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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




Cell Press

Publication Date





187 - 195


Amino Acid Sequence Animal Apolipoproteins B/*genetics Base Sequence Binding Sites Catalysis Comparative Study Cytidine Deaminase/genetics/*metabolism DNA Mutational Analysis Human Metalloproteins/genetics/*metabolism Models, Molecular Molecular Sequence Data Phylogeny Protein Binding *RNA Editing RNA-Binding Proteins/genetics/*metabolism Rats Sequence Deletion Sequence Homology, Amino Acid Structure-Activity Relationship Support, Non-U.S. Gov't Zinc/metabolism