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The site-specific C to U editing of apolipoprotein B100 (apoB100) mRNA requires a 27 kDa protein (p27) with homology to cytidine deaminase. Here, we show that p27 is a zinc-containing deaminase, which operates catalytically like the E. coli enzyme that acts on monomeric substrate. In contrast with the bacterial enzyme that does not bind RNA, p27 interacts with its polymeric apoB mRNA substrate at AU sequences adjacent to the editing site. This interaction is necessary for editing. RNA binding is mediated through amino acid residues involved in zinc coordination, in proton shuttling, and in forming the alpha beta alpha structure that encompasses the active site. However, certain mutations that inactivate the enzyme do not affect RNA binding. Thus, RNA binding does not require a catalytically active site. The acquisition of polymeric substrate binding provides a route for the evolution of this editing enzyme from one that acts on monomeric substrates.

Original publication




Journal article



Publication Date





187 - 195


APOBEC-1 Deaminase, Amino Acid Sequence, Animals, Apolipoprotein B-100, Apolipoproteins B, Base Sequence, Binding Sites, Catalysis, Cytidine Deaminase, DNA Mutational Analysis, Humans, Metalloproteins, Models, Molecular, Molecular Sequence Data, Phylogeny, Protein Binding, RNA Editing, RNA-Binding Proteins, Rats, Sequence Deletion, Sequence Homology, Amino Acid, Structure-Activity Relationship, Zinc