Mutagenesis of apobec-1, the catalytic subunit of the mammalian apolipoprotein B mRNA editing enzyme, reveals distinct domains that mediate cytosine nucleoside deaminase, RNA binding, and RNA editing activity

Apolipoprotein (apo) B48 is synthesized by mammalian small intestine as a result of post-transcriptional RNA editing. This process is mediated by an enzyme complex containing a catalytic subunit, apobec-1, which is homologous to other cytidine deaminases, particularly in a domain (H/C)-(A/V)-E-(X)_24-30-P-C-(X)₂-C which coordinates zinc. apobec-1, expressed as a glutathione S-transferase fusion protein, demonstrates both apoB RNA editing and cytidine deaminase activity. His⁶¹, Cys⁹³, and Cys⁹⁶, the putative zinc-coordinating residues, were mutated to Arg, Ser, and Ser, respectively, with loss of RNA editing activity and either great reduction or abolition of cytidine deaminase activity. Mutation of the catalytically active Glu⁶³ residue to Gln and Pro⁹² to Leu abolished both cytidine deaminase and RNA editing activity. The conservative His⁶¹ → Cys mutation, which should coordinate zinc, retained both editing and cytidine deaminase activity. Thus, zinc binding is required for both apoB RNA editing and cytidine deaminase activity. Mutation of the first four leucines within the heptad repeat of the leucine-rich region (LRR) of apobec-1 resulted in reduced RNA editing but preservation of wild-type cytidine deaminase activity. GST/APOBEC-1 was also demonstrated to cross-link to apoB RNA. Mutation of His⁶¹ → Arg abolished RNA binding, while the Glu⁶³ → Gln and Cys⁹⁶ → Ser mutant proteins showed wild-type levels of RNA binding. The remaining mutants had reduced levels of activity. Overexpression of wild-type apobec-1 in McA 7777 cells resulted in a 5-6-fold increase in editing of endogenous apoB. Transfection of the His⁶¹ → Cys, LRR, and Cys⁹³ → Ser mutants increased endogenous editing 2-3-fold, while Glu⁶³ → Gln and His⁶¹ → Arg mutants acted as dominant negatives, reducing endogenous editing. These data suggest that apobec-1 has distinct functional domains which modulate activity in the context of the apoB mRNA editing enzyme.