage site are 5'-GUACG-alpha-hyroxy-alpha,beta-unsaturated aldehyde and pGAGGA-3', demonstrating that the enzyme catalyzes a p-elimination reaction. The substrate specificity of the enzyme is extremely high: it acts only at the apurinic site in the sarcin/ricin domain of intact ribosomes, not on deproteinized rRNA or DNA containing apurinic sites, The amino acid sequences of five endopeptidase LysC-liberated peptides from the purified enzyme were determined and used to obtain a cDNA sequence. The open reading frame encodes a protein of 456 amino acids, and a homology search revealed a related rice protein. Similar enzyme activities were also found in other plants that express ribosome-inactivating proteins. We believe that RALyase is part of a complex self-defense mechanism.