The binding of apobec-1 to mammalian apo B RNA is stabilized by the presence of complementation factors which are required for post-transcriptional editing.

C to U RNA editing in mammalian intestinal apolipoprotein B mRNA creates an in-frame translational stop and the synthesis of a truncated protein called apo B48. This site specific cytidine deamination is mediated by an enzyme complex of which the catalytic component (apobec-1) is a 27 kDa zinc-binding protein. apobec-1, expressed in bacteria, will bind to mammalian apo B RNA as well as a number of other AU-rich RNA templates. Apo B RNA-binding activity can be competed by the addition of tRNA, an effect which can be overcome by the addition of complementation factors such as chick enterocyte S-100 extracts. Thus, apobec-1 may be a non-specific RNA binding protein which requires the presence of complementation factors to stabilize and enhance its binding in the setting of the holo-enzyme.