Cytidine Deaminase Activity Research Articles

Antiviral Function of APOBEC3G Can Be Dissociated from Cytidine Deaminase Activity

The antiretroviral activity of the cellular enzyme APOBEC3G has been attributed to the excessive deamination of cytidine (C) to uridine (U) in minus strand reverse transcripts, a process resulting in guanosine (G) to adenosine (A) hypermutation of plus strand DNAs. The HIV-1 Vif protein counteracts APOBEC3G by inducing proteasomal degradation and exclusion from virions through recruitment of a cullin5 ECS E3 ubiquitin ligase complex. APOBEC3G belongs to the APOBEC protein family, members of which possess consensus (H/C)-(A/V)-E-(X)24-30-P-C-(X)2-C cytidine deaminase motifs. Earlier analyses of APOBEC-1 have defined specific residues that are important for zinc coordination, proton transfer, and, therefore, catalysis within this motif. Because APOBEC3G contains two such motifs, we used site-directed mutagenesis of conserved residues to assess each region's contribution to anti-HIV-1 activity. Surprisingly, whereas either the N- or C-terminal domain could confer antiviral function in tissue culture-based infectivity assays, only an intact C-terminal motif was essential for DNA mutator activity. These findings reveal the nonequivalency of APOBEC3G's N- and C-terminal domains and imply that APOBEC3G-mediated DNA editing may not always be necessary for antiviral activity. Accordingly, we propose that APOBEC3G can achieve an anti-HIV-1 effect through an undescribed mechanism that is distinct from cytidine deamination.

Impact of cytidine deaminase activity on intrinsic resistance to cytarabine in carcinoma cells

Cytarabine (araC) is a highly active antimetabolite against hematological malignancy while the agent shows limited activity against carcinomas. In this study, we focused on cellular transport and catalysis of the nucleoside in order to elucidate the mechanism of intrinsic resistance to araC in carcinomas. Activities of two metabolizing enzymes for araC, deoxycytidine kinase (DCK) and cytidine deaminase (CDA), and cellular transport of the agent were examined in 9 carcinoma cell lines. These variables in carcinoma lines were compared with those in 14 araC-sensitive leukemia lines and one leukemia line with acquired resistance. The mean IC50 in 9 carcinoma lines was 3 x 10(3)-fold higher than that in 14 leukemia lines (4.6 x 10(3) vs. 1.3 microM, p<0.01). A cell line with acquired resistance (U937R), which was established from U937 monocytoid leukemia cells, showed more than 10(3)-fold higher IC50 than the parent cells (1.6 x 10(3) vs. 1.3 microM). The resistance in carcinomas was associated with higher CDA activity and lower influx when compared to araC sensitive leukemias. Especially, these two types of malignant cell lines were clearly distinguished by CDA activity. The acquired resistance in U937R cells was followed by increase in cytidine deaminase (CDA) activity, decrease in DCK activity and decrease in influx of the drug. In conclusion, carcinomas are intrinsically resistant to cytarabine through high CDA activity and low cellular transport, but not low DCK activity. This finding suggests that treatment of carcinoma with deoxycytidine analogues should conquer the high CDA activity.

Ring Finger Protein ZIN Interacts with Human Immunodeficiency Virus Type 1 Vif

Virion infectivity factor (Vif) protein of human immunodeficiency virus type 1 (HIV-1) is essential for the productive infection of primary human CD4 T lymphocytes and macrophages. Vif overcomes the HIV-inhibitory effects of cellular factor APOBEC3G, which has cytidine deaminase activity. We previously reported the isolation of a Vif-interacting ring finger protein, Triad 3, from a human leukocyte cDNA library, using the yeast two-hybrid system. The full-length cellular protein homologue of Triad 3 has been recently identified as the zinc finger protein inhibiting NF-kappaB (ZIN). Sequence analysis indicates that Triad 3 protein contains all four major ring-like motifs of ZIN. We report here that ZIN binds to purified Vif in vitro and that Triad 3/ZIN interacts with HIV-1 Vif in transfected human 293T cells, as demonstrated by coimmunoprecipitation. To test the biological relevance of this interaction, we produced infectious HIV-1 NL4.3 in the presence or absence of cotransfected ZIN. HIV-1 NL4.3 virus stocks produced in the presence of exogenously expressed ZIN were twofold less infectious in a single-cycle infectivity assay than virus produced in the absence of exogenous ZIN. It was further shown that cells infected with HIV NL4.3 virus stocks produced in the presence of exogenously expressed ZIN were impaired in viral DNA synthesis by twofold. The impairment in viral reverse transcription and the reduction in single-cycle viral infectivity were both shown to be dependent on the presence of Vif in the virus producer cells. The possible mechanisms by which ZIN interferes with the early events of HIV-1 replication are discussed.

Human Apolipoprotein B mRNA-editing Enzyme-catalytic Polypeptide-like 3G (APOBEC3G) Is Incorporated into HIV-1 Virions through Interactions with Viral and Nonviral RNAs

Apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like 3G (APOBEC3G) is a host cytidine deaminase that is packaged into virions and confers resistance to retroviral infection. APOBEC3G deaminates deoxycytidines in minus strand DNA to deoxyuridines, resulting in G to A hypermutation and viral inactivation. Human immunodeficiency virus type 1 (HIV-1) virion infectivity factor counteracts the antiviral activity of APOBEC3G by inducing its proteosomal degradation and preventing virion incorporation. To elucidate the mechanism of viral suppression by APOBEC3G, we developed a sensitive cytidine deamination assay and analyzed APOBEC3G virion incorporation in a series of HIV-1 deletion mutants. Virus-like particles derived from constructs in which pol, env, and most of gag were deleted still contained high levels of cytidine deaminase activity; in addition, coimmunoprecipitation of APOBEC3G and HIV-1 Gag in the presence and absence of RNase A indicated that the two proteins do not interact directly but form an RNase-sensitive complex. Viral particles lacking HIV-1 genomic RNA which were generated from the gag-pol expression constructs pC-Help and pSYNGP packaged APOBEC3G at 30-40% of the wild-type level, indicating that interactions with viral RNA are not necessary for incorporation. In addition, viral particles produced from an nucleocapsid zinc finger mutant contained approximately 1% of the viral genomic RNA but approximately 30% of the cytidine deaminase activity. The reduction in APOBEC3G incorporation was equivalent to the reduction in the total RNA present in the nucleocapsid mutant virions. These results indicate that interactions with viral proteins or viral genomic RNA are not essential for APOBEC3G incorporation and suggest that APOBEC3G interactions with viral and nonviral RNAs that are packaged into viral particles are sufficient for APOBEC3G virion incorporation.

Non-redundancy of cytidine deaminases in class switch recombination.

Class switch recombination (CSR), somatic hypermutation, and gene conversion are immunoglobulin diversification mechanisms that are strictly dependent on the activity of the activation-induced cytidine deaminase (AID). The precise role and substrate(s) of AID in these processes remain to be well defined. The closest homologue of AID is APOBEC-1, a bona fide mRNA-editing enzyme, which shares with AID the ability to deaminate cytidines within single-stranded DNA in vitro and in prokaryotic cells. To determine whether APOBEC-1 can therefore substitute for AID in activated B cells, we expressed human AID, a catalytic mutant thereof, and rat APOBEC-1 in AID-deficient murine B cells. Whereas AID rescued CSR, neither the inactive mutant nor APOBEC-1 could complement AID deficiency. This indicates that cytidine deaminase activity is necessary but not sufficient to initiate CSR, and suggests that AID is specifically targeted to its cognate substrate, the immunoglobulin genes or a distinct mRNA, by an as-yet-unknown mechanism.

Production of infectious human immunodeficiency virus type 1 does not require depletion of APOBEC3G from virus-producing cells

BackgroundThe human immunodeficiency virus Vif protein overcomes the inhibitory activity of the APOBEC3G cytidine deaminase by prohibiting its packaging into virions. Inhibition of APOBEC3G encapsidation is paralleled by a reduction of its intracellular level presumably caused by the Vif-induced proteasome-dependent degradation of APOBEC3G.ResultsIn this report we employed confocal microscopy to study the effects of Vif on the expression of APOBEC3G on a single cell level. HeLa cells dually transfected with Vif and APOBEC3G expression vectors revealed efficient co-expression of the two proteins. Under optimal staining conditions approximately 80% of the transfected cells scored double-positive for Vif and APOBEC3G. However, the proportion of double-positive cells observed in identical cultures varied dependent on the fixation protocol and on the choice of antibodies used ranging from as low as 40% to as high as 80% of transfected cells. Importantly, single-positive cells expressing either Vif or APOBEC3G were observed both with wild type Vif and a biologically inactive Vif variant. Thus, the lack of APOBEC3G in some Vif-expressing cells cannot be attributed to Vif-induced degradation of APOBEC3G. These findings are consistent with our results from immunoblot analyses that revealed only moderate effects of Vif on the APOBEC3G steady state levels. Of note, viruses produced under such conditions were fully infectious demonstrating that the Vif protein used in our analyses was both functional and expressed at saturating levels.ConclusionsOur results suggest that Vif and APOBEC3G can be efficiently co-expressed. Thus, depletion of APOBEC3G from Vif expressing cells as suggested previously is not a universal property of Vif and thus is not imperative for the production of infectious virions.

Forced Expression of Cytidine Deaminase Confers Sensitivity to Capecitabine

Objective: Cytidine deaminase (CDD) is involved in the metabolism of new pyrimidine analogues, capecitabine (N⁴-pentyloxycarbonyl-5′-deoxy-5-fluorocytidine) and gemcitabine (2′,2′-difluorodeoxycytidine). The purpose of the present study was to directly examine the role of CDD in tumor cells themselves in mediating the sensitivity to capecitabine compared with gemcitabine. Methods: The human bladder cancer cell line T24 was transfected with human CDD2 cDNA by the lipofectin method. Results: Transfection of CDD2 cDNA did not change the levels of thymidine phosphorylase, dihydropyrimidine dehydrogenase and thymidylate synthase (TS) but increased the CDD activity significantly (p < 0.01). Forced expression of CDD made T24 sensitive to 5′-deoxy-5-fluorocytidine (5′DFCR) in vitro and capecitabine in vivo, but resistant to gemcitabine both in vitro and in vivo. Tetrahydrouridine, a specific CDD inhibitor, abrogated the changes in the in vitro sensitivity to 5′DFCR and gemcitabine by transfection of CDD2 cDNA. Transfection of CDD2 cDNA resulted in a significant increase in cellular 5-fluorouracil level (p < 0.01) and inhibition of TS activity (p < 0.01) after treatment with 5′DFCR in vitro. Conclusions: The present study clearly showed direct evidence for the contribution of CDD in tumor cells themselves to the sensitivities to capecitabine and gemcitabine.

C-Terminal Deletion of AID Uncouples Class Switch Recombination from Somatic Hypermutation and Gene Conversion

Class-switch recombination (CSR), somatic hypermutation (SHM), and antibody gene conversion are distinct DNA modification reactions, but all are initiated by activation-induced cytidine deaminase (AID), an enzyme that deaminates cytidine residues in single-stranded DNA. Here we describe a mutant form of AID that catalyzes SHM and gene conversion but not CSR. When expressed in E. coli, AIDΔ189-198 is more active in catalyzing cytidine deamination than wild-type AID. AIDΔ189-198 also promotes high levels of gene conversion and SHM when expressed in eukaryotic cells, but fails to induce CSR. These results underscore an essential role for the C-terminal domain of AID in CSR that is independent of its cytidine deaminase activity and that is not required for either gene conversion or SHM.

Cutting edge: the G-U mismatch glycosylase methyl-CpG binding domain 4 is dispensable for somatic hypermutation and class switch recombination.

Affinity maturation of the humoral response is accomplished by somatic hypermutation and class switch recombination (CSR) of Ig genes. Activation-induced cytidine deaminase likely initiates these processes by deamination of cytidines in the V and switch regions of Ig genes. This activity is expected to produce G-U mismatches that can be substrates for MutS homolog 2/MutS homolog 6 heterodimers and for uracil DNA glycosylase. However, G-T and G-U mismatches are also substrates of the methyl-CpG binding domain 4 (Mbd4) glycosylase. To determine whether Mbd4 functions downstream of activation-induced cytidine deaminase activity, we examined somatic hypermutation and CSR in Mbd4(-/-) mice. In this study, we report that CSR, as analyzed by an in vitro switch assay and by in vivo immunizations, is unaffected in Mbd4(-/-) mice. In addition, the hypermutated JH2 to JH4 region in Peyer's patch B cells showed no effects as a result of Mbd4 deficiency. These data indicate that the Mbd4 glycosylase does not significantly contribute to mechanisms of Ab diversification.

A novel cytidine deaminase AIDs in the delivery of error-prone polymerases to immunoglobulin genes

Immunoglobulin genes undergo somatic hypermutation in activated B lymphocytes. The mutations are targeted to the variable region of the gene, apparently by co-transcriptional deposition of a mutation complex. The novel activation induced deaminase (AID) gene is required for this process and recent work indicates that it is the cytidine deaminase activity of the AID protein rather than an associated RNA editing function that is required [Curr. Biol. 12 (2002) 1748; Nature 419 (2002) 1; Nature 418 (2002) 99]. The authors review the implications of this work and argue that deamination of deoxy-cytidine to deoxy-uracil seems to initiate several pathways in which error-prone polymerases are postulated to create mutations.

Serum adenosine deaminase and cytidine deaminase activities in patients with systemic lupus erythematosus.

We evaluated serum total adenosine deaminase, its isoenzymes adenosine deaminase-1 and adenosine deaminase-2, and cytidine deaminase activities in 24 patients with active systemic lupus erythematosus, and in 26 healthy control subjects, and found the means +/- SD values to be 21.38 +/- 5.96 IU/l, 3.74 +/- 2.12 IU/l, 17.72 +/- 5.02 IU/l and 17.89 +/- 4.62 IU/l, respectively in the patients, and 14.97+/- 4.71 IU/l, 4.01 +/- 1.35 IU/l, 10.91 +/- 3.91 IU/l and 7.39 +/- 3.97 IU/l, respectively in the control subjects. When compared to the healthy controls, serum total adenosine deaminase, adenosine deaminase-2 and cytidine deaminase levels were significantly higher (p<0.001) in systemic lupus erythematosus patients, but the decrease of adenosine deaminase-1 level was not statistically significant (p>0.05). The increased adenosine deaminase-2 may be of macrophage origin. It closely correlated with clinical signs of active systemic lupus erythematosus. The membranes of polymorphonuclear neutrophils may be damaged, and cytidine deaminase may be released into serum. In conclusion, serum total adenosine deaminase, adenosine deaminase-2 and cytidine deaminase activities may serve as useful indicators for evaluating disease activity in patients with active systemic lupus erythematosus.

ARCD-1, an apobec-1-related cytidine deaminase, exerts a dominant negative effect on C to U RNA editing.

Mammalian apolipoprotein B (apoB) C to U RNA editing is catalyzed by a multicomponent holoenzyme containing a single catalytic subunit, apobec-1. We have characterized an apobec-1 homologue, ARCD-1, located on chromosome 6p21.1, and determined its role in apoB mRNA editing. ARCD-1 mRNA is ubiquitously expressed; phylogenetic analysis reveals it to be a distant member of the RNA editing family. Recombinant ARCD-1 demonstrates cytidine deaminase and apoB RNA binding activity but does not catalyze C to U RNA editing, either in vitro or in vivo. Although not competent itself to mediate deamination of apoB mRNA, ARCD-1 inhibits apobec-1-mediated C to U RNA editing. ARCD-1 interacts and heterodimerizes with both apobec-1 and apobec-1 complementation factor (ACF) and localizes to both the nucleus and cytoplasm of transfected cells. Together, the data suggest that ARCD-1 is a novel cytidine deaminase that interacts with apobec-1 and ACF to inhibit apoB mRNA editing, possibly through interaction with other protein components of the apoB RNA editing holoenzyme.

Clinical pharmacokinetics of capecitabine.

Capecitabine is a novel oral fluoropyrimidine carbamate that is preferentially converted to the cytotoxic moiety fluorouracil (5-fluorouracil; 5-FU) in target tumour tissue through a series of 3 metabolic steps. After oral administration of 1250 mg/m2, capecitabine is rapidly and extensively absorbed from the gastrointestinal tract [with a time to reach peak concentration (tmax) of 2 hours and peak plasma drug concentration (Cmax) of 3 to 4 mg/L] and has a relatively short elimination half-life (t(1/2)) [0.55 to 0.89 h]. Recovery of drug-related material in urine and faeces is nearly 100%. Plasma concentrations of the cytotoxic moiety fluorouracil are very low [with a Cmax of 0.22 to 0.31 mg/L and area under the concentration-time curve (AUC) of 0.461 to 0.698 mg x h/L]. The apparent t(1/2) of fluorouracil after capecitabine administration is similar to that of the parent compound. Comparison of fluorouracil concentrations in primary colorectal tumour and adjacent healthy tissues after capecitabine administration demonstrates that capecitabine is preferentially activated to fluorouracil in colorectal tumour, with the average concentration of fluorouracil being 3.2-fold higher than in adjacent healthy tissue (p = 0.002). This tissue concentration differential does not hold for liver metastasis, although concentrations of fluorouracil in liver metastases are sufficient for antitumour activity to occur. The tumour-preferential activation of capecitabine to fluorouracil is explained by tissue differences in the activity of cytidine deaminase and thymidine phosphorylase, key enzymes in the conversion process. As with other cytotoxic drugs, the interpatient variability of the pharmacokinetic parameters of capecitabine and its metabolites, 5'-deoxy-5-fluorocytidine and fluorouracil, is high (27 to 89%) and is likely to be primarily due to variability in the activity of the enzymes involved in capecitabine metabolism. Capecitabine and the fluorouracil precursors 5'-deoxy-5-fluorocytidine and 5'-deoxy-5-fluorouridine do not accumulate significantly in plasma after repeated administration. Plasma concentrations of fluorouracil increase by 10 to 60% during long term administration, but this time-dependency is assumed to be not clinically relevant. A potential drug interaction of capecitabine with warfarin has been observed. There is no evidence of pharmacokinetic interactions between capecitabine and leucovorin, docetaxel or paclitaxel.

Optimizing antimetabolite-based chemotherapy for the treatment of childhood acute lymphoblastic leukaemia.

Optimizing antimetabolite-based chemotherapy for the treatment of childhood acute lymphoblastic leukaemia.

Coexpression of rat glutathione S-transferase A3 and human cytidine deaminase by a bicistronic retroviral vector confers in vitro resistance to nitrogen mustards and cytosine arabinoside in murine fibroblasts.

The transfer of drug resistance genes into hematopoietic cells is an experimental approach to protect patients from drug-induced myelosuppression. Because anti-cancer drugs are often administered in combination to increase their clinical efficacy, vectors that express two drug resistance genes are being developed to broaden the spectrum of chemoprotection. We have constructed a bicistronic vector, MFG/GST-IRES-CD (MFG/GIC) coexpressing rat glutathione S-transferase (GST) A3 isoform (rGST Yc1) and human cytidine deaminase (CD). Murine NIH 3T3 fibroblast cells transduced with this vector were evaluated for their resistance to nitrogen mustards and cytosine nucleoside analogs. GIC-transduced polyclonal cell populations (GIC cells) demonstrated marked increases in selenium-independent glutathione peroxidase (peroxidase) and CD activities, as well as increased resistance to melphalan (2.3-fold), chlorambucil (3.4-fold), and cytosine arabinoside (Ara-C) (8.1-fold). After selection with Ara-C, the peroxidase and CD activities of GIC cells were augmented 2.6- and 2.9-fold, respectively, in comparison with unselected cells, and the resistance to melphalan, chlorambucil, and Ara-C was further increased to 3.7-, 5.9-, and 53-fold, respectively. Melphalan selection of GIC cells likewise augmented their peroxidase (2.3-fold) and CD (1.9-fold) activities. GIC cells proliferated in the simultaneous presence of melphalan and Ara-C at drug concentrations that completely inhibited the growth of untransduced cells. The growth rate of unselected GIC cells exposed to the drug combination averaged 18% that of drug-free cultures. The growth rate of GIC cells exposed to the drug combination increased to 30% of controls after Ara-C selection and to 50% after melphalan selection. Our results suggest that retroviral transfer of MFG/GIC may be useful for chemoprotection against the toxicities of nitrogen mustards and cytosine nucleoside analogs.

Molecular cloning of apobec-1 complementation factor, a novel RNA-binding protein involved in the editing of apolipoprotein B mRNA.

The C-to-U editing of apolipoprotein B (apo-B) mRNA is catalyzed by a multiprotein complex that recognizes an 11-nucleotide mooring sequence downstream of the editing site. The catalytic subunit of the editing enzyme, apobec-1, has cytidine deaminase activity but requires additional unidentified proteins to edit apo-B mRNA. We purified a 65-kDa protein that functionally complements apobec-1 and obtained peptide sequence information which was used in molecular cloning experiments. The apobec-1 complementation factor (ACF) cDNA encodes a novel 64.3-kDa protein that contains three nonidentical RNA recognition motifs. ACF and apobec-1 comprise the minimal protein requirements for apo-B mRNA editing in vitro. By UV cross-linking and immunoprecipitation, we show that ACF binds to apo-B mRNA in vitro and in vivo. Cross-linking of ACF is not competed by RNAs with mutations in the mooring sequence. Coimmunoprecipitation experiments identified an ACF-apobec-1 complex in transfected cells. Immunodepletion of ACF from rat liver extracts abolished editing activity. The immunoprecipitated complexes contained a functional holoenzyme. Our results support a model of the editing enzyme in which ACF binds to the mooring sequence in apo-B mRNA and docks apobec-1 to deaminate its target cytidine. The fact that ACF is widely expressed in human tissues that lack apobec-1 and apo-B mRNA suggests that ACF may be involved in other RNA editing or RNA processing events.

Cytidine deaminase - the methodological relevance of AraC deamination for ex vivo experiments using cultured cell lines, fresh leukemic blasts, and normal bone marrow cells.

The clinical effects of cytosine arabinoside (AraC) are highly dependent on schedule and dose. Many regimens administered to patients are derived from artificial model systems involving permanent leukemic cell lines. The differences in pharmacokinetics between the in vivo situation and such cell lines are largely neglected. However, cytidine deaminase activity in particular has a major impact on AraC pharmacokinetics by degrading AraC to its inactive metabolite AraU, and it has been shown to be of prognostic relevance in the treatment of acute myeloid leukemia. This study therefore investigated cytidine deaminase activities and AraC deamination in a variety of the most commonly used leukemic cell lines and fresh blasts and their impact on the results of an in vitro model system. It was found that cells from different cell lines (BLIN, CEM, HL60, K562, RAJI, REH, U937) vary greatly in cytidine deaminase activity (e.g., 1.89 nmol per min/mg in K562 versus 0.01 in BLIN cells) and degrade between 18.5 (BLIN) and 96.5% (REH) of AraC to AraU in the incubation medium. This degradation results in highly different AraC exposures for different cells (e.g., AUC of 960 ng per h/ml in REH versus 4048 ng per h/ml in BLIN cells) in spite of identical starting concentrations of the drug. Formation of AraCTP as the main cytotoxic metabolite of AraC is significantly influenced by the differences in cell type-dependent cytidine deaminase activity (e.g., 35.6 ng/10(7) cells in REH versus 180.2 ng/10(7) cells in BLIN cells). In contrast to permanent cell lines, fresh leukemic blasts and normal bone marrow mononuclear cells featured low AraC degradation in the model system.

New insights into the pyrimidine salvage pathway of Saccharomyces cerevisiae: requirement of six genes for cytidine metabolism.

Cytidine metabolism in the yeast Saccharomyces cerevisiae was analyzed by genetic and biochemical approaches. Disruption of a unique ORF (Genbank accession No. U 20865) bearing homology with eucaryotic or bacterial cytidine deaminases abolished cytidine deaminase activity and resulted in 5-fluorocytidine resistance. The gene encoding cytidine deaminase will be referred to as CDD1 (Genbank accession number AF080089). The ability to isolate mutants resistant to 5-fluorocytidine which mapped to five other loci demonstrated the existence of a complex cytidine metabolic network. Deciphering this network revealed several original features:(1) cytidine entry is mediated by the purine-cytosine transporter (Fcy2p),(2) cytidine is cleaved into cytosine by the uridine nucleosidase (Urh1p),(3) cytidine is phosphorylated into CMP by the uridine kinase (Urk1p),(4) a block in cytosine deaminase (Fcy1p), but not in cytidine deaminase (Cdd1p), constitutes a limiting step in cytidine utilisation as a UMP precursor.

Psoriasis Upregulated Phorbolin-1 Shares Structural but not Functional Similarity to the mRNA-Editing Protein Apobec-1

Earlier studies of psoriatic and normal primary keratinocytes treated with phorbol 12-myristate-1-acetate identified two low-molecular-weight proteins, termed phorbolin-1 (20 kDa; pI 6.6) and phorbolin-2 (17.6 kDa; pI 6.5). As a first step towards elucidating the role of these proteins in psoriasis, we report here the molecular cloning and chromosomal mapping of phorbolin-1 and a related cDNA that codes for a protein exhibiting a similar amino acid sequence. The phorbolins were mapped to position 22q13 immediately centromeric to the c-sis proto-oncogene. Transient expression of the phorbolin-1 cDNA in COS cells and by in vitro transcription/translation, yielded polypeptides that comigrated with phorbolins-1 and -2. Comparative sequence analysis revealed 22% overall identity and a similarity of 44% of the phorbolins to apobec-1, the catalytic subunit of the mammalian apolipoprotein B mRNA editing enzyme; however, recombinant-expressed phorbolin-1 exhibited no cytidine deaminase activity, using either a monomeric nucleoside or apolipoprotein B cRNA as substrate, and failed to bind an AU-rich RNA template. Whereas the precise function of the phorbolins remains to be elucidated, the current data suggest that it is unlikely to include a role in the post-transcriptional modification of RNA in a manner analogous to that described for apobec-1.

Molecular Modelling of the Biosynthesis of the Rna‐Editing Enzyme Apobec‐1, Responsible for Generating the Alternative forms of Apolipoprotein B

We discovered in 1987 that the shorter form of apolipoprotein B (B48) synthesized in the intestine is due to the action, previously unrecognized in mammalian cells, of an mRNA-editing process, and more recently we demonstrated that this was due to a specific enzyme (APOBEC-1) with cytidine deaminase activity. We show here, by sequence alignment, molecular modelling and mutagenesis, that APOBEC-1 is a cytidine deaminase, responsible for editing apoB mRNA, and that is related in crystal structure to the cytidine deaminase of Escherichia coli (ECCDA). The two enzymes are both homodimers with composite active sites formed with loops from each monomer. In the sequence of APOBEC-1, three gaps compared with ECCDA match the size and contour of the minimal RNA substrate. We propose a model in which the asymmetric binding of one active site to the substrate cytidine which is positioned by the downstream binding of the product uridine and that this helps to target the other active site for deamination.