DIMERIC STRUCTURE OF A HUMAN APOLIPOPROTEIN-B MESSENGER-RNA EDITING PROTEIN AND CLONING AND CHROMOSOMAL LOCALIZATION OF ITS GENE

Apolipoprotein B (apoB) mRNA editing consists of a posttranscriptional C --> U conversion involving the first base of the codon CAA encoding glutamine-2153 to UAA, a stop codon, in apoB mRNA. Using a cloned rat cDNA as a probe, we cloned the cDNA and genomic sequences of the gene for a human apoB mRNA editing protein. Expression of the cDNA in HepG 2 cells results in editing of the intracellular apoB mRNA. By fluoresc ence in situ hybridization, we localized the gene for the editing prot ein to chromosome band 12p13.1-p13.2. By Northern blot analysis, it wa s shown that the human editing protein mRNA is expressed exclusively i n the small intestine. The cDNA sequence predicts a translation produc t of 236-aa residues. By attaching an epitope tag sequence to the C te rminus of the editing protein, we examined the polymerization state of the editing protein synthesized in vitro. We found that the editing p rotein undergoes spontaneous polymerization. The migration of the huma n apoB mRNA editing protein on an HPLC column and the stoichiometry of polymeric epitope-tagged to untagged protein indicate that the protei n exists as a dimer. Dimerization does not require glycosylation of a consensus N-linked glycosylation sequence present in the protein and i s not mediated by disulfide bridge formation. The human apoB mRNA edit ing protein is a cytidine deaminase showing structural homology to som e known mammalian and bacteriophage deoxycytidylate deaminases. The la tter enzymes exist as homopolymers. The fact that the apoB mRNA editin g protein also exists as a homodimer has important implications for th e mechanism of apoB mRNA editing in humans.