Ah receptor in spleen of rodent and primate species: detection by binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin

In many species systemic toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is manifested by a generalized wasting syndrome accompanied by a variety of specific organ changes including atrophy of the thymus and spleen. TCDD toxicity in most tissues is thought to be mediated by the Ah receptor. Although the spleen is a prime target for TCDD toxicity, the possible presence of Ah receptor in the spleen has not previously been investigated. Specific binding of [3H]TCDD to Ah receptor in spleen cytosols was assessed by velocity sedimentation on sucrose gradients. Ah receptor was detected in spleen cytosols from adult Rhesus monkeys (mean +/- SEM, 36 +/- 8 fmol/mg cytosol protein), fetal Rhesus monkeys (9 +/- 6), Sprague-Dawley rats (20 +/- 5), C57BL/6J mice (18 +/- 2), New Zealand white rabbits (19 +/- 2), and Hartley guinea pigs (15 +/- 2). Ah receptor was not detectable in spleen cytosol from genetically "nonresponsive" DBA/2J mice or from Golden Syrian hamsters, a species resistant to toxicity of TCDD. Molecular properties of Ah receptor from spleen were similar to those of the receptor from liver of the same species. The high Ah receptor content in spleen cytosols from those species that are most susceptible to TCDD toxicity is consistent with the view that the Ah receptor mediates TCDD toxicity in spleen as well as in other tissues.     

Conversion of human erythrocyte acetylcholinesterase from an amphiphilic to a hydrophilic form by phosphatidylinositol-specific phospholipase C and serum phospholipase D

Each catalytic subunit in the amphiphilic dimer of human erythrocyte acetylcholinesterase (AChE) is anchored in the plasma membrane exclusively by a glycoinositol phospholipid. In contrast to erythrocyte AChEs in other mammalian species, the human enzyme is resistant to direct cleavage by phosphatidylinositol-specific phospholipase C (PtdIns-specific PLC). The resistance is due to the existence of an additional fatty acyl chain on the inositol ring which blocks the action of PtdIns-specific PLC [Roberts et al. (1988) J. Biol. Chem. 263, 18766-18775]. In this report, nondenaturing polyacrylamide gel electrophoresis was applied to permit rapid and unambiguous distinction between amphiphilic AChE, in which each catalytic subunit binds one nonionic detergent micelle, and hydrophilic AChE, which does not interact with detergent. Deacylation of human erythrocyte AChE by an alkaline treatment with hydroxylamine rendered the amphiphilic AChE susceptible to PtdIns-specific PLC with the consequent release of hydrophilic AChE. Although serum anchor-specific phospholipase D (PLD) cleaves the intact human erythrocyte AChE anchor, this treatment, as judged by nondenaturing electrophoresis, did not release hydrophilic AChE. Hydroxylamine treatment before or after PLD digestion was necessary to achieve the conversion. These observations indicate that binding of a single detergent micelle was maintained when any of the three fatty acyl or alkyl groups in the human erythrocyte AChE anchor phospholipid were retained. For proteins that can be identified following nondenaturing gel electrophoresis, these procedures provide methods both for detecting glycoinositol phospholipid anchors resistant to PtdIns-specific PLC and for indicating fatty acyl and/or alkyl chains in these anchors.     

Properties of the interaction between phosphofructokinase and actin

The interaction of rabbit skeletal muscle phosphofructokinase (PFK) with actin is characterized in terms of the binding of PFK to actin in the presence and absence of tropomyosin and troponin, the effect of PFK on actin polymerization, and the involvement of adenylates in the binding of PFK to actin. The thin filament proteins, tropomyosin and troponin, are associated with skeletal muscle actin and reduce the binding of PFK to actin, thus influencing the probable distribution of PFK in skeletal muscle. The binding of PFK to actin is inhibited by ATP and ADP but not by fructose 6-phosphate or fructose 2,6-bisphosphate. This specific inhibition, plus evidence from fluorescence quenching and photoaffinity labeling, suggests that actin binds at the adenosine activation sites of PFK. Light scattering measurements used to monitor actin polymerization indicate that PFK dramatically increases the level of light scattering produced by the polymerization of actin, indicative of a superaggregate of PFK and actin. PFK inhibits the polymerization of actin when polymerization is induced by low concentrations of added salts. Although PFK binds to actin with high affinity, it seems to have little effect on the high shear viscosity of actin filaments.     

Structural study of the sugar chains of human platelet thrombospondin

The asparagine-linked sugar chains of human platelet thrombospondin were released as oligosaccharides by hydrazinolysis. About 12 mol of sugar chains was released from one thrombospondin molecule. This was converted to radioactive oligosaccharides by sodium borotritide reduction after N-acetylation, and separated into one neutral and four acidic fractions by paper electrophoresis. More than 90% of the oligosaccharides were recovered in the acidic fraction. The acidic oligosaccharides were mostly converted to neutral oligosaccharides by sialidase treatment, indicating that they are sialyl derivatives. The neutral and sialidase-treated acidic oligosaccharides were further fractionated by Bio-Gel P-4 column chromatography. Structural study of each oligosaccharide by sequential exoglycosidase digestion and methylation analysis revealed that the thrombospondin contains mono-, bi-, tri-, and tetraantennary complex-type sugar chains in addition to a small amount of high-mannose type. Approximately 70% of the complex-type sugar chains was fucosylated at asparagine-linked N-acetylglucosamine residue and 19% of the biantennary complex-type sugar chains was bisected.     

Secondary structures of rat lipolytic enzymes: circular dichroism studies and relation to hydrophobic moments

To explore the secondary structures of lingual and pancreatic lipases, circular dichroism measurements were performed. Maximum average ellipticities were used to calculate the percentage of alpha-helices, beta-sheets, and random coils. Lingual lipase had an ellipticity of -20235 +/- 140 deg cm2/dmol (mean +/- SE) at 220 nm suggesting 60% alpha-helix, 20% beta-sheet and 20% random coil structure, but the mean ellipticity for pancreatic lipase was -14093 +/- 82 deg cm2/dmol (mean +/- SE) at 210 nm suggesting a 34.8% alpha-helical, 25% beta-sheet and 40% random coil secondary structure. An alpha-helical stretch of residues with a large hydrophobic moment ("globular" alpha-helix by hydrophobic moment plot) from amino acids 382 through 389 at the COOH-terminal end of lingual lipase was noted. This sequence, absent in pancreatic lipase, may account for the avid binding of lingual lipase to fat emulsion particles.     

NMR studies of differences in the conformations and dynamics of ligand complexes formed with mutant dihydrofolate reductases

Two mutants of Lactobacillus casei dihydrofolate reductase, Trp 21----Leu and Asp 26----Glu, have been prepared by using site-directed mutagenesis methods, and their ligand binding and structural properties have been compared with those of the wild-type enzyme. 1H, 13C, and 31P NMR studies have been carried out to characterize the structural changes in the complexes of the mutant and wild-type enzymes. Replacement of the conserved Trp 21 by a Leu residue causes a decrease in activity of the enzyme and reduces the NADPH binding constant by a factor of 400. The binding of substrates and substrate analogues is only slightly affected. 1H NMR studies of the Trp 21----Leu enzyme complexes have confirmed the original resonance assignments for Trp 21. In complexes formed with methotrexate and the mutant enzyme, the results indicate some small changes in conformation occurring as much as 14 A away from the site of substitution. For the enzyme-NADPH complexes, the chemical shifts of nuclei in the bound coenzyme indicate that the nicotinamide ring binds differently in complexes with the mutant and the wild-type enzyme. There are complexes where the wild-type enzyme has been shown to exist in solution as a mixture of conformations, and studies on the corresponding complexes with the Trp 21----Leu mutant indicate that the delicately poised equilibria can be perturbed. For example, in the case of the ternary complex formed between enzyme, trimethoprim, and NADP+, two almost equally populated conformations (forms I and II) are seen with the wild-type enzyme but only form II (the one in which the nicotinamide ring of the coenzyme is extended away from the enzyme structure and into the solvent) is observed for the mutant enzyme complex. It appears that the Trp 21----Leu substitution has a major effect on the binding of the nicotinamide ring of the coenzyme. For the Asp 26----Glu enzyme there is a change in the bound conformation of the substrate folate. Further indications that some conformational adjustments are required to allow the carboxylate of Glu 26 to bind effectively to the N1 proton of inhibitors such as methotrexate and trimethoprim come from the observation of a change in the dynamics of the bound trimethoprim molecule as seen from the increased rate of the flipping of the 13C-labeled benzyl ring and the increased rate of the N1-H bond breaking.     

Nonrecombinant Meiosis I Nondisjunction in Saccharomyces Cerevisiae Induced by Trna Ochre Suppressors

The presence of the tRNA ochre suppressors SUP11 and SUP5 is found to induce meiosis I nondisjunction in the yeast Saccharomyces cerevisiae. The induction increases with increasing dosage of the suppressor and decreases in the presence of an antisuppressor. The effect is independent of the chromosomal location of SUP11. Each of five different chromosomes monitored exhibited nondisjunction at frequencies of 0.1%-1.1% of random spores, which is a 16-160-fold increase over wild-type levels. Increased nondisjunction is reflected by a marked increase in tetrads with two and zero viable spores. In the case of chromosome III, for which a 50-cM map interval was monitored, the resulting disomes are all in the parental nonrecombinant configuration. Recombination along chromosome III appears normal both in meioses that have no nondisjunction and in meioses for which there was nondisjunction of another chromosome. We propose that a proportion of one or more proteins involved in chromosome pairing, recombination or segregation are aberrant due to translational read-through of the normal ochre stop codon. Hygromycin B, an antibiotic that can suppress nonsense mutations via translational read-through, also induces nonrecombinant meiosis I nondisjunction. Increases in mistranslation, therefore, increase the production of aneuploids during meiosis. There was no observable effect of SUP11 on mitotic chromosome nondisjunction; however some disomes caused SUP11 ade2-ochre strains to appear white or red, instead of pink.     

Cloning and characterization of the HpaII methylase gene.

The HpaII restriction-modification system from Haemophilus parainfluenzae recognizes the DNA sequence CCGG. The gene for the HpaII methylase has been cloned into E. coli and its nucleotide sequence has been determined. The DNA of the clones is fully protected against cleavage by the HpaII restriction enzyme in vitro, indicating that the methylase gene is active in E. coli. The clones were isolated in an McrA-strain of E. coli; attempts to isolate them in an McrA+ strain were unsuccessful. The clones do not express detectable HpaII restriction endonuclease activity, suggesting that either the endonuclease gene is not expressed well in E. coli, or that it is not present in its entirety in any of the clones that we have isolated. The derived amino acid sequence of the HpaII methylase shows overall similarity to other cytosine methylases. It bears a particularly close resemblance to the sequences of the HhaI, BsuFI and MspI methylases. When compared with three other methylases that recognize CCGG, the variable region of the HpaII methylase, which is believed to be responsible for sequence specific recognition, shows some similarity to the corresponding regions of the BsuFI and MspI methylases, but is rather dissimilar to that of the SPR methylase.     

Towards a physical map of the Drosophila melanogaster genome: mapping of cosmid clones within defined genomic divisions.

A physical map of the D. melanogaster genome is being constructed, in the form of overlapping cosmid clones that are assigned to specific polytene chromosome sites. A master library of ca. 20,000 cosmids is screened with probes that correspond to numbered chromosomal divisions (ca. 1% of the genome); these probes are prepared by microdissection and PCR-amplification of individual chromosomes. The 120 to 250 cosmids selected by each probe are fingerprinted by Hinfl digestion and gel electrophoresis, and overlaps are detected by computer analysis of the fingerprints, permitting us to assemble sets of contiguous clones (contigs). Selected cosmids, both from contigs and unattached, are then localized by in situ hybridization to polytene chromosomes. Crosshybridization analysis using end probes links some contigs, and hybridization to previously cloned genes relates the physical to the genetic map. This approach has been used to construct a physical map of the 3.8 megabase DNA in the three distal divisions of the x chromosome. The map is represented by 181 canonical cosmids, of which 108 clones in contigs and 32 unattached clones have been mapped individually by in situ hybridization to chromosomes. Our current database of in situ hybridization results also includes the beginning of a physical map for the rest of the genome: 162 cosmids have been assigned by in situ hybridization to 129 chromosomal subdivisions elsewhere in the genome, representing 5 to 6 megabases of additional mapped DNA.