Tissue-Specific Differences in the Role of RNA 3′ of the Apolipoprotein B mRNA Mooring Sequence in Editosome Assembly

Journal of Biological Chemistry

Lehmann

Lin

et al. 2004

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Two novel mRNA transcripts have been identified that result from species-and tissue-specific, alternative polyadenylation and splicing of the pre-mRNA encoding the apolipoprotein B (apoB) editing catalytic subunit 1 (APOBEC-1) complementation factor (ACF) family of related proteins. The alternatively processed mRNAs encode 43-and 45-kDa proteins that are components of the previously identified p44 cluster of apoB RNA binding, editosomal proteins. Recombinant ACF45 displaced ACF64 and ACF43 in mooring sequence RNA binding but did not demonstrate strong binding to APOBEC-1. In contrast, ACF43 bound strongly to APOBEC-1 but demonstrated weak binding to mooring sequence RNA. Consequently ACF45/43 complemented APOBEC-1 in apoB mRNA editing with less efficiency than full-length ACF64. These data, together with the finding that all ACF variants were co-expressed in rat liver nuclei (the site of apoB mRNA editing), suggested that ACF variants might compete with one another for APOBEC-1 and apoB mRNA binding and thereby contribute to the regulation of apoB mRNA editing. In support for this hypothesis, the ratio of nuclear ACF65/64 to ACF45/43 decreased when hepatic editing was inhibited by fasting and increased when editing was re-stimulated by refeeding. These findings suggested a new model for the regulation of apoB mRNA editing in which the catalytic potential of editosomes is modulated at the level of their assembly by alterations in the relative abundance of multiple related RNA-binding auxiliary proteins and the expression level of APOBEC-1.Alternative mRNA polyadenylation, splicing, and mRNA editing are mechanisms of post-transcriptional RNA processing that introduce diversity in mammalian mRNAs, thereby regulating the number of structurally and functionally different proteins encoded by a single gene (reviewed in Refs. 1 and 2). Base modification RNA editing may involve the enzymatic deamination of single nucleotides within splice site consensus motifs (3) or in protein coding sequence, often with physiologically significant effects (4). Adenosine to inosine editing of mRNAs, such as that occurring on mRNAs encoding the glutamate-gated calcium channel and serotonin 2C receptor subunits, is mediated by a family of adenosine deaminases active on RNAs that function independently of cofactors to deaminate their cognate target within double-stranded RNA (reviewed in Ref. 5). Multiple cytidine deaminases active on RNA have been predicted through their homology with the zinc-dependent cytidine deaminase domain of the mRNA editing enzyme APO-BEC-1 1 (Ref. 6 and references therein); however, only APO-BEC-1 (7, 8) and CDD1 (9) have been shown to deaminate cytidines within mRNA. Unlike adenosine deaminases active on RNA, cytidine deaminases active on RNA minimally require RNA-binding auxiliary proteins for their activity on RNA.Cytidine to uridine deamination of nucleotide 6666 of apolipoprotein B mRNA occurs in the small intestine of all, and the livers of some, mammalian species (10 -12). The glutaminespecifying CAA codon...

Section: Acf45 and Acf43 Are Members Of The P44 Cluster Of Editosomalmentioning

confidence: 99%

Identification of Novel Alternative Splice Variants of APOBEC-1 Complementation Factor with Different Capacities to Support Apolipoprotein B mRNA Editing

Journal of Biological Chemistry

Lehmann

Lin

et al. 2004

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“…Cytidine deamination in itro and in i o required a multiprotein complex or ' editosome ' [22][23][24][25]. Editosome assembly in cells [22,26] or in itro [11,15,22,27] required a complex protein composition with an aggregate size of 27 S. Minimally, in itro editing activity required a homodimer of APOBEC-1 (catalytic subunit for cytidine-to-uridine editing of apolipoprotein B mRNA-1), the zinc-dependent catalytic subunit of cytidine deaminase [28,29] and a 65 kDa RNA binding protein, ACF\ASP (APOBEC-1 complementation factor\APOBEC-1 stimulatory protein) [30,31]. APOBEC-1 demonstrated a weak and non-specific RNA binding activity to AU-rich apoB sequences [32] and alone could not edit apoB mRNA.…”

Section: Introductionmentioning

confidence: 99%

“…In terms of the potential effect of pre-mRNA splicing on apoB mRNA editing, some of the auxiliary proteins implicated in apoB mRNA editing, including heterogeneous ribonucleoprotein (hnRNP) C, hnRNP D, APOBEC-1binding protein and KSRP (KH type splicing regulatory protein) have a role in pre-mRNA splicing and\or hnRNP structure [31,[36][37][38]. Tissue-specific heterogeneity in auxiliary protein expression and function [16,27], and the widespread expression of the auxiliary proteins, even in cells that lack apobec-1 or apoB mRNA [23,24,26,30,33], has suggested an additional role(s) for these auxiliary proteins.…”

Section: Introductionmentioning

confidence: 99%

Commitment of apolipoprotein B RNA to the splicing pathway regulates cytidine-to-uridine editing-site utilization

Smith

2001

Biochem. J.

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A tripartite motif located in the centre of the 7.5 kb exon 26 of apolipoprotein B (apoB) mRNA directs editosome assembly and site-specific cytidine-to-uridine editing at nucleotide 6666. apoB mRNA editing is a post-transcriptional event, occurring primarily at the time exon 26 is spliced or at a time after splicing, but before nuclear export. We show, through reporter RNA constructs, that RNA splice sites suppress editing of precursor RNAs when placed proximal or distal to the editing site. Processed RNAs were edited more efficiently than precursor RNAs. Mutation of both the splice donor and acceptor sites was necessary for RNAs to be edited efficiently. The results suggested that commitment of pre-mRNA to the splicing and/or nuclear-export pathways may play a role in regulating editing-site utilization. The HIV-1 Rev-Rev response element ('RRE') interaction was utilized to uncouple the commitment of precursor RNAs to the spliceosome assembly pathway and associated nuclear-export pathway. Under these conditions, unspliced reporter RNAs were edited efficiently. We propose that pre-mRNA passage through the temporal or spatial restriction point where they become committed to spliceosome assembly contributes regulatory information for subsequent editosome activity.

“…Editosome assembly in cells [22,26] or in itro [11,15,22,27] required a complex protein composition with an aggregate size of 27 S. Minimally, in itro editing activity required a homodimer of APOBEC-1 (catalytic subunit for cytidine-to-uridine editing of apolipoprotein B mRNA-1), the zinc-dependent catalytic subunit of cytidine deaminase [28,29] and a 65 kDa RNA binding protein, ACF\ASP (APOBEC-1 complementation factor\APOBEC-1 stimulatory protein) [30,31]. APOBEC-1 demonstrated a weak and non-specific RNA binding activity to AU-rich apoB sequences [32] and alone could not edit apoB mRNA.…”

Section: Introductionmentioning

confidence: 99%

“…In terms of the potential effect of premRNA splicing on apoB mRNA editing, some of the auxiliary proteins implicated in apoB mRNA editing, including heterogeneous ribonucleoprotein (hnRNP) C, hnRNP D, APOBEC-1-binding protein and KSRP (KH type splicing regulatory protein) have a role in pre-mRNA splicing and\or hnRNP structure [31,[36][37][38]. Tissue-specific heterogeneity in auxiliary protein expression and function [16,27], and the widespread expression of the auxiliary proteins, even in cells that lack apobec-1 or apoB mRNA [23,24,26,30,33], has suggested an additional role(s) for these auxiliary proteins.…”

Section: Introductionmentioning

confidence: 99%

Commitment of apolipoprotein B RNA to the splicing pathway regulates cytidine-to-uridine editing-site utilization

Smith

2001

Biochemical Journal

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