Essential component of the apolipoprotein B mRNA editing enzyme complex which is responsible for the postranscriptional editing of a CAA codon for Gln to a UAA codon for stop in APOB mRNA. Binds to APOB mRNA and is probably responsible for docking the catalytic subunit, APOBEC1, to the mRNA to allow it to deaminate its target cytosine. The complex also seems to protect the edited APOB mRNA from nonsense-mediated decay (By similarity). Part of the apolipoprotein B mRNA editing complex with APOBEC1. Found in a complex with APOBEC1 and CELF2/CUGBP2. Interacts APOBEC1. Interacts with TNPO2; TNPO2 may be responsible for transport of A1CF into the nucleus. Interacts with SYNCRIP. Interacts with CELF2/CUGBP2 (By similarity). Predominantly nuclear where it localizes to heterochromatin. Also cytoplasmic where it is found at the outer surface of the endoplasmic reticulum. Shuttles between the nucleus and cytoplasm. May be transported into the nucleus by the nuclear import protein TNPO2/TRN2 or by APOBEC1. Isoforms 1 and 2 are widely expressed while isoforms 3 and 4 are restricted to liver and small intestine. The RRM domains are necessary but not sufficient for binding to APOB mRNA. Additional residues in the pre-RRM and C-terminal regions are required for RNA-binding and for complementing APOBEC1 activity (By similarity).