ZINC-DEPENDENT CYTIDINE DEAMINASE ACTIVITY ASSOCIATED WITH THE CLONEDHUMAN APOLIPOPROTEIN-B MESSENGER-RNA EDITING PROTEIN

Recently, cDNAs were cloned from rats and humans that encode proteins which edit apoB mRNA when complemented with chicken enterocyte extract . These editing proteins, designated rat editing protein (REPR) and hu man editing protein (HEPR), respectively, are thought to be catalytic subunits of the editosomes since they share sequence similarity to cyt osine nucleoside/nucleotide deaminases from bacteria, yeasts and mamma ls and have been suspected of carrying out cytidine deamination at the apoB mRNA. Whereas REPR has been characterized as having cytidine dea minase activity, the human counterpart HEPR has not been determined to have the same activity, Here, we show that recombinant HEPR expressed in E. coli manifested cytidine deaminase activity. This activity was inhibited by 1, 10-o-phenanthroline, a zinc-specific chelating agent, and was recovered by an addback of zinc chloride. On the other hand, g lutathione S-transferase (GST)/HEPR fusion protein could not convert c ytidine to uridine, suggesting the possibility that the active site wi th a putative zinc-chelating pocket at the NH2-terminal half of HEPR m ay be masked by the fused GST. All these results are in complete agree ment with and further explain the previous observation that apoB mRNA editing is a zinc-dependent process.