Molecular cloning of apobec-1 complementation factor, a novel RNA-binding protein involved in the editing of apolipoprotein B mRNA

The C-to-U editing of apolipoprotein B (apo-B) mRNA is catalyzed by a multi protein complex that recognizes an 11-nucleotide mooring sequence downstrea m of the editing site. The catalytic subunit of the editing enzyme, apobec- 1, has cytidine deaminase activity but requires additional unidentified pro teins to edit apo-B mRNA. We purified a 65-kDa protein that functionally co mplements apobec-1 and obtained peptide sequence information which was used in molecular cloning experiments. The apobec-1 complementation factor (ACF ) cDNA encodes a novel 64.3-kDa protein that contains three nonidentical RN A recognition motifs. ACF and apobec-1 comprise the minimal protein require ments for apo-B mRNA editing in vitro. By UV cross-linking and immunoprecip itation, we show that ACF binds to apo-B mRNA in vitro and in vivo. Cross-l inking of ACF is not competed by RNAs with mutations in the mooring sequenc e. Coimmunoprecipitation experiments identified an ACF-apobec-1 complex in transfected cells. Immunodepletion of ACF from rat liver extracts abolished editing activity. The immunoprecipitated complexes contained a functional holoenzyme. Our results support a model of the editing enzyme in which ACF binds to the mooring sequence in apo-B mRNA and docks apobec-1 to deaminate its target cytidine. The fact that ACF is widely expressed in human tissue s that lack apobec-1 and apo-B mRNA suggests that ACF may be involved in ot her RNA editing or RNA processing events.