HYPEREDITING OF MULTIPLE CYTIDINES OF APOLIPOPROTEIN-B MESSENGER-RNA BY APOBEC-1 REQUIRES AUXILIARY PROTEIN(S) BUT NOT A MOORING SEQUENCE MOTIF

An RNA-binding cytidine deaminase (APOBEC-1) and unidentified auxiliar y protein(s) are required for apolipoprotein (apo) B mRNA editing. A s equence motif on apoB mRNA (''mooring sequence,'' nucleotides 6671-668 1) is obligatory for the editing of cytidine 6666 (C-6666), the only c ytidine on apoB mRNA converted to uridine in normal animals. Transgeni c animals with hepatic overexpression of APOBEC-1 develop liver tumors , and other non-apoB mRNAs are edited, suggesting a loss of the normal ly precise specificity. In this study, we examined apoB mRNA from thes e transgenic animals to determine if cytidines aside from C-6666 are e dited. Multiple cytidines downstream from C-6666 in apoB mRNA were edi ted extensively by the overexpressed APOBEC-1. This pathophysiological ''hyperediting'' could be mimicked in vitro by incubating a synthetic apoB RNA substrate with the transgenic mouse liver extracts. Multiple cytidines in the synthetic apoB RNA were edited by recombinant APOBEC -1 but only with supplementation of the auxiliary protein(s). Mutation s in the mooring sequence markedly decreased the normal editing of C-6 666 but, surprisingly, increased the hyperediting of downstream cytidi nes. Furthermore, cytidines in an apoB RNA substrate lacking the moori ng sequence were also edited in vitro. These results indicate that the hyperediting of apoB mRNA by overexpressed APOBEC-1 depends upon auxi liary protein(s) but is independent of the mooring sequence motif. The se results suggest that hyperediting may represent the first step in a two-step recognition model for normal apoB mRNA editing.