Enhanced Diagnostic Tools
The C-to-U editing of apolipoprotein B (apo-B) mRNA is catalyzed by a multiprotein complex that recognizes
an 11-nucleotide mooring sequence downstream of the editing site. The catalytic subunit of the editing enzyme,
apobec-1, has cytidine deaminase activity but requires additional unidentified proteins to edit apo-B mRNA.
We purified a 65-kDa protein that functionally complements 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 RNA recognition motifs. ACF and apobec-1
comprise the minimal protein requirements for apo-B mRNA editing in vitro. By UV cross-linking and
immunoprecipitation, we show that ACF binds to apo-B mRNA in vitro and in vivo. Cross-linking of ACF is
not competed by RNAs with mutations in the mooring sequence. 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 tissues that lack apobec-1 and
apo-B mRNA suggests that ACF may be involved in other RNA editing or RNA processing events.
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