Apolipoprotein B mRNA editing enzyme, catalytic polypeptide 1 also known as C->U-editing enzyme APOBEC-1 is a protein that in humans is encoded by the APOBEC1gene.[1]
APOBEC-1 (A1) has been linked with cholesterol control, cancer development and inhibition of viral replication.[2] Its function relies on introducing a stop codon into apolipoprotein B (ApoB) mRNA, which alters lipid metabolism in the gastrointestinal tract. The editing mechanism is highly specific. A1’s deamination of the cytosine base yields uracil, which creates a stop codon in the mRNA.
A1 has been linked with both positive and negative health effects. In rodents, it has wide tissue distribution where as in humans, it is only expressed in the small intestine.[3]
ApoB is essential in the assembly of very low density lipoproteins from lipids, in the liver and small intestine.[3] By editing ApoB, it forces only the smaller expression, ApoB48 to be expressed, which greatly inhibits lipoprotein production. However, A1 is currently found only at extremely low levels in the human liver and intestine, while it is highly expressed in rodents. In humans, A1 is found exclusively in gastrointestinal epithelial cells.[2]
Mechanism
A1 modifies the cytosine base at position 6666 on the ApoB mRNA strand through a deamination.[5] An A1 dimer first binds to ACF, which forms the binding complex that is then able to eliminate the amine group from cytosine.
File:Dimerization Portion of APOBEC1.pngThese residues (Leu-182 to Pro-191) are necessary for dimerization of APOBEC1, which is necessary to form the correct enzyme complex with ACF. During experimentation, substituted leucine and isoleucine residues significantly reduced the deamination of cytosine.
ACF binds to the mooring sequence, which puts A1 in position to edit the correct residue.[6] By converting cytosine to uracil, A1 changes the codon from CAA, which codes for glutamine during transcription, to UAA, a stop codon.[7] This stop codon yields the much shorter protein ApoB48 instead of ApoB100, as the mRNA is predisposed to transcript.[8] The editing amount, or expression, of A1 performs is correlated with the insulin concentration in the nucleus, the site of modification.[9][10] Tests involving A1 mutants with various deleted amino acid sequences have shown that editing activity is dependent on residues 14 to 35. Like all APOBEC proteins, A1 coordinates a zinc atom with two cysteine and one histidine residues that serve as a Lewis acid. Hydrolytic deamination of the cytosine amine group then occurs, catalyzed by the proton transfer from the nearby glutamic acid residue, and the enzymatic structure is conserved by a proline residue.[6]
File:C-to-U mechanism.pngPossible mechanism for C-to-U modification using Zinc complex with H-66, Cys-93, and Cys-96.
Structure
The structure of A1 relies on three dimensional folds induced by a zinc complex.[11] These folds allow the enzyme to access the RNA specifically. Deletion tests with mutant strands have shown that residues 181 to 210 are integral to mRNA editing, and there is most likely a beta-turn at proline residues 190 and 191.[6] Specifically, L182, I185, and L189 are integral to the complex’s function, most likely due to their importance to dimerization.[6] Substituting these residues has no predicted impact on secondary structure, so the significant decrease in editing activity is best explained by the alteration of the side-chains, which are integral to dimer structure.[6] Amino acid replacements at these sites deactivated deamination. The C-terminal of enzyme structure is more strongly expressed in the nucleus, hence the site of modification, while the 181 to 210 residues indicate that the enzyme is in the cytoplasm. These are regulatory factors.[12]
File:APOBEC1 Catalytic Site.pngAPOBEC1 catalytic active site, residue regionResidues 59-70, 82-95Linking glycine represents residues 71-81, which are not related to activation
Disease relevance
The low levels of A1 in humans are one reason why high lipid intake is damaging to health. ApoB48 is essential for the assembly and secretion of triglyceride-rich chylomicrons, which are necessary as a response to high-fat intake. ApoB100 are metabolized in the bloodstream to LDL cholesterol,[13] high levels of which are associated with artherosclerosis.[14] While A1 has a negligible impact on human lipid synthesis, at high concentrations it can be genotoxic. Its diffusion toward the nucleic membrane can lead it to mutate DNA sequences that are actively transcribed on the genome. In single growth assays, A1 has been found to impact HIV replications. Additionally, A1 has reduced Hepatitis B virus (HBV) DNA replication, although the mechanism is still not known. The antiviral properties of A1 extend to both DNA and RNA due to its deamination function, which can hinder DNA replication and consequently suppress further infection by HIV or HBV.[15] There has also been evidence that A1 also edits at NF1, related to tumors in nerve cells.[16]
↑ 3.03.1Teng BB, Ochsner S, Zhang Q, Soman KV, Lau PP, Chan L (1999). "Mutational analysis of apolipoprotein B mRNA editing enzyme (APOBEC1). structure-function relationships of RNA editing and dimerization". J. Lipid Res. 40 (4): 623–35. PMID10191286.
↑Jarmuz A, Chester A, Bayliss J, Gisbourne J, Dunham I, Scott J, Navaratnam N (2002). "An anthropoid-specific locus of orphan C to U RNA-editing enzymes on chromosome 22". Genomics. 79 (3): 285–96. doi:10.1006/geno.2002.6718. PMID11863358.
↑Yang Y, Ballatori N, Smith HC (2002). "Apolipoprotein B mRNA editing and the reduction in synthesis and secretion of the atherogenic risk factor, apolipoprotein B100 can be effectively targeted through TAT-mediated protein transduction". Mol. Pharmacol. 61 (2): 269–76. doi:10.1124/mol.61.2.269. PMID11809850.
↑von Wronski MA, Hirano KI, Cagen LM, Wilcox HG, Raghow R, Thorngate FE, Heimberg M, Davidson NO, Elam MB (1998). "Insulin increases expression of apobec-1, the catalytic subunit of the apolipoprotein B mRNA editing complex in rat hepatocytes". Metab. Clin. Exp. 47 (7): 869–73. doi:10.1016/s0026-0495(98)90128-7. PMID9667237.
↑Yang Y, Sowden MP, Yang Y, Smith HC (2001). "Intracellular trafficking determinants in APOBEC-1, the catalytic subunit for cytidine to uridine editing of apolipoprotein B mRNA". Exp. Cell Res. 267 (2): 153–64. doi:10.1006/excr.2001.5255. PMID11426934.
↑MacGinnitie AJ, Anant S, Davidson NO (1995). "Mutagenesis of apobec-1, the catalytic subunit of the mammalian apolipoprotein B mRNA editing enzyme, reveals distinct domains that mediate cytosine nucleoside deaminase, RNA binding, and RNA editing activity". J. Biol. Chem. 270 (24): 14768–75. doi:10.1074/jbc.270.24.14768. PMID7782343.
↑Nakamuta M, Chang BH, Zsigmond E, Kobayashi K, Lei H, Ishida BY, Oka K, Li E, Chan L (1996). "Complete phenotypic characterization of apobec-1 knockout mice with a wild-type genetic background and a human apolipoprotein B transgenic background, and restoration of apolipoprotein B mRNA editing by somatic gene transfer of Apobec-1". J. Biol. Chem. 271 (42): 25981–8. doi:10.1074/jbc.271.42.25981. PMID8824235.
↑Blanc V, Navaratnam N, Henderson JO, Anant S, Kennedy S, Jarmuz A, Scott J, Davidson NO (March 2001). "Identification of GRY-RBP as an apolipoprotein B RNA-binding protein that interacts with both apobec-1 and apobec-1 complementation factor to modulate C to U editing". J. Biol. Chem. 276 (13): 10272–83. doi:10.1074/jbc.M006435200. PMID11134005.
↑Lau PP, Chan L (Dec 2003). "Involvement of a chaperone regulator, Bcl2-associated athanogene-4, in apolipoprotein B mRNA editing". J. Biol. Chem. 278 (52): 52988–96. doi:10.1074/jbc.M310153200. PMID14559896.
↑Lau PP, Chang BH, Chan L (April 2001). "Two-hybrid cloning identifies an RNA-binding protein, GRY-RBP, as a component of apobec-1 editosome". Biochem. Biophys. Res. Commun. 282 (4): 977–83. doi:10.1006/bbrc.2001.4679. PMID11352648.
Wedekind JE, Dance GS, Sowden MP, Smith HC (2003). "Messenger RNA editing in mammals: new members of the APOBEC family seeking roles in the family business". Trends Genet. 19 (4): 207–16. doi:10.1016/S0168-9525(03)00054-4. PMID12683974.
Espinosa R, Funahashi T, Hadjiagapiou C, Le Beau MM, Davidson NO (1994). "Assignment of the gene encoding the human apolipoprotein B mRNA editing enzyme (APOBEC1) to chromosome 12p13.1". Genomics. 24 (2): 414–5. doi:10.1006/geno.1994.1645. PMID7698776.
Navaratnam N, Bhattacharya S, Fujino T, Patel D, Jarmuz AL, Scott J (1995). "Evolutionary origins of apoB mRNA editing: catalysis by a cytidine deaminase that has acquired a novel RNA-binding motif at its active site". Cell. 81 (2): 187–95. doi:10.1016/0092-8674(95)90328-3. PMID7736571.
Lau PP, Zhu HJ, Nakamuta M, Chan L (1997). "Cloning of an Apobec-1-binding protein that also interacts with apolipoprotein B mRNA and evidence for its involvement in RNA editing". J. Biol. Chem. 272 (3): 1452–5. doi:10.1074/jbc.272.3.1452. PMID8999813.
Oka K, Kobayashi K, Sullivan M, Martinez J, Teng BB, Ishimura-Oka K, Chan L (1997). "Tissue-specific inhibition of apolipoprotein B mRNA editing in the liver by adenovirus-mediated transfer of a dominant negative mutant APOBEC-1 leads to increased low density lipoprotein in mice". J. Biol. Chem. 272 (3): 1456–60. doi:10.1074/jbc.272.3.1456. PMID8999814.
Hirano K, Min J, Funahashi T, Baunoch DA, Davidson NO (1997). "Characterization of the human apobec-1 gene: expression in gastrointestinal tissues determined by alternative splicing with production of a novel truncated peptide". J. Lipid Res. 38 (5): 847–59. PMID9186903.
Fujino T, Navaratnam N, Scott J (1998). "Human apolipoprotein B RNA editing deaminase gene (APOBEC1)". Genomics. 47 (2): 266–75. doi:10.1006/geno.1997.5110. PMID9479499.
Lellek H, Kirsten R, Diehl I, Apostel F, Buck F, Greeve J (2000). "Purification and molecular cloning of a novel essential component of the apolipoprotein B mRNA editing enzyme-complex". J. Biol. Chem. 275 (26): 19848–56. doi:10.1074/jbc.M001786200. PMID10781591.
Blanc V, Navaratnam N, Henderson JO, Anant S, Kennedy S, Jarmuz A, Scott J, Davidson NO (2001). "Identification of GRY-RBP as an apolipoprotein B RNA-binding protein that interacts with both apobec-1 and apobec-1 complementation factor to modulate C to U editing". J. Biol. Chem. 276 (13): 10272–83. doi:10.1074/jbc.M006435200. PMID11134005.
Lau PP, Chang BH, Chan L (2001). "Two-hybrid cloning identifies an RNA-binding protein, GRY-RBP, as a component of apobec-1 editosome". Biochem. Biophys. Res. Commun. 282 (4): 977–83. doi:10.1006/bbrc.2001.4679. PMID11352648.
Anant S, Henderson JO, Mukhopadhyay D, Navaratnam N, Kennedy S, Min J, Davidson NO (2001). "Novel role for RNA-binding protein CUGBP2 in mammalian RNA editing. CUGBP2 modulates C to U editing of apolipoprotein B mRNA by interacting with apobec-1 and ACF, the apobec-1 complementation factor". J. Biol. Chem. 276 (50): 47338–51. doi:10.1074/jbc.M104911200. PMID11577082.
Lau PP, Villanueva H, Kobayashi K, Nakamuta M, Chang BH, Chan L (2001). "A DnaJ protein, apobec-1-binding protein-2, modulates apolipoprotein B mRNA editing". J. Biol. Chem. 276 (49): 46445–52. doi:10.1074/jbc.M109215200. PMID11584023.
Anant S, Mukhopadhyay D, Sankaranand V, Kennedy S, Henderson JO, Davidson NO (2001). "ARCD-1, an apobec-1-related cytidine deaminase, exerts a dominant negative effect on C to U RNA editing". Am. J. Physiol., Cell Physiol. 281 (6): C1904–16. PMID11698249.
Dance GS, Sowden MP, Cartegni L, Cooper E, Krainer AR, Smith HC (2002). "Two proteins essential for apolipoprotein B mRNA editing are expressed from a single gene through alternative splicing". J. Biol. Chem. 277 (15): 12703–9. doi:10.1074/jbc.M111337200. PMID11815617.
