An N-terminal domain of herpes simplex virus type Ig E is capable of forming stable complexes with gI

Using limited proteolytic analyses, we show that gE present in soluble herpes simplex virus type 1 gE-gI complexes is cleaved into a C-terminal (CgE) and an N-terminal (NgE) domain. The domain boundary is in the vicinity of residue 188 of mature gE. NgE, but not CgE, forms a stable complex with soluble gI.

Use of chimeric proteins to investigate the role of transporter associated with antigen processing (TAP) structural domains in peptide binding and translocation

The transporter associated with antigen processing (TAP) comprises two subunits, TAP1 and TAP2, each containing a hydrophobic membrane-spanning region (MSR) and a nucleotide binding domain (NBD). The TAP1/TAP2 complex is required for peptide translocation across the endoplasmic reticulum membrane. To understand the role of each structural unit of the TAP1/TAP2 complex, we generated two chimeras containing TAP1 MSR and TAP2 NBD (T1MT2C) or TAP2 MSR and TAP1 NBD (T2MT1C). We show that TAP1/T2MT1C, TAP2/T1MT2C, and T1MT2C/T2MT1C complexes bind peptide with an affinity comparable to wild-type complexes. By contrast, TAP1/T1MT2C and TAP2/T2MT1C complexes, although observed, are impaired for peptide binding. Thus, the MSRs of both TAP1 and TAP2 are required for binding peptide. However, neither NBD contains unique determinants required for peptide binding. The NBD-switched complexes, T1MT2C/T2MT1C, TAP1/T2MT1C, and TAP2/T1MT2C, all translocate peptides, but with progressively reduced efficiencies relative to the TAP1/TAP2 complex. These results indicate that both nucleotide binding sites are catalytically active and support an alternating catalytic sites model for the TAP transport cycle, similar to that proposed for P-glycoprotein. The enhanced translocation efficiency of TAP1/T2MT1C relative to TAP2/T1MT2C complexes correlates with enhanced binding of the TAP1 NBD-containing constructs to ATP-agarose beads. Preferential ATP interaction with TAP1, if occurring in vivo, might polarize the transport cycle such that ATP binding to TAP1 initiates the cycle. However, our observations that TAP complexes containing two identical TAP NBDs can mediate translocation indicate that distinct properties of the nucleotide binding site per se are not essential for the TAP catalytic cycle.

Walker A lysine mutations of TAP1 and TAP2 interfere with peptide translocation but not peptide binding

We generated mutants of the transporter associated with antigen-processing subunits TAP1 and TAP2 that were altered at the conserved lysine residue in the Walker A motifs of the nucleotide binding domains (NBD). In other ATP binding cassette transporters, mutations of the lysine have been shown to reduce or abrogate the ATP hydrolysis activity and in some cases impair nucleotide binding. Mutants TAP1(K544M) and TAP2(K509M) were expressed in insect cells, and the effects of the mutations on nucleotide binding, peptide binding, and peptide translocation were assessed. The mutant TAP1 subunit is significantly impaired for nucleotide binding relative to wild type TAP1. The identical mutation in TAP2 does not significantly impair nucleotide binding relative to wild type TAP2. Using fluorescence quenching assays to measure the binding of fluorescent peptides, we show that both mutants, in combination with their wild type partners, can bind peptides. Since the mutant TAP1 is significantly impaired for nucleotide binding, these results indicate that nucleotide binding to TAP1 is not a requirement for peptide binding to TAP complexes. Peptide translocation is undetectable for TAP1.TAP2(K509M) complexes, but low levels of translocation are detectable with TAP1(K544M).TAP2 complexes. These results suggest an impairment in nucleotide hydrolysis by TAP complexes containing either mutant TAP subunit and indicate that the presence of one intact TAP NBD is insufficient for efficient catalysis of peptide translocation. Taken together, these results also suggest the possibility of distinct functions for TAP1 and TAP2 NBD during a single translocation cycle.

Non-dietary risk factors for gastric dilatation-volvulus in large and giant breed dogs

OBJECTIVE

To identify non-dietary risk factors for gastric dilatation-volvulus (GDV) in large breed and giant breed dogs.

DESIGN

Prospective cohort study.

ANIMALS

1,637 dogs > or = 6 months old of the following breeds: Akita, Bloodhound, Collie, Great Dane, Irish Setter, Irish Wolfhound, Newfoundland, Rottweiler, Saint Bernard, Standard Poodle, and Weimaraner.

PROCEDURE

Owners of dogs that did not have a history of GDV were recruited at dog shows, and the dog's length and height and the depth and width of its thorax and abdomen were measured. Information concerning the dog's medical history, genetic background, personality, and diet was obtained from the owners, and owners were contacted by mail and telephone at approximately 1-year intervals to determine whether dogs had developed GDV or died. Incidence of GDV, calculated on the basis of dog-years at risk for dogs that were or were not exposed to potential risk factors, was used to calculate the relative risk of GDV.

RESULTS AND CLINICAL RELEVANCE

Cumulative incidence of GDV during the study was 6% for large breed and giant breed dogs. Factors significantly associated with an increased risk of GDV were increasing age, having a first-degree relative with a history of GDV, having a faster speed of eating, and having a raised feeding bowl. Approximately 20 and 52% of cases of GDV among the large breed and giant breed dogs, respectively, were attributed to having a raised feed bowl.

Pairing of the nucleotide binding domains of the transporter associated with antigen processing

The transporter associated with antigen processing (TAP) comprises two structurally related subunits, TAP1 and TAP2, that form stable complexes in endoplasmic reticulum (ER) membranes. TAP complexes function in the translocation of peptides from the cytosol into the ER lumen for presentation by major histocompatibility complex class I molecules. Each TAP subunit contains an N-terminal membrane-spanning region with multiple membrane-spanning segments, and a C-terminal, cytosolic nucleotide binding region. To study the nature of the interactions occurring on the cytosolic face of TAP1/TAP2 complexes, we investigated quaternary associations mediated by two C-terminal fragments of human TAP1 (T1c, residues 452-748 and T1ctr, residues 472-748) and two C-terminal fragments of human TAP2 (T2c, residues 399-686 and T2ctr, residues 433-686). Each of these constructs contains the core nucleotide binding region as well as a long or short N-terminal extension. We show stable complex formation between T1c and T2c but not between T1ctr and T2ctr. The mechanistic implications of these results are discussed. We also show that each of the constructs except T1ctr interacts with wild type TAP1 and TAP2, indicating possibilities for homodimerization of TAP1 and TAP2, or of oligomerization of TAP1/TAP2 heterodimers on membranes.

Incidence of and breed-related risk factors for gastric dilatation-volvulus in dogs

OBJECTIVE

To compare incidence of and breed-related risk factors for gastric dilatation-volvulus (GDV) among 11 dog breeds (Akita, Bloodhound, Collie, Great Dane, Irish Setter, Irish Wolfhound, Newfoundland, Rottweiler, Saint Bernard, Standard Poodle, and Weimaraner).

DESIGN

Prospective cohort study.

ANIMALS

1,914 dogs.

PROCEDURE

Owners of dogs that did not have a history of GDV were recruited at dog shows, and the dog's length and height and depth and width of the thorax and abdomen were measured. Information concerning the dogs' medical history, genetic background, personality, and diet was obtained from owners, and owners were contacted by mail and telephone at approximately 1-year intervals to determine whether dogs had developed GDV or died. Incidence of GDV based on the number of dog-years at risk was calculated for each breed, and breed-related risk factors were identified.

RESULTS AND CLINICAL RELEVANCE

Incidence of GDV for the 7 large (23 to 45 kg [50 to 99 lb]) and 4 giant (> 45 kg [> 99 lb]) breeds was 23 and 26 cases/1,000 dog-years at risk, respectively. Of the 105 dogs that developed GDV, 30 (28.6%) died. Incidence of GDV increased with increasing age. Cumulative incidence of GDV was 5.7% for all breeds. The only breed-specific characteristic significantly associated with a decreased incidence of GDV was an owner-perceived personality trait of happiness.

Characterization of the interaction between the herpes simplex virus type I Fc receptor and immunoglobulin G

Herpes simplex virus type I (HSV-1) virions and HSV-1-infected cells bind to human immunoglobulin G (hIgG) via its Fc region. A complex of two surface glycoproteins encoded by HSV-1, gE and gI, is responsible for Fc binding. We have co-expressed soluble truncated forms of gE and gI in Chinese hamster ovary cells. Soluble gE-gI complexes can be purified from transfected cell supernatants using a purification scheme that is based upon the Fc receptor function of gE-gI. Using gel filtration and analytical ultracentrifugation, we determined that soluble gE-gI is a heterodimer composed of one molecule of gE and one molecule of gI and that gE-gI heterodimers bind hIgG with a 1:1 stoichiometry. Biosensor-based studies of the binding of wild type or mutant IgG proteins to soluble gE-gI indicate that histidine 435 at the CH2-CH3 domain interface of IgG is a critical residue for IgG binding to gE-gI. We observe many similarities between the characteristics of IgG binding by gE-gI and by rheumatoid factors and bacterial Fc receptors such as Staphylococcus aureus protein A. These observations support a model for the origin of some rheumatoid factors, in which they represent anti-idiotypic antibodies directed against antibodies to bacterial and viral Fc receptors.

Fc receptors and their interactions with immunoglobulins

Receptors for the Fc domain of immunoglobulins play an important role in immune defense. There are two well-defined functional classes of mammalian receptors. One class of receptors transports immunoglobulins across epithelial tissues to their main sites of action. This class includes the neonatal Fc receptor (FcRn), which transports immunoglobulin G (IgG), and the polymeric immunoglobulin receptor (pIgR), which transports immunoglobulin A (IgA) and immunoglobulin M (IgM). Another class of receptors present on the surfaces of effector cells triggers various biological responses upon binding antibody-antigen complexes. Of these, the IgG receptors (Fc gamma R) and immunoglobulin E (IgE) receptors (Fc epsilon R) are the best characterized. The biological responses elicited include antibody-dependent, cell-mediated cytotoxicity, phagocytosis, release of inflammatory mediators, and regulation of lymphocyte proliferation and differentiation. We summarize the current knowledge of the structures and functions of FcRn, pIgR, and the Fc gamma R and Fc epsilon RI proteins, concentrating on the interactions of the extracellular portions of these receptors with immunoglobulins.

Extended repertoire of permissible peptide ligands for HLA-B*2702

Recognition of self peptides bound to the class I major histocompatibility complex molecule HLA-B27 is thought to trigger proliferation of autoreactive T cells and result in autoimmune arthritic diseases. Previous work from other laboratories established that a predominant feature of endogenous peptides eluted from purified B27 is an arginine at position 2. We studied the binding of peptides containing both natural and unnatural amino acids by the subtype HLA-B*2702, with the goal of gaining insight into peptide binding by this B27 subtype that is associated with susceptibility to arthritic disease. A soluble from of B*2702 was depleted of endogenous peptides. We tested the binding of peptides substituted with cysteine, homocysteine, or an alpha-amino-epsilon-mercapto hexanoic acid side chain (Amh) instead of the naturally occurring arginine at position 2, to determine whether the peptide sulfhydryl residue could be covalently linked to cysteine 67 in the B*2702 binding cleft. Although none of the altered peptide sequences bound covalently to B*2702, the affinities of the homocysteine- and Amh-substituted peptides were close to that of the native peptide sequence. Substitutions at position 2 with other side chains, such as glutamine and methionine, also resulted in peptides that bound with only slightly reduced affinity. These results demonstrate that peptide side chains other than arginine at position 2 can be accomodated within the B*2702 peptide binding site with only minor reductions in affinity. This extended repertoire of permissible B27-binding peptides should be taken into account for a consideration of disease-associated peptide sequences.

Effects of receptor dimerization on the interaction between the class I major histocompatibility complex-related Fc receptor and IgG

The neonatal Fc receptor (FcRn) transports maternal IgG from ingested milk in the gut to the bloodstream of newborn mammals. An FcRn dimer was observed in crystals of the receptor alone and of an FcRn-Fc complex, but its biological relevance was unknown. Here we use surface plasmon resonance-based biosensor assays to assess the role of FcRn dimerization in IgG binding. We find high-affinity IgG binding when FcRn is immobilized on a biosensor chip in an orientation facilitating dimerization but not when its orientation disrupts dimerization. This result supports a model in which IgG-induced dimerization of FcRn is relevant for signaling the cell to initiate endocytosis of the IgG-FcRn complex.

Analysis of the pH dependence of the neonatal Fc receptor/immunoglobulin G interaction using antibody and receptor variants

The neonatal Fc receptor (FcRn) binds maternal immunoglobulin G (IgG) from ingested milk in the gut (pH 6.0-6.5) and delivers it to the bloodstream of the newborn (pH 7.0-7.5). A soluble version of FcRn reproduces the physiological pH-dependent interaction with IgG, showing high-affinity binding at pH 6.0-6.5 but weak or no binding at pH 7.0-7.5. We have studied the pH dependence of the FcRn/IgG interaction using a surface plasmon resonance assay to measure kinetic and equilibrium constants. We show that the affinity of FcRn for IgG is reduced about 2 orders of magnitude as the pH is raised from 6.0 to 7.0. A hill put analysis suggests that several titrating residues participate in the pH-dependent affinity transition. Histidine side chains are likely candidate for residues that titrate between pH 6.0 and 7.0, and previous biochemical and structural work identified several histidines on the Fc portion of IgG that are located at the FcRn binding site. Using mutant IgG molecules and IgG subtype variants that differ in the number of histidines at the IgG/FcRn interface, we demonstrate that IgG histidines located at the junction between the CH2 and CH3 domains (residues 310 and 433) contribute to the pH-dependent affinity transition. Experiments with a mutant FcRn molecule show that two histidines on the FcRn heavy chain (residues 250 and 251) also contribute to the pH dependence of the FcRn/IgG interaction. There results are interpreted using the crystal structures of FcRn and an FcRn/Fc complex.

Investigation of the interaction between the class I MHC-related Fc receptor and its immunoglobulin G ligand

The neonatal Fc receptor (FcRn) is structurally similar to class I major histocompatibility molecules. FcRn transports maternal immunoglobulin G (IgG) from ingested milk into the blood. IgG is bound at the pH of milk (pH 6.0-6.5) in the gut and released at the pH of blood (pH 7.5). We find that alteration of a histidine pair within the alpha 3 domain of FcRn and of a nearby loop (the FcRn counterpart of the class I CD8-binding loop) affects the affinity for IgG. Inhibition studies suggest the involvement of the FcRn B2-microglobulin domain in IgG binding. Fragment B of protein A inhibits FcRn binding to IgG, localizing the binding site on Fc for FcRn to the CH2-CH3 domain interface. Three histidines present at the CH2-CH3 domain interface of Fc could be partially responsible for the pH-dependent interaction between FcRn and IgG.

The class I major histocompatibility complex related Fc receptor shows pH-dependent stability differences correlating with immunoglobulin binding and release

Maternal immunoglobulin G (IgG) in milk is transported to the bloodstream of newborn rodents via an Fc receptor (FcRn) expressed in the gut. The receptor shows a striking structural similarity to class I major histocompatibility complex (MHC) molecules, being composed of a related heavy chain and the identical light chain (beta 2-microglobulin). FcRn binds IgG at the pH of milk in the proximal intestine (pH 6.0-6.5) and releases it at the pH of blood (pH approximately 7.5). We have compared the stability of a soluble form of FcRn in these two pH ranges and find that the heterodimer is markedly more stable at the permissive pH for IgG binding. Using the rate of beta 2m exchange as a correlate of heterodimer stability, we find that exchange is more than 10 times slower at pH 6.1 compared to pH 7.8. Thermal denaturation profiles of FcRn heterodimers at pH 8.0 indicate a two-step, sequential heavy-chain (Tm = 52 degrees C) and beta 2m (Tm = 67 degrees C) denaturation. By contrast, at pH 6.0, a single transition is observed, centered at 62 degrees C, corresponding to denaturation of both chains. The striking difference in stability does not appear to be correlated with the binding of peptide as in class I MHC molecules, because analysis of purified FcRn by acid dissociation and sequencing suggests that FcRn is not associated with cellular peptides. These results are indicative of pH-dependent conformational changes in the FcRn heterodimer, which may be related to its physiological function.

The use of alternative substrates in the characterization of actin-methylating and carnosine-methylating enzymes

Actin isolated from nearly every eukaryotic species contains approximately 1 mol 3-methylhistidine/mol protein. His73 in actin has been shown, by protein sequencing, to be the site of methylation. The methylation occurs enzymically and post-translationally. A rabbit skeletal muscle myofibrillary fraction has previously been shown to contain a histidine methyltransferase activity that is actin specific. Detailed study of this enzyme has been hampered by lack of a suitable substrate assay. Naturally occurring actins are poor substrates for the enzyme, presumably due to prexistent methylation at His73. In this study, two potential alternative substrates have been investigated. These are a chicken beta-actin expressed in Escherichia coli as a fusion protein with 80 amino acids of an influenza protein, NS1, and a synthetic peptide, Tyr-Pro-Ile-Glu-His-Gly-Ile-Ile-Thr, corresponding to residues 69-77 of actin. Both substrates were covalently methylated at histidine residues in the presence of S-adenosylmethionine and partially purified enzyme fractions from rabbit muscle. In methylation experiments employing the fusion actin in the form of inclusion bodies, 3-methylhistidine is the major product, as is the case when soluble muscle or non-muscle actin is used. However, for the synthetic peptide, the methylated product primarily contained 1-methylhistidine and only a small amount of the isomeric 3-methylhistidine. Further investigations revealed that the peptide was recognized by carnosine N-methyltransferase, another histidine methyltransferase found in muscle tissue. Carnosine N-methyltransferase appears to copurify with the actin-methylating enzyme in preliminary fractionation experiments. Separation of the two methyltransferase activities is described.

Perinatal risk factors in neonatal infections

Perinatal risk factors were studied among 50 cases of neonatal septicemia and 200 matched normal neonates during one year period. The consanguinity among parents, birth order and sex of the baby did not increase the risk for developing septicemia. There was significant increase in the risk for septicemia when the duration of labour was more than 24 hours (P less than 0.01), time interval between rupture of membrane and delivery of baby was more than 12 hours (P less than 0.001), liquor was meconium stained or foul smelling (P less than 0.001) and delivery was operative (P less than 0.01). The neonatal factors identified with risk for septicemia were preterm delivery (P less than 0.01), low birth weight (P less than 0.01), birth asphyxia (P less than 0.001) assisted ventilation (P less than 0.001) and intravenous alimentation (P less than 0.02). Identification of high risk pregnancies and appropriate management can minimize many of the above risk factors which in turn will reduce the occurrence of neonatal sepsis.

Neonatal septicaemia among inborn and outborn babies in a referral hospital

Neonatal sepsis was studied among one hundred neonates (50 hospital born and 50 outborn babies) over one year period. The incidence of neonatal septicaemia was 15.5 per 1000 live births in the hospital. Among outborn babies it accounted for 6.1% of total pediatric admissions and 43.7% of sick neonates referred from outside. Low birth weight and prematurity were important predisposing factors in both the groups. Blood culture was positive among 32% of outborn and 34% of inborn babies. Coagulase-negative Staphylococcus, Klebsiella and Acinetobacter were the common causative organisms. All isolated organisms were sensitive to Gentamicin whereas 75% of them were resistant to Ampicillin. Mortality among outborn neonates (32%) was much higher in comparison to (10%) hospital born babies. Early identification of high risk antenatal cases and neonates and appropriate referral can bring down mortality and morbidity from neonatal sepsis.

Preliminary X-ray crystallographic analysis of holotoxin from Bordetella pertussis

Pertussis (whooping cough) is a serious infectious disease caused by the bacterium Bordetella pertussis. One of the major virulence factors is a protein known as pertussis toxin, which is composed of six subunits, with a total molecular weight of 106,000. Enzymatic transfer of ADP-ribose from NAD to a family of GTP-binding proteins is effected by the largest subunit (S1 or the A monomer), while binding of host cells and entry of S1 to the interior is a function of the other subunits (the B oligomer). The holotoxin crystallizes in the orthorhombic space group P2(1)2(1)2(1), with unit cell dimensions a = 98.4 A, b = 164.2 A and c = 195.2 A. The crystals are suitable for high-resolution X-ray diffraction analysis.

Seizure with ciprofloxacin and theophylline combined therapy

Ciprofloxacin has been reported to cause theophylline toxicity by inhibiting theophylline metabolism. A 93-year-old woman without a known seizure history, while on ciprofloxacin and theophylline combined therapy, experienced a grand mal seizure. Her serum theophylline concentration at the time was 20 micrograms/mL. On previous occasion of theophylline toxicity, she had a serum theophylline concentration of 27 micrograms/ml but the patient did not experience any seizure. Several reports suggest that the combination of theophylline and ciprofloxacin has an additive inhibitory effect on gamma-aminobutyric acid (GABA) sites. Inhibition of the binding of GABA to its receptor sites has been related to the convulsant effects of other drugs. The seizure in our patient may have been caused by altered pharmacokinetics and pharmacodynamics brought about by combined therapy of theophylline and ciprofloxacin.

Analytical determination of methylated histidine in proteins: actin methylation

The methylation of histidine in actin from various muscle and nonmuscle sources has been studied by formation of phenylthiocarbamyl derivatives and subsequent reverse-phase high-pressure liquid chromatographic separation and analysis of actin hydrolyzates. All the actin species examined were found to contain 3-methylhistidine. This method has also been used in assays for the enzyme(s) responsible for methylation of rabbit skeletal muscle actin and to investigate the formation of other methylated residues in vitro. 3-Methyl-histidine is the major methylation product in this in vitro reaction.