Disproportionate relationship between APOBEC-1 expression and apolipoprotein B mRNA editing activity

Apolipoprotein B (apoB) mRNA editing is a site-specific (nucleotide 6666) c ytidine to uridine transition catalyzed by a cytidine deaminase, APOBEC-1, in the context of a multiprotein complex referred to as the C/U editosome, This report quantifies for the first time the effect of altering APOBEC-1 p rotein abundance on the proportion of edited apoB mRNAs using transfected M cArdle rat hepatoma cells which had been sorted by how cytometry into popul ations expressing different levels of green fluorescent protein-APO-BEC-1 c himera, GFP-APOBEC. A correlation was observed in which increased expressio n of GFP-APOBEC protein resulted in a higher proportion of edited apoB mRNA . The number of enzyme molecules required to increase the proportion of edi ted apoB RNAs was disproportionately high relative to that which might have been predicted from a typical catalytic relationship. Moreover, editing of apoB mRNA at inappropriate sites (promiscuous editing) occurred in respons e to overexpressing GFP-APOBEC. The data suggest that experimental manipula tion of APOBEC-1 abundance in the absence of other regulatory consideration s will always result in some level of promiscuous editing. Coordinate expre ssion of APOBEC-1 and the auxiliary proteins and/or regulation of their int eractions may be required to increase editing activity without losing editi ng-site fidelity, (C) 1999 Academic Press.