Apolipoprotein B-48 Is the Product of a Messenger RNA with an Organ-Specific In-Frame Stop Codon

Abstract

The primary structure of human apolipoprotein (apo) B-48 has been deduced and shown by a combination of DNA excess hybridization, sequencing of tryptic peptides, cloned complementary DNAs, and intestinal messenger RNAs (mRNAs) to be the product of an intestinal mRNA with an in-frame UAA stop codon resulting from a C to U change in the codon CAA encoding Gln2153 in apoB-100 mRNA. The carboxyl-terminal Ile2152 of apoB-48 purified from chylous ascites fluid has apparently been cleaved from the initial translation product, leaving Met2151 as the new carboxyl-terminus. These data indicate that approximately 85% of the intestinal mRNAs terminate within approximately 0.1 to 1.0 kilobase downstream from the stop codon. The other approximately 15% have lengths similar to hepatic apoB-100 mRNA even though they have the same in-frame stop codon. The organ-specific introduction of a stop codon to a mRNA appears unprecedented and might have implications for cryptic polyadenylation signal recognition and RNA processing.

ACTH reduces the rise in ApoB-48 levels after fat intake

“…This postprandial lipidemia is mainly accounted for by intestinal production of chylomicrons, which contain ApoB-48. Intestinal ApoB-48 is homologous to the N-terminal 2152 amino acids of hepatic ApoB-100 (21), due to their common origin in a single, human ApoB gene, APOB (22), and is formed through editing of full length ApoB mRNA occurring predominantly in the intestine in humans (23,24). Both ApoBs are essential structure proteins for assembly and secretion of chylomicrons and VLDL, respectively.…”

Identification of domains in apobec-1 complementation factor required for RNA binding and apolipoprotein-B mRNA editing

“…RNA-protein interactions play a critical role in regulating eukaryotic gene expression at the posttranscriptional level+ RNA-binding proteins mediate the processes of mRNA splicing, polyadenylation, transport to the cytoplasm, translation, and turnover+ In mRNA editing, the sequence of the mRNA is changed after transcription by the insertion, deletion, or modification of nucleotides Gott & Emeson, 2000)+ By altering the coding capacity of the transcript, editing generates alternative forms of the protein that have different biological functions+ In mammals, the characterized examples of mRNA editing involve single nucleotide changes that are generated by site-specific deamination reactions that convert A r I or C r U (Smith & Sowden, 1996)+ Several mammalian mRNAs, including the glutamate and serotonin 5-HT 2c receptors, and hepatitis delta virus, undergo A r I editing events (Bass, 1997;Gott & Emeson, 2000)+ These conversions are catalyzed by a family of adenosine deaminases that act on double-stranded RNA (ADAR)+ The ADAR enzymes function as a single polypeptide that can bind to the substrate RNA and deaminate the target adenosines in the absence of other factors (Smith & Sowden, 1996;Bass, 1997;Gott & Emeson, 2000)+ To date, there is only one example of C r U editing for which the editing machinery has been defined+ The enzyme complex that edits mammalian apolipoprotein-B (apo-B) mRNA deaminates a genomically encoded cytidine at nt 6666 (Chan et al+, 1997)+ This modification converts the codon CAA encoding glutamine in the fulllength protein (apo-B100) to a premature stop codon, UAA (Chen et al+, 1987;Powell et al+, 1987)+ The edited mRNA codes for a truncated apo-B protein (apo-B48)+ In several species, including humans, editing of apo-B mRNA is restricted to the intestine whereas the liver synthesizes only the full-length protein (Greeve et al+, 1993)+ The two forms of apo-B perform distinct roles in the synthesis, metabolism, and transport of plasma lipoproteins and in susceptibility to atherosclerosis (Chan, 1992;Innerarity et al+, 1996)+ The editing of apo-B mRNA requires sequences on apo-B mRNA that are recognized by the editing enzyme complex+ An 11-nt mooring sequence (nt 6671-6681) located downstream of C 6666 is critical for editing, and mutations in this sequence abolish or down regulate editing in vitro (Shah et al+, 1991;Backus & Smith, 1991, 1992Driscoll et al+, 1993)+ The native editing holoenzyme has not been purified, but a minimal complex that edits apo-B mRNA in vitro has been defined+ The catalytic subunit of the enzyme, apobec-1, is a cytidine deaminase that alone is not competent to edit Navaratnam et al+, 1993)+ We recentl...…”

Absence of APOBEC-1 mediated mRNA editing in human carcinomas

“…Editing from cytidine to uridine at nucleotide position 6666 in the apo B mRNA creates a premature translational stop codon and leads to the generation of the carboxyterminal truncated apo B-48 (Chen et al, 1987;Powell et al, 1987). In humans and many other mammalian species, the apo B mRNA is completely edited in the intestine but remains unedited in the liver (Greeve et al, 1993).…”