Asn Substitution Research Articles

Structure and Function of Cholesteryl Ester Transfer Protein in the Tree Shrew

Cholesteryl ester transfer protein (CETP) plays an important role in reverse cholesterol transport (RCT). To study on the structure and function of CETP in the tree shrew, a kind of animal resistant to atherosclerosis, we completed the cloning of the full-length tree-shrew CETP cDNA sequence based on the reported partial sequence. The full-length cDNA of tree shrew CETP was 1,704bp and the deduced protein of the cDNA showed a sequence identity of 81, 80 and 74%, respectively, with the human, monkey and rabbit CETP. The level of CETP mRNA in the liver was much more abundant than that in the other tissues. A mutant protein with a substitution of Asn at position 110 by Gln was found to possess an impaired secretion property compared with the wild-type tree shrew CETP. The mutant proteins, respectively, with a substitution of Pro at position 344 by Ser and a substitution of Gln at position 452 by Arg displayed similar secretion ability, but a decreased cholesteryl ester transfer capability compared with the wild type (48 and 26% lower, respectively). These findings demonstrate that liver is the main tissue synthesizing CETP in the tree shrew. Asn at position 110 plays an important role in the secretion of tree shrew CETP. The residues at position 344 and 452 play essential roles in cholesteryl ester transferring process.

Highest prevalence of the Mycobacterium tuberculosis Beijing genotype isolates in patients newly diagnosed with tuberculosis in the Novosibirsk oblast, Russian Federation

In order to assess the genetic diversity of Mycobacterium tuberculosis in the Novosibirsk Region and determine profiles of resistance, 106 M. tuberculosis isolates were analysed. Fifty (47 %) isolates were identified using variable number tandem repeat typing as being in the Beijing family, of which eight (16 %) were type M2 isolates with the genetic profile 233325153533424 and eight (16 %) were type M11 isolates with the genetic profile 233325173533424, both of which are widespread in Russia. Mutations associated with resistance to isoniazid and rifampicin were identified. Of 48 isolates with resistance to isoniazid, 42 (87.5 %) contained a Ser(315)→Thr substitution in the katG gene and one contained a T→A substitution at position -34 of the promoter region of the ahpC gene. Of 31 isolates with resistance to rifampicin, 19 (61 %) each contained a mutation (TCG→TTG) at codon 531 of the rpoB gene. Two isolates each contained a mutation (GAC→GTC) at codon 516 of the rpoB gene and two others each contained a substitution at codon 526 of the rpoB gene, leading to a His→Asn substitution in one case and a His→Asp substitution in another case. One isolate contained a mutation (CTG→CCG) at codon 533 of the rpoB gene. An association between the Beijing genotype and multidrug resistance was demonstrated (R = 0.2, P = 0.032). However, it was interesting to note that a significant proportion (46 %) of isolates were sensitive to all drugs tested.

Structure–function analysis of amino acid 74 of human RAMP1 and RAMP3 and its role in peptide interactions with adrenomedullin and calcitonin gene-related peptide receptors

The receptors for calcitonin gene-related peptide (CGRP) and adrenomedullin (AM) are complexes of the calcitonin receptor-like receptor (CLR) and receptor activity-modifying proteins (RAMP). The CGRP receptor is a CLR/RAMP1 pairing whereas CLR/RAMP2 and CLR/RAMP3 constitute two subtypes of AM receptor: AM 1 and AM 2, respectively. Previous studies identified Glu74 in RAMP3 to be important for AM binding and potency. To further understand the importance of this residue and its equivalent in RAMP1 (Trp74) we substituted the native amino acids with several others. In RAMP3, these were Trp, Phe, Tyr, Ala, Ser, Thr, Arg and Asn; in RAMP1, Glu, Phe, Tyr, Ala and Asn substitutions were made. The mutant RAMPs were co-expressed with CLR in Cos7 cells; receptor function in response to AM, AM 2/intermedin and CGRP was measured in a cAMP assay and cell surface expression was determined by ELISA. Phe reduced AM potency in RAMP3 but had no effect in RAMP1. In contrast, Tyr had no effect in RAMP3 but enhanced AM potency in RAMP1. Most other substitutions had a small effect on AM potency in both receptors whereas there was little impact on CGRP or AM 2 potency. Overall, these data suggest that the geometry and charge of the residue at position 74 contribute to how AM interacts with the AM 2 and CGRP receptors and confirms the role of this position in dictating differential AM pharmacology at the AM 2 and CGRP receptors.

Follistatin-like 1 Suppresses Sensory Afferent Transmission by Activating Na+,K+-ATPase

Excitatory synaptic transmission is modulated by inhibitory neurotransmitters and neuromodulators. We found that the synaptic transmission of somatic sensory afferents can be rapidly regulated by a presynaptically secreted protein, follistatin-like 1 (FSTL1), which serves as a direct activator of Na(+),K(+)-ATPase (NKA). The FSTL1 protein is highly expressed in small-diameter neurons of the dorsal root ganglion (DRG). It is transported to axon terminals via small translucent vesicles and secreted in both spontaneous and depolarization-induced manners. Biochemical assays showed that FSTL1 binds to the α1 subunit of NKA and elevates NKA activity. Extracellular FSTL1 induced membrane hyperpolarization in cultured cells and inhibited afferent synaptic transmission in spinal cord slices by activating NKA. Genetic deletion of FSTL1 in small DRG neurons of mice resulted in enhanced afferent synaptic transmission and sensory hypersensitivity, which could be reduced by intrathecally applied FSTL1 protein. Thus, FSTL1-dependent activation of NKA regulates the threshold of somatic sensation.

Surface-enhanced Raman scattering studies on bombesin, its selected fragments and related peptides adsorbed at the silver colloidal surface

SERS studies presented in this work on BN 8–14 , [ d-Phe 6 ,β-Ala 11 ,Phe 13 ,Nle 14 ]BN 6–14 , [ d-Tyr 6 , β-Ala 11 ,Phe 13 ,Nle 14 ]BN 6–14 , BN and its modified analogues, as well as NMB, NMC, and PG-L show that these molecules at pH 8.3 bind to a colloidal silver surface mainly through Trp 8 and Met 14 residues. Trp 8 adsorbs at the surface almost perpendicularly. Met 14 appears on the surface mainly as a P C-G conformer. His 12, as is evident from the spectra, practically does not take part in the adsorption process. Substitution of l-leucine at the 13 position of amino acid sequence with l-phenylalanine does not change substantially the pattern of the adsorption mechanism; however, substitution of phenylalanine at the 12 position (instead of l-histidine) causes changes in the SERS spectra that show that Phe 12 takes parallel orientation to the surface upon adsorption of [ d-Phe 12 ]BN, while in the case of [Tyr 4 , d-Phe 12 ]BN this residue is perpendicular to the surface and influences the orientation of the bound Trp 8. On the other hand, substitution of Asn with Tyr in the 6 position in nonapeptide fragment causes changes in the adsorption mechanism. In this case, the discussed fragment binds to the silver colloidal surface by Tyr 6, Trp 8, and Met 14. The SERS spectrum of NMC is very similar to that of BN; although it differs by the binding orientation of the amide bond towards the surface. Appearance of Phe 13 in NMB and PG-L causes that this residue competes successfully with Trp 8 forcing it to take tilted orientation. As seen from the enhancement of the characteristic Phe vibrations this moiety in NMB and PG-L adsorbs on the silver surface in a tilted fashion. This arrangements cause that the 8–14 peptide chain in all these studied compounds takes almost a parallel orientation to the surface while the 1–5 fragment of the peptide chain is removed from the silver surface vicinity.

Generation of a humanized anti-glypican 3 antibody by CDR grafting and stability optimization

Glypican 3 (GPC3), a glycosylphosphatidylinositol-anchored heparan sulfate proteoglycan, is expressed in a majority of hepatocellular carcinoma tissues. The murine monoclonal antibody GC33 that specifically binds to the COOH-terminal part of GPC3 causes strong antibody-dependent cellular cytotoxicity against hepatocellular carcinoma cells and exhibits strong antitumor activity in the xenograft models. To apply GC33 for clinical use, we generated a humanized GC33 from complementarity-determining region grafting with the aid of both the hybrid variable region and two-step design methods. The humanized antibody bound to GPC3 specifically and induced antibody-dependent cellular cytotoxicity as effectively as a chimeric GC33 antibody. To improve stability of the humanized GC33, we further optimized humanized GC33 by replacing the amino acid residues that may affect the structure of the variable region of a heavy chain. Substitution of Glu6 with Gln in the heavy chain significantly improved the stability under high temperatures. GC33 also has the risk of deamidation of the -Asn-Gly- sequence in the complementarity-determining region 1 of the light chain. As substitution of Asn diminished the antigen binding, we changed the neighboring Gly to Arg to avoid deamidation. The resulting humanized anti-GPC3 antibody was as efficacious as chimeric GC33 against the HepG2 xenograft and is now being evaluated in clinical trials.

Hemoglobin Yamagata: Hemoglobin Variant Detected by HbA1c Test

Hemoglobin (Hb) Yamagata is a rare Hb variant, which has been reported only twice-one case each in Japan and Korea. This variant arises from a Lys --> Asn substitution due to a mutation of AAA to AAC or AAT at codon 133 of the beta-globin gene. This study reports the third case of a patient detected with Hb Yamagata [HBB: c.399A>T; p.Lys133Asn] and discusses the effect of this variant on HbA1c measurement. This variant was detected in a 70-yr-old Korean man with diabetes mellitus during a routine follow-up. The HbA1c concentration determined using Variant ll Turbo (Bio-Rad, USA) was abnormally high at 47.9%. It was impossible to measure the HbA1c level accurately using Variant ll Thalassemia Mode (Bio-Rad, USA). However, the HbA1c levels analyzed by HLC-723 G7 (Tosoh, Japan), Cobas Integra (Roche, Switzerland) and NycoCard (Axis-Shield, Norway) were 5.0%, 8.0%, and 7.9%, respectively. This study shows that Hb Yamagata interferes with the accurate measurement of HbA1c levels in a diabetic patient. Taking these findings into consideration, we think that an immunoassay or affinity chromatography can be used as an alternate method for measuring the HbA1c level in a patient with this variant. In conclusion, a patient can be inferred to have an Hb variant if the HbA1c concentration is abnormally high or low or if there is a discrepancy between the results obtained using different methods, and if the clinical status of the patient suggests the presence of abnormal Hb. Subsequently, the HbA1c values can be determined by methods based on different principles.

On-resin cyclization and antimicrobial activity of Laterocidin and its analogues

Total synthesis of naturally occurring Laterocidin and its alanine-scanning analogues with the side chain carboxyl group of Aspartate linked to Rink resin, and the α-carboxyl group of Aspartate protected by Dmab group as a temporary blocking group for on-resin head-to-tail cyclization of the linear precursor is reported. Total synthesis of cyclodecapeptide antibiotics from Bacillus laterosporus , laterocidin and its analogues, was accomplished for the first time by solid-phase peptide synthesis followed by traceless on-resin cyclization of the linear precursors with protection of α-carboxyl group on the Asp residue by Dmab as a temporary blocking group for on-resin head-to-tail cyclization, in which the carboxyl group of side chain of Asp was linked to Rink resin. Single alanine substitution or Asn substitution (Asp 10 →Asn 10 ) demonstrated improvements in antibacterial activity. Of note, d -Phe 2 and Pro 4 play an important role in on-resin head-to-tail cyclization and exerting antibacterial activity of Laterocidin.

Discovery and Investigation of Misincorporation of Serine at Asparagine Positions in Recombinant Proteins Expressed in Chinese Hamster Ovary Cells

Misincorporation of amino acids in proteins expressed in Escherichia coli has been well documented but not in proteins expressed in mammalian cells under normal recombinant protein production conditions. Here we report for the first time that Ser can be incorporated at Asn positions in proteins expressed in Chinese hamster ovary cells. This misincorporation was discovered as a result of intact mass measurement, peptide mapping analysis, and tandem mass spectroscopy sequencing. Our analyses showed that the substitution was not related to specific protein molecules or DNA codons and was not site-specific. We believe that the incorporation of Ser at sites coded for Asn was due to mischarging of tRNA(Asn) rather than to codon misreading. The rationale for substitution of Asn by Ser and not by other amino acids is also discussed. Further investigation indicated that the substitution was due to the starvation for Asn in the cell culture medium and that the substitution could be limited by using the Asn-rich feed. These observations demonstrate that the quality of expressed proteins should be closely monitored when altering cell culture conditions.

Novel CYP17A1 mutation in a Japanese patient with combined 17 α-hydroxylase/17,20-lyase deficiency

Combined 17 α-hydroxylase/17,20-lyase deficiency is caused by a defect of P450c17 that catalyzes both 17 α-hydroxylase and 17,20-lyase reactions in adrenal glands and gonads. In the present study, we analyzed the CYP17A1 gene in a Japanese girl with 17 α-hydroxylase/17,20-lyase deficiency. The patient was referred to us for clitoromegaly at the age of 3 years. The karyotype was 46,XY. The patient was diagnosed as having 17 α-hydroxylase/17,20-lyase deficiency based on the clinical and laboratory findings. Analysis of the CYP17A1 gene revealed a compound heterozygous mutation. One mutation was a deletion of codon 53 or 54 encoding Phe (TTC) in exon 1 (ΔF54) on a maternal allele, which has been previously shown to partially abolish both 17 α-hydroxylase and 17,20-lyase activities. The other was a novel missense mutation resulting in a substitution of Asn (AAC) for His (CAC) at codon 373 in exon 6 (H373N) on a paternal allele. Functional expression study demonstrated that the H373N mutation almost completely eliminates enzymatic activity. Previous studies have demonstrated that replacement of histidine by leucine at position 373 causes complete loss of both 17 α-hydroxylase and 17,20-lyase activities with a defect in heme binding due to a global alteration of P450c17 structure, indicating the importance of H373 for P450c17 structure and function. Together, these results indicate that the patient is a compound heterozygote for the ΔF54 and H383N mutations and that these mutations inactivate both 17 α-hydroxylase and 17,20-lyase activities and give rise to clinically manifest combined 17 α-hydroxylase/17,20-lyase deficiency.

Mutagenesis of the fusion peptide-like domain of hepatitis C virus E1 glycoprotein: involvement in cell fusion and virus entry

BackgroundEnvelope (E) glycoprotein E2 of the hepatitis C virus (HCV) mediates binding of the virus to target cell receptors. Nevertheless, the precise role of E1 in viral entry remains elusive.MethodsTo understand the involvement of the fusion peptide-like domain positioned at residues 264 to 290 within envelope glycoprotein E1 in HCV infection, mutants with Ala and Asn substitutions for residues conserved between HCV and E proteins of flaviviruses or the fusion proteins of paramyxoviruses were constructed by site-directed mutagenesis and their effects on membrane fusion and viral infectivity were examined.ResultsNone of these mutations affected the synthesis or cell surface expression of envelope proteins, nor did they alter the formation of a non-covalent E1-E2 heterodimer or E2 binding to the large extracellular loop of CD81. The Cys residues located at positions 272 and 281 were unlikely involved in intra- or intermolecular disulfide bond formation. With the exception of the G267A mutant, which showed increased cell fusion, other mutants displayed reduced or marginally inhibited cell fusion capacities compared to the wild-type (WT) E1E2. The G267A mutant was also an exception in human immunodeficiency virus type 1 (HIV-1)/HCV E1E2 pseudotyping analyses, in that it showed higher one-cycle infectivity; all other mutants exhibited greatly or partially reduced viral entry versus the WT pseudotype. All but the G278A and D279N mutants showed a WT-like profile of E1E2 incorporation into HIV-1 particles. Since C272A, C281A, G282A, and G288A pseudotypes bound to Huh7 cells as effectively as did the WT pseudotype, the reduced infectivity of these pseudotypes was due to their ability to inhibit cell fusion.ConclusionOur results indicate that specific residues, but not the structure, of this fusion peptide-like domain are required for mediating cell fusion and viral entry.

Effect of Leucine and Lysine substitution on the antimicrobial activity and evaluation of the mechanism of the HPA3NT3 analog peptide

In this study, a HPA3NT3-analog (FKKLKKLFKKILKLK-NH2) peptide was designed. In this analog, two Trp residues (positions 12, 14) were replaced with Leu, and Arg and Asn (positions 3, 13) were replaced with Lys to investigate the role of amino acid substitution and increased cationicity on antimicrobial and hemolytic activities. In fungal and Gram-negative bacterial cells, HPA3NT3-analog activity was unchanged or slightly enhanced when compared to the HPA3NT3 peptide. In addition, a twofold decrease in activity against Gram-positive bacteria was observed. The HPA3NT3-analog also induced less hemolysis (4.2%) than the HPA3NT3 peptide (71%) at 200 microM. Circular dichroism (CD) spectra revealed that the HPA3NT3-analog peptide had an unordered structure in buffer and egg yolk L-2-phosphatidyl choline (EYPC), but adapted an alpha-helical conformation in 50% 2,2,2-trifluoroethanol (TFE) and negatively charged egg yolk L-2-phosphatidyl glycerol (EYPG), while the parent peptide showed an ordered structure in the EYPC. Additionally, the HPA3NT3-analog peptide induced the leakage of calcein from egg yolk L-2-phosphatidyl ethanolamine (EYPE)/EYPG (7:3 w/w) large unilamellar vesicles (LUVs); however, the activity was slightly weaker than that of the HPA3NT3 peptide. The molecular dynamics (MD) structures revealed that the amino acid substitutions induced a significant variation in peptide structure. These results suggest that the substitutions of Arg and Asn with Lys and two Trp with Leu resulted in small changes in HPA3NT3-analog activity and significant decreases in hemolytic activity.

Multiple allelic forms of acetohydroxyacid synthase are responsible for herbicide resistance in Setaria viridis

In weed species, resistance to herbicides inhibiting acetohydroxyacid synthase (AHAS) is often conferred by genetic mutations at one of six codons in the AHAS gene. These mutations provide plants with various levels of resistance to different chemical classes of AHAS inhibitors. Five green foxtail [Setaria viridis (L.) Beauv.] populations were reported in Ontario with potential resistance to the AHAS-inhibiting herbicide imazethapyr. The objectives of this study were to confirm resistance, establish the resistance spectrum for each of the five populations, and determine its genetic basis. Dose response curves were generated for whole plant growth and enzyme activity, and the AHAS gene was sequenced. Resistance was confirmed by determining the resistance factor to imazethapyr in the five resistant green foxtail populations for whole plant dose response experiments (21- to 182-fold) and enzyme assays (15- to 260-fold). All five imazethapyr-resistant populations showed cross-resistance to nicosulfuron and flucarbazone while only three populations had cross-resistance to pyrithiobac. Sequence analyses revealed single base-pair mutations in the resistant populations of green foxtail. These mutations were coded for Thr, Asn, or Ile substitution at Ser(653). In addition, a new mutation was found in one population that coded for an Asp substitution at Gly(654). There is an agreement between the spectra of resistance observed and the type of resistance known to be conferred by these substitutions. Moreover, it indicates that, under similar selection pressure (imazethapyr), a variety of mutations can be selected for different populations, making the resistance pattern difficult to predict from herbicide exposure history.

Establishment of a screening system for essential genes from the pathogenic yeastCandida glabrata: identification of a putativeTEM1homologue

To establish a system for screening and identification of essential genes from the pathogenic haploid yeast Candida glabrata by using temperature-sensitive (ts) mutants. Based on the general concepts that ts mutations are generated within essential genes in the genome by virtue of point mutation, we attempted to establish a system where essential genes were screened and identified from the C. glabrata genomic DNA library by the complementation of ts point mutations. By using this system, we successfully identified a putative TEM1 homologue as an essential gene by the complementation of a point mutation (-GAT-/-AAT- corresponding to Asp-143/Asn substitution) within its coding region in a ts mutant, T-3. We were able to establish a system for screening and identification of the essential genes, such as the TEM1 homologue, from the pathogenic yeast C. glabrata, as the gene that complements ts mutation. The identification of essential genes, by using the present system, may provide novel potential antifungal targets.

130 Trend of First-Line and Second-Line Eradication Therapy for Helicobacter pylori Infection in Korea

G A A b st ra ct s to yield these reduction products that are directly toxic to the bacterium. Although mutational inactivation of the rdxA gene is strongly associated with the development of resistance to MNZ, a number of MNZ-resistant strains have also been reported that show no evident change in the rdxA gene (Aliment. Pharmacol. Ther. 24:81-87, 2006), suggesting the potential existence of other mechanisms of resistance. We assumed that these resistant strains exhibit enhanced ability to defend themselves against superoxide radicals by overexpressing bacterial superoxide dismutase (SodB). The present study was designed to examine the expression of sodB and the function of the ferric uptake regulator (Fur) that acts as a transcriptional repressor protein exhibiting iron-dependent binding to the sodB promoter. Methods. H. pylori strain ATCC 700392 was used as the MNZ-susceptible strain and the MNZ-resistant strains used for the study (KS0033, KS0048 and KS0145) were isolated from patients at Keio University Hospital. The expression of sodB under exposure to MNZ was measured by quantitative RT-PCR, and the levels in the susceptible strain and resistant strains were compared. The complete fur of resistant strains was amplified by PCR, cloned into the pCR2.1-TOPO vector, and then sequenced. The Fur proteins were aligned using Genetyx and the protein structures were modeled using Swiss-Model and DeepView-SwisspdbViewer. Results.While sodB expression was scarcely induced by MNZ in ATCC 700392, overexpression of sodB by about 1.7, 2.6 and 2.1-fold was observed in KS0033, KS0048 and KS0145, respectively, under MNZ exposure. Thus, these MNZ-resistant strains showed enhanced sodB expression, as compared with that in ATCC 700392, under exposure to MNZ. The MNZ-resistant strains had a mutant-type of Fur protein, in which Asn 118 was replaced by His (N118H). From the results of the homologymodeling, Asn 118 was predicted to be located in the 5th alpha-helix region that projected outward. It is conceivable that the N118H of Fur affected the affinity of Fur for iron or the sodB promoter region, resulting in sodB overexpression. Conclusion. Fur mutation in H. pylori, resulting in substitution of Asn 118 by His, is associated with the development of resistance to MNZ.

A glycine-leucine-rich peptide structurally related to the plasticins from skin secretions of the frog Leptodactylus laticeps (Leptodactylidae)

A glycine-leucine-rich peptide was isolated from norepinephrine-stimulated skin secretions of the Sante Fe frog Leptodactylus laticeps (Leptodactylidae) whose primary structure (Gly-Leu-Val-Asn-Gly-Leu-Leu-Ser-Ser-Val-Leu-Gly-Gly-Gly-Gln-Gly-Gly-Gly-Gly-Leu-Leu-Gly-Gly-Ile-Leu) contains the (GXXXG) 3 motif found in the plasticins, previously identified only in phyllomedusid frogs (Hylidae). Circular dichroism studies showed that the secondary structure of the peptide, termed plasticin-L1, was markedly solvent-dependent displaying a random coil conformation in water, a β-sheet structure in methanol, and an α-helical conformation in 50% trifluoroethanol–water. A synthetic replicate of the peptide did not inhibit the growth of Escherichia coli or Staphylococcus aureus or lyse human erythrocytes at concentrations up to 500 μM. At relatively high concentrations (≥1 μM), the peptide produced a significant ( P < 0.05), although modest (139% of basal rate at 3 μM), increase in the rate of glucose-induced release of insulin from rat clonal BRIN-BD11 β cells without increasing the rate of release of lactate dehydrogenase. A peptide, termed ocellatin-L2 was also identified in the skin secretion that was identical to the previously described ocellatin-L1 except for the substitution Asn 23 → Asp. Ocellatin-L2 was devoid of antimicrobial and hemolytic activity but also showed significant activity in stimulating insulin release from BRIN-BD11 cells (181% of basal rate at 3 μM).

Detection of the Pandemic O25-ST131 Human Virulent Escherichia coli CTX-M-15-Producing Clone Harboring the qnrB2 and aac ( 6 ′)- Ib-cr Genes in a Dog

Detection of the Pandemic O25-ST131 Human Virulent <i>Escherichia coli</i> CTX-M-15-Producing Clone Harboring the <i>qnrB2</i> and <i>aac</i> ( <i>6</i> ′)- <i>Ib-cr</i> Genes in a Dog

Asp-to-Asn Substitution at the First Position of the DxD TOPRIM Motif of Recombinant Bacterial Topoisomerase I Is Extremely Lethal to E. coli

The TOPRIM domain found in many nucleotidyl transferases contains a DxD motif involved in magnesium ion coordination for catalysis. Medium- to high-copy-number plasmid clones of Yersinia pestis topoisomerase I (YpTOP) with Asp-to-Asn substitution at the first aspartate residue (D117N) of this motif could not be generated in Escherichia coli without second-site mutation even when expression was under the control of the tightly regulated BAD promoter and suppressed by 2% glucose in the medium. Arabinose induction of a single-copy YpTOP-D117N mutant gene integrated into the chromosome resulted in ∼ 10 5-fold of cell killing in 2.5 h. Attempt to induce expression of the corresponding E. coli topoisomerase I mutant (EcTOP-D111N) encoded on a high-copy-number plasmid resulted in either loss of viability or reversion of the clone to wild type. High-copy-number plasmid clones of YpTOP-D119N and EcTOP-D113N with the Asn substitution at the second Asp of the TOPRIM motif could be stably maintained, but overexpression also decreased cell viability significantly. The Asp-to-Asn substitutions at these TOPRIM residues can selectively decrease Mg 2+ binding affinity with minimal disruption of the active-site geometry, leading to trapping of the covalent complex with cleaved DNA and causing bacterial cell death. The extreme sensitivity of the first TOPRIM position suggested that this might be a useful site for binding of small molecules that could act as topoisomerase poisons.

A new HLA‐DR4 allele, DRB1*0474, with an unusual residue at position 77

We report here a new DR4 allele, DRB1*0474, identified in a volunteer hematopoietic stem cell donor of the Swiss National Registry. DRB1*0474 differs from DRB1*040701 by two nucleotide residues resulting in a single Thr --> Asn substitution at codon 77.

Efficient catalytic turnover of cytochrome P450cam is supported by a T252N mutation

A Thr (or Ser) residue on the I-helix is a highly conserved structural feature of cytochrome P450 enzymes. It is believed to be indispensable as a proton delivery shuttle in the oxygen activation process. Previous work showed that P450cin (CYP176A1), which contains an Asn instead of the conserved Thr, is fully functional in the catalytic oxidation of cineole [D.B. Hawkes, G.W. Adams, A.L. Burlingame, P.R. Ortiz de Montellano, J.J. De Voss, J. Biol. Chem. 277 (2002) 27725–27732]. To determine whether the substitution of Asn for Thr is specific or general, the conserved Thr252 in P450cam (CYP101) was mutated to generate the T252N, T252N/V253T, and T252A mutants. Steady-state kinetic analysis of the oxidation of camphor by these mutants indicated that the T252N and T252N/V253T mutants have comparable turnover numbers but higher Km values relative to the wild-type enzyme. Spectroscopic binding assays indicate that the higher Km values reflect a decrease in the camphor binding affinity. Non-productive H2O2 generation was negligible with the T252N and T252N/V253T mutants, but, as previously observed, was dominant in the T252A mutant. Our results, and a structure model based on the crystal structures of the ferrous dioxygen complexes of P450cam and its T252A mutant, suggest that Asn252 can stabilize the ferric hydroperoxy intermediate, preventing premature release of H2O2 and enabling addition of the second proton to the distal oxygen to generate the catalytic ferryl species.