Multiple protein domains determine the cell type-specific nuclear distribution of the catalytic subunit required for apolipoprotein B mRNA editing.
Apolipoprotein B (apoB) mRNA editing catalyzed by apoB mRNA editing catalytic subunit 1 (APOBEC-1) has been proposed to be a nuclear process. To test this hypothesis, the subcellular distribution of hemagglutinin- (HA) tagged APOBEC-1 expressed in transiently transfected hepatoma cells was determined by indirect immunofluorescence microscopy. HA-APOBEC-1 was detected in both the nucleus and cytoplasm of rat and human hepatoma cells. Mutagenesis of APOBEC-1 demonstrated that the N-terminal 56 amino acids (1-56) were necessary for the nuclear distribution of APOBEC-1, but this region did not contain a functional nuclear localization signal (NLS). However, we identified a 24-amino acid domain in the C terminus of APOBEC-1 with characteristics of a cytoplasmic retention signal (CRS) or a nuclear export signal (NES). These data suggest, therefore, that the nuclear import of APOBEC-1 may not be mediated by a positive NLS; rather, it may be achieved by overcoming the effect of a CRS/NES. We also demonstrated that the nuclear distribution of APOBEC-1 occurred only in cell lines that were capable of editing apoB RNA. We propose that the cellular distribution of APOBEC-1 is determined by multiple domains within this protein, and a nuclear localization of the enzyme may be regulated by cell type-specific factors that render these cells uniquely editing competent.
Yang
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Department of Biochemistry, University of Rochester, NY 14642, USA.
Yang
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Smith
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D000071479:APOBEC-1 Deaminase; D000595:Amino Acid Sequence; D000818:Animals; D001055:Apolipoproteins B / Q000235:genetics*; D016466:CHO Cells; D019556:COS Cells; D002384:Catalysis; D002467:Cell Nucleus / Q000378:metabolism*; D006224:Cricetinae; D003564:Cytidine Deaminase / Q000378:metabolism*; D006801:Humans; D008969:Molecular Sequence Data; D017393:RNA Editing; D013347:Subcellular Fractions / Q000378:metabolism; D014407:Tumor Cells, Cultured; D000098830:AICDA (Activation-Induced Cytidine Deaminase)
D001055:Apolipoproteins B; D000098830:AICDA (Activation-Induced Cytidine Deaminase); D000071479:APOBEC-1 Deaminase; C000605089:APOBEC1 protein, human; C000605091:Apobec1 protein, rat; D003564:Cytidine Deaminase