Photoaffinity labeling and partial purification of the beta cell sulfonylurea receptor using a novel, biologically active glyburide analog

An iodinated analog of the sulfonylurea, glyburide, has been synthesized which can be labeled to high specific activity and used to photolabel the sulfonylurea receptor. 5-Iodo-2-hydroxy-"glyburide", has an iodo group replacing the chlorine at position 5 and a methoxy residue replacing the hydroxy group at position 2 on the benzamido ring. This analog retains biologic activity stimulating insulin secretion from a hamster beta cell line (HIT cells) at the same ED50 (0.4 nM) as glyburide. Scatchard analysis demonstrated high and low affinity binding sites on HIT cell membranes (Kd values of 0.36 nM and 277 nM and Bmax values of 1.6 and 100 pmol/mg of membrane protein, respectively). Competitive binding assays with unlabeled glyburide or 5-iodo-2-hydroxyglyburide yield Ki values of 0.5 and 1.0 nM, respectively. The analog can be covalently linked by ultraviolet irradiation to a membrane protein of Mr = 140,000. The photolabeling is completely blocked by unlabeled glyburide or the analog. Two other species of Mr = 65,000 and 43,000 are also photolabeled; these may be the low affinity sites. After photolabeling, the receptor has been purified partially by chromatographic procedures and is suitable for obtaining peptide sequence. The 140,000 molecular weight protein is identified as the sulfonylurea receptor since its binding constant, 0.36 nM, is closely correlated with its ability to stimulate insulin secretion (ED50 congruent to 0.4 nM).     

The extracellular domains of MHC class II molecules determine their processing requirements for antigen presentation

We have evaluated the relative contributions of the extracellular and cytoplasmic domains of MHC class II molecules in determining the Ag-processing requirements for class II-restricted Ag presentation to T cells. Hybrid genes were constructed to encode a heterodimeric I-Ak molecule in which the extracellular portion of the molecule resembled wild type I-Ak but where the connecting stalk, transmembrane and cytoplasmic domains of both the alpha- and beta-chain were derived from the class I molecule H-2Dd. Mutant I-Ak molecules were expressed as heterodimeric membrane glycoproteins reactive with mAb specific for wild type I-Ak. Fibroblast and B lymphoma cells expressing either wild type or mutant I-Ak molecules were able to process and present hen egg lysozyme (HEL) and conalbumin to Ag-specific, I-Ak-restricted, T cell hybridomas or clones. The mutant-expressing cells presented native and peptide Ag less efficiently than the wild type-expressing cells, suggesting that the disparity in presentation efficiency was not due to a difference in Ag processing. CD4 interaction was intact on the mutant I-Ak molecules. Presentation of native Ag by mutant and wild type-I-Ak-expressing cells was abolished by preincubation with chloroquine, or after paraformaldehyde fixation. After transfection of a cDNA encoding the gene for HEL, neither mutant nor wild type-I-Ak-expressing cells presented endogenously synthesized HEL to a specific T hybrid. Newly synthesized mutant I-Ak molecules were associated with invariant chain. These data demonstrate the ability of hybrid class II molecules to associate intracellularly with invariant chain and degraded foreign Ag in a conventional class II-restricted processing pathway indicating that the extracellular domains of class II molecules play a dominant role in controlling these Ag-processing requirements.     

Regional chromosomal assignment of the human glucocorticoid receptor gene to 5q31

Summary The gene for the human glucocorticoid receptor, previously mapped to chromosome 5, has been further localised to 5q31 by in situ hybridisation using a biotinylated 4.3-kb cDNA probe.     

Recurrences of trisomy 18 and trisomy 13 after trisomy 21

Summary Between 40 years and 43 years of age, a woman had three consecutive pregnancies with different prenatally diagnosed autosomal trisomies. This is compatible with the view that the predisposition to non-disjunction is not chromosome-specific.     

Isolation and characterization of a human variable copy number tandem repeat at Xcen-p11.22

A subclone (M27B) has been isolated from a cosmid randomly selected from a library enriched for human X-chromosomal material. The subclone is extensively single-copy sequence, but also contains three complete copies and one partial copy of a 26-bp repeat, within which exists an inverted repeat having the potential to form a cruciform loop structure. Genomic sequences in this repeat region are apparently refractory to cloning, and rearrangements occurring during this process result in deletions. M27B detects multiple X-linked restriction fragments for a wide range of enzymes including MspI and HpaII. We have assigned the locus recognized by the probe (DXS255) to Xcen-Xp11.4 by mapping with a somatic cell hybrid panel and further refined its localization to Xp11.22 by in situ hybridization. The repeat sequence that is presumed to be responsible for the hypervariability observed does not show close similarity to other variable copy number tandem repeats described.     

Cloning and characterization of a phospholipase gene from Erwinia chrysanthemi EC16.

A single gene (plcA) was cloned from a cosmid library of Erwinia chrysanthemi EC16 DNA that encoded an extracellular phospholipase. The gene was subcloned and DNA sequence data showed the presence of a single open reading frame encoding a protein with a predicted size of 39kDa. The coding region was G+C-rich and the protein had a predicted basic isoelectric point. The protein showed no significant homology with others in the PIR library, including other phospholipases. When overexpressed in Escherichia coli cells, the plcA gene directed production of a c. 39kDa protein that was largely localized in the periplasm, but its N-terminal amino acid sequence was that of the native protein predicted from DNA sequence data. Unlike the wild-type bacterium, an E. chrysanthemi EC16 marker exchange mutant of the plcA gene did not secrete extracellular phospholipase activity into the medium. However, no detectable change was observed in terms of the virulence of the mutant strain on potato tubers or chrysanthemum stems.     

Purification and characterization of a mitochondrial endonuclease from Drosophila melanogaster embryos.

A mitochondrial endonuclease from Drosophila melanogaster embryos was purified to near homogeneity by successive fractionation with DEAE-cellulose and heparin--avidgel-F, followed by FPLC chromatography on mono S, Superose 12 and a second mono S column. This enzyme digests double-stranded DNA more efficiently than heat-denatured DNA. The endonuclease activity has a molecular mass of 44 kDa, as determined under native conditions using a gel-filtration Superose 12 column. The prominent peptide detected by SDS/polyacrylamide gel electrophoresis likewise has a molecular mass of 44 kDa, suggesting a monomeric protein. The enzyme has an absolute requirement for divalent cations, preferring Mg2+ over Mn2+. No activity could be detected when these cations were replaced by Ca2+ or Zn2+. The pH optimum for this enzyme activity is 6.5-7.4 and its isoelectric point is 4.9. Both single-strand and double-strand breaks are introduced simultaneously into a supercoiled substrate in the presence of MgCl2 or MnCl2. Endonuclease-treated DNA serves as a substrate for DNA polymerase I from Escherichia coli, suggesting that 3'-OH termini are generated during cleavage. The enzyme is free from any detectable DNA exonuclease activity but not from RNase activity. Partial inhibition by antibodies raised against mitochondrial endonucleases derived from bovine heart and Saccharomyces cerevisiae have revealed a potential structural homology between these nucleases.     

Mapping of Xp21 translocation breakpoints in and around the DMD gene by pulsed field gel electrophoresis

Balanced translocations with a breakpoint in the Xp21 region are likely to disrupt the giant Duchenne muscular dystrophy (DMD) locus and can be demonstrated in females suffering from the disease. Pulsed field gel electrophoresis allows the positioning of these breakpoints by detecting junction fragments on the derived chromosomes; DNA probes hybridizing to these fragments may be located as many as several hundred kilobases away from the breakpoints. By using this approach, 11 translocation breakpoints from the Xp21 region have been analyzed. The localization of three previously examined breakpoints was confirmed. Six other breakpoints, including a breakpoint flanking the DMD gene and not associated with the DMD phenotype, could be positioned relative to SfiI sites on a 3.5-Mb restriction map of the region.     

Desensitization of membrane-bound Torpedo acetylcholine receptor by amine noncompetitive antagonists and aliphatic alcohols: studies of [3H]acetylcholine binding and 22Na+ ion fluxes

Measurements of the kinetics of binding of [3H]acetylcholine ([3H]AcCh) to membrane-bound nicotinic AcCh receptors from Torpedo electric tissue have been used to characterize the effects of a series of amine and alcohol noncompetitive antagonists on receptor conformational equilibria. The receptor exists in multiple, interconvertible conformations distinguished by agonist binding affinity. In the absence of cholinergic ligands, certain aromatic amines including proadifen, dimethisoquin, and lidocaine, as well as propanol and butanol, produce a dose-dependent increase in the fraction of receptors (f) in a high-affinity conformation from a value of fmax approximately 0.17 in the absence of drug to fmax approximately 0.9. Not all noncompetitive antagonists produce that same value of fmax. For histrionicotoxin (HTX), fmax approximately 0.3, and the aromatic amine adiphenine did not alter f while tetracaine actually decreased f to 0.1. The high-affinity receptor conformation stabilized by noncompetitive antagonists was characterized by (1) the rate constant (krec) for receptor reisomerization upon removal of stabilizing ligand and (2) the rate constant (kdis) for dissociation of [3H]AcCh-receptor complexes. On the basis of these criteria, the high-affinity receptor conformation stabilized by amine and alcohol noncompetitive blockers is the same as that stabilized by agonist. At 4 degrees C, krec = (2.2 +/- 0.2) X 10(-3) s-1 and kdis = 4 X 10(-2) s-1. Since HTX and adiphenine produced only a small conformational perturbation, their effects on the actions of proadifen and 2-propanol were examined. HTX and adiphenine antagonized the conformational perturbation caused by proadifen, while mixtures of HTX and 2-propanol produced additive effects. Effects of noncompetitive blockers were also assayed in terms of the inhibition of agonist-induced efflux of 22Na+ from Torpedo vesicles. Exposure to proadifen in the absence of agonist produced a reversible inhibition (desensitization) of the flux response, and recovery from desensitization occurred at the same rate as the reisomerization from the high-affinity receptor state.(ABSTRACT TRUNCATED AT 400 WORDS)