Secretion of immunoglobulin M assembly intermediates in the presence of reducing agents

There are several demonstrations that misfolded or unassembled proteins are not transported along the secretory pathway, but are retained intracellularly, generally in the endoplasmic reticulum. For instance, B lymphocytes synthesize but do not secrete IgM, and only the polymeric form of IgM is secreted by plasma cells. The C-terminal cysteine of the mu heavy chain of secreted IgM (residue 575) is involved in the intracellular retention of unpolymerized IgM subunits. Here we report that the addition of reducing agents to the culture medium, at concentrations which do not affect cell viability, terminal glycosylation, or retention of proteins in the endoplasmic reticulum through the KDEL mechanism, induces secretion of IgM assembly intermediates by both B and plasma cells. Free joining (J) chains, which are not normally secreted by plasma cells unless as part of IgM or IgA, are also secreted in the presence of reducing agents. We propose a role for free thiol groups in preventing the unhindered transport of proteins through the secretory pathway. Under the scheme, assembly intermediates interact through their thiol groups between themselves and/or with unknown proteins of the endoplasmic reticulum. Such interactions may be prevented by altering the intracellular redox potential or by site-directed mutagenesis of the relevant cysteine residue(s).

Phenotypic heterogeneity in osteogenesis imperfecta: the mildly affected mother of a proband with a lethal variant has the same mutation substituting cysteine for alpha 1-glycine 904 in a type I procollagen gene (COL1A1)

A proband with a lethal variant of osteogenesis imperfecta (OI) has been shown to have, in one allele in a gene for type I procollagen (COL1A1), a single base mutation that converted the codon for alpha 1-glycine 904 to a codon for cysteine. The mutation caused the synthesis of type I procollagen that was posttranslationally overmodified, secreted at a decreased rate, and had a decreased thermal stability. The results here demonstrate that the proband's mother had the same single base mutation as the proband. The mother had no fractures and no signs of OI except for short stature, slightly blue sclerae, and mild frontal bossing. As a child, however, she had the triangular facies frequently seen in many patients with OI. On repeated subculturing, the proband's fibroblasts grew more slowly than the mother's, but they continued to synthesize large amounts of the mutated procollagen in passages 7-14. In contrast, the mother's fibroblasts synthesized decreasing amounts of the mutated procollagen after passage 11. Also, the relative amount of the mutated allele in the mother's fibroblasts decreased with passage number. In addition, the ratio of the mutated allele to the normal allele in leukocyte DNA from the mother was half the value in fibroblast DNA from the proband. The simplest interpretation of the data is that the mother was mildly affected because she was a mosaic for the mutation that produced a lethal phenotype in one of her three children.

Redox regulation of fos and jun DNA-binding activity in vitro

The proto-oncogenes c-fos and c-jun function cooperatively as inducible transcription factors in signal transduction processes. Their protein products, Fos and Jun, form a heterodimeric complex that interacts with the DNA regulatory element known as the activator protein-1 (AP-1) binding site. Dimerization occurs via interaction between leucine zipper domains and serves to bring into proper juxtaposition a region in each protein that is rich in basic amino acids and that forms a DNA-binding domain. DNA binding of the Fos-Jun heterodimer was modulated by reduction-oxidation (redox) of a single conserved cysteine residue in the DNA-binding domains of the two proteins. Furthermore, a nuclear protein was identified that reduced Fos and Jun and stimulated DNA-binding activity in vitro. These results suggest that transcriptional activity mediated by AP-1 binding factors may be regulated by a redox mechanism.

Protein folding

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Macrophages regulate intracellular glutathione levels of lymphocytes. Evidence for an immunoregulatory role of cysteine

Macrophages consume cystine and generate approximately equivalent amounts of acid-soluble thiol. Stimulation of macrophages with bacterial lipopolysaccharide (LPS) or tumor necrosis factor (TNF) strongly augments the amount of thiol released into the culture supernatant. Cysteine constitutes most of the acid-soluble thiol. The intracellular glutathione level and the DNA synthesis activity in mitogenically stimulated lymphocytes are strongly increased by either exogenously added cysteine, or (syngeneic) macrophages. This cysteine dependency is observed even in the presence of relatively high extracellular cystine concentration as they occur in the blood plasma. The extracellular cysteine concentration also has a strong influence on the intracellular glutathione concentration, viability, and DNA synthesis of cycling T cell clones. Moreover, the cysteine concentration in the culture medium on Day 3 and Day 4 of a 5-day allogeneic mixed lymphocyte culture (i.e., in the late phase of incubation) has a strong influence on the generation of cytotoxic T cell activity, indicating that regulatory effects of cysteine are not restricted to the early phase of the blastogenic response. The inhibitory effect of cysteine starvation on the DNA synthesis of the T cell clones and on the activation of cytotoxic T lymphocytes can be explained essentially by the depletion of intracellular glutathione, since similar effects are observed after treatment with buthionine sulfoximine (BSO), a specific inhibitor of the glutathione biosynthesis. BSO has practically no influence, however, on the N alpha-benzyloxycarbonyl Ne-t-butyloxycarbonyl-L-lysine-thiobenzyl-ester (BLT)-esterase activity and hemolytic activity of the cell lysates from cytotoxic T cells against sheep red blood cells (perforin activity). Taken together, our experiments indicate that cysteine has a regulatory role in the immune system analogous to the hormone-like lymphokines and cytokines. It is released by macrophages at a variable and regulated rate and regulates immunologically relevant functions of lymphocytes in the vicinity.

The role of protein-thiol mixed disulfides in cataractogenesis

Protein-thiol mixed disulfides in lenses have been implicated in the mechanism of protein-protein disulfide and other cross-linking leading to protein aggregation. The methodology for the detection and quantitation of protein-thiol mixed disulfides has been successfully established in our laboratory. Examination of mixed disulfides at different stages during development of a cataract may give relevant information on the mechanism of cataractogenesis, and whether oxidation is a part of that mechanism. In this study we investigated the involvement of mixed disulfides in cataract formation by using the H2O2-exposed lens as a model. Rat lenses, after being exposed to 0.5 mM H2O2 in culture showed an inverse relationship between the GSH loss and the protein-GSH formation with no effect on the protein-cysteine level. The H2O2-induced protein modification was also demonstrated indirectly by isoelectric focusing. The rate of protein-GSH production is dependent on the time of exposure and the concentration of H2O2. Age also plays some role as the lens GSH level decreases and the protein-thiol mixed disulfides increase as the animal becomes older. Lenses of older rats did not display more susceptibility to H2O2-induced mixed disulfide formation. The two protein-thiol mixed disulfides have a well-defined pattern of distribution in the rat lens. Most of the protein-GSH was found in the cortex and the water-soluble protein fraction whereas more protein-cysteine was found in the nucleus and water-insoluble protein fraction. Lens of older rat has more protein-cysteine as well as more water-insoluble proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel peroxisomal nonspecific lipid-transfer protein from Candida tropicalis. Gene structure, purification and possible role in beta-oxidation

We have sequenced the nucleotides of the gene POX18 that encodes PXP-18, a major peroxisomal polypeptide inducible by oleic acid in the yeast Candida tropicalis. POX18 had a single open reading frame of 127 amino acids. Some 33% of the amino acid sequence of the predicted basic polypeptide (13,805 Da), was identical to that of the nonspecific lipid-transfer protein (sterol carrier protein 2) from rat liver. PXP-18, purified to near homogeneity from isolated peroxisomes, had an amino-terminal sequence identical to that of the predicted polypeptide except for the initiator methionine, and had nonspecific lipid-transfer activity comparable to that of its mammalian equivalents. Unexpectedly, PXP-18 lacked the cysteine residue thought to be essential for the activity of this protein in mammals. RNA blot analysis showed that the POX18 gene was expressed exclusively in cells grown on oleic acid, suggesting that PXP-18 has a role in the beta-oxidation of long-chain fatty acids. PXP-18 modulated acyl-coenzyme A oxidase activity at low pH.

Short related sequences in the cytoplasmic domains of CD4 and CD8 mediate binding to the amino-terminal domain of the p56lck tyrosine protein kinase

We report that the cytoplasmic domains of the T-lymphocyte glycoproteins CD4 and CD8 alpha contain short related amino acid sequences that are involved in binding the amino-terminal domain of the intracellular tyrosine protein kinase, p56lck. Transfer of as few as six amino acid residues from the cytoplasmic domain of the CD8 alpha protein to the cytoplasmic domain of an unrelated protein conferred p56lck binding to the hybrid protein in HeLa cells. The common sequence motif shared by CD4 and CD8 alpha contains two cysteines, and mutation of either cysteine in the CD4 sequence eliminated binding of p56lck.p56lck also contains two cysteine residues within its CD4-CD8 alpha-binding domain, and both are critical to the interaction with CD4 or CD8 alpha. Because the interaction does not involve disulfide bond formation, a metal ion could stabilize the complex.

Guilt by association: HLA-B27 and ankylosing spondylitis

The remarkable association between HLA-B27 and ankylosing spondylitis (AS) remains an enigma. While previous reviews have discussed the controversies surrounding the involvement of bacteria in the etiology of this disease and the sequence variability between subtypes of HLA-B27, concepts of disease mechanism remain ill-defined. In this article Richard Benjamin and Peter Parham synthesize new data on the structure and function of HLA class I molecules into possible mechanisms that might underly the pathogenesis of AS.

Apolipoprotein E2-Dunedin (228 Arg replaced by Cys): an apolipoprotein E2 variant with normal receptor-binding activity

Homozygosity for the apolipoprotein (apo) E variant apoE2(158 Arg----Cys) invariably gives rise to dysbetalipoproteinemia, and when associated with obesity or a gene for hyperlipidemia, results in type III hyperlipoproteinemia. The association of the E2/2 phenotype with type IV/V hyperlipoproteinemia rather than type III hyperlipoproteinemia in identical twin brothers led us to investigate the primary structure of their apoE. Lipoprotein electrophoresis on agarose gels confirmed the presence of increased very low density lipoproteins (VLDL) and chylomicrons but little, if any, beta-VLDL, indicating that these subjects did not have dysbetalipoproteinemia. When the apoE from these twins was subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis on a system that can distinguish apoE2(158 Arg----Cys) from all other known apoE variants, it gave rise to two components. One had the unique mobility of apoE2(158 Arg----Cys), and one migrated in the position of the other variants of apoE (and normal apoE3), indicating that the brothers were heterozygous for apoE2(158 Arg----Cys) and a second apoE2 isoform. Cysteamine modification and isoelectric focusing showed that, like apoE2(158 Arg----Cys), the second apoE2 isoform also contained two cysteine residues. The structural mutation in the second apoE2 isoform was determined by peptide sequencing. Like normal apoE3, this variant had arginine at position 158, but differed from apoE3 by the substitution of cysteine for arginine at position 228. Total apoE isolated from the brothers had the same receptor-binding activity in a competitive binding assay as a 1:1 mixture of normal apoE3 and apoE2(158 Arg----Cys).(ABSTRACT TRUNCATED AT 250 WORDS)

Critical spacing between two essential cysteine residues in the interdomain linker of the Bradyrhizobium japonicum NifA protein

A special sequence motif in the Bradyrhizobium japonicum NifA protein, consisting of two functionally essential cysteines separated by four other amino acids (Cys-aa4-Cys), has been proposed to be part of a potential metal-binding site [(1988) Nucleic Acids Res. 16, 2207-2224]. Using the techniques of oligonucleotide-directed mutagenesis, we report here that several of the four intervening amino acids can be replaced by others without loss of NifA function. The deletion of one amino acid to give a Cys-aa3-Cys motif renders the protein inactive whereas the creation of a Cys-aa5-Cys motif (one amino acid inserted) still leads to a partially active NifA protein.

Cystein 402 of HIV gp 120 is essential for CD4-binding and resistance of gp 120 to intracellular degradation

A DNA fragment encoding the CD4-binding region of human immunodeficiency virus type 1 (HIV) gp 120 was excised from an SV40-based expression vector containing gp 160, and subcloned into phage M13 for site-directed mutagenesis. Mutant vectors were constructed and CV-1 cells were transfected with constructs, where Cys402 was substituted for a serine, and metabolically labelled with [3H]-N-acetylglucosamine (GlcN). Radioimmunoprecipitation with an hyperimmunserum, specific for gp 120/gp 160, and subsequent SDS-polyacrylamide gel electrophoresis demonstrated presence of gp 160, whereas gp 120 was replaced by [3H]-GlcN-labelled material, migrating as a diffuse band corresponding to 80-105k, suggesting increased sensitivity of mutant env gene products to proteolysis after cleavage to gp 120. Wild type gp 120 and gp 160 bound to CD4, whereas neither gp 160 nor gp 120 from mutant-transfected cell lysates did bind to CD4. Altogether the results indicated that Cys402, probably by participating in a disulfide bridge, is essential for (i) the CD4-binding ability of env gene products and for (ii) the physical stability of gp 120.

Expression and site-specific mutagenesis of phospholamban. Studies of residues involved in phosphorylation and pentamer formation

Full-length cDNAs encoding either dog cardiac or rabbit skeletal muscle phospholamban were expressed transiently in COS-1 cells. The expressed protein displayed the mobility of a pentamer when dissolved in sodium dodecyl sulfate and separated in polyacrylamide gels, and of a monomer when boiled prior to polyacrylamide gel separation. Site-specific mutagenesis was used to analyze the roles of several amino acids in the structure and function of the protein. Ser16 and Thr17 were shown to be phosphorylated uniquely by cAMP- and calmodulin-dependent protein kinases, respectively, confirming earlier observations on the native protein (Simmerman, H. K. B., Collins, J. H., Theibert, J.L., Wegener, A.D., and Jones, L.R. (1986) J. Biol. Chem. 261, 13333-13341). Arg13 and Arg14 were shown to be essential for both types of phosphorylation, and Arg9 was shown to be essential for calmodulin-dependent phosphorylation. In studies of pentamer stability, mutation of Gln22-Gln23 to Ala-Ala or Glu-Glu, of Gln26-Asn27 to Glu-Asp, or of Gln29-Asn30 to Glu-Asp had no effect on thermal stability of the pentamer, suggesting that hydrogen bonding involving these residues in domain IB is not important for pentamer stability. By contrast, mutation of Cys36, Cys41, and Cys46 in transmembrane domain II to Ser, Ala, or Phe diminished the stability of the pentamer when microsomal proteins were dissociated in sodium dodecyl sulfate and separated by polyacrylamide gel electrophoresis. In particular, the Cys41 to Phe mutant existed as a monomer at ambient temperature. These results suggest that the intramembranous cysteine residues are important for pentamer formation even though they are not disulfide-bonded.

Studies on fatty-acid-binding proteins. The purification of rat liver fatty-acid-binding protein and the role of cysteine-69 in fatty acid binding

1. A new, simple and high-yield procedure is described for the purification of hepatic fatty-acid-binding protein from rat liver using naphthylaminodecyl-agarose as an affinity column. 2. Cysteine-69 is shown to react slowly, but quantitatively, with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB), indicating that the thiol group is free, but may be buried within the protein. 3. Fatty acids do not affect the DTNB reactivity of this cysteine residue; however, cysteine reactivity is enhanced in the presence of haem and oleoyl-CoA. 4. Fatty-acid-binding protein that has been modified with DTNB is still able to bind the fluorescent fatty acid 11-(dansylamino)undecanoic acid, indicating that cysteine-69 may be remote from the fatty-acid-binding site.

Coronavirus genome: prediction of putative functional domains in the non-structural polyprotein by comparative amino acid sequence analysis

Amino acid sequences of 2 giant non-structural polyproteins (F1 and F2) of infectious bronchitis virus (IBV), a member of Coronaviridae, were compared, by computer-assisted methods, to sequences of a number of other positive strand RNA viral and cellular proteins. By this approach, juxtaposed putative RNA-dependent RNA polymerase, nucleic acid binding ("finger"-like) and RNA helicase domains were identified in F2. Together, these domains might constitute the core of the protein complex involved in the primer-dependent transcription, replication and recombination of coronaviruses. In F1, two cysteine protease-like domains and a growth factor-like one were revealed. One of the putative proteases of IBV is similar to 3C proteases of picornaviruses and related enzymes of como- nepo- and potyviruses. Search of IBV F1 and F2 sequences for sites similar to those cleaved by the latter proteases and intercomparison of the surrounding sequence stretches revealed 13 dipeptides Q/S(G) which are probably cleaved by the coronavirus 3C-like protease. Based on these observations, a partial tentative scheme for the functional organization and expression strategy of the non-structural polyproteins of IBV was proposed. It implies that, despite the general similarity to other positive strand RNA viruses, and particularly to potyviruses, coronaviruses possess a number of unique structural and functional features.

Site-directed mutagenesis of beta-adrenergic receptors. Identification of conserved cysteine residues that independently affect ligand binding and receptor activation

Using site-directed mutagenesis of the human beta 2-adrenergic receptor and continuous expression in B-82 cells, the role of 3 conserved cysteines in transmembrane domains and 2 conserved cysteines in the third extracellular domain in receptor function was examined. Cysteine was replaced with serine in each mutant receptor as this amino acid is similar to cysteine in size but it cannot form disulfide linkages. Replacement of cysteine residues 77 and 327, in the second and seventh transmembrane-spanning domains, respectively, had no effect on ligand binding or the ability of the receptor to mediate isoproterenol stimulation of adenylate cyclase. Substitution of cysteine 285, in the sixth transmembrane domain of the receptor, produced a mutant receptor with normal ligand-binding properties but a significantly attenuated ability to mediate stimulation of adenylate cyclase. Mutation of cysteine residues 190 and 191, in the third extracellular loop of the beta 2 receptor, had qualitatively similar effects on ligand binding and isoproterenol-mediated stimulation of adenylate cyclase. Replacement of either of these residues with serine produced mutant receptors that displayed a marked loss in affinity for both beta-adrenergic agonists and antagonists. Replacement of both cysteine 190 and 191 with serine had an even greater effect on the ability of the receptor to bind ligands. Consistent with the loss of Ser190 and/or Ser191 mutant receptor affinity for agonists was a corresponding shift to the right in the dose-response curve for isoproterenol-induced increases in intracellular cyclic AMP concentrations in cells expressing the mutant receptors. These data implicate one of the conserved transmembrane cysteine residues in the human beta 2-adrenergic receptor in receptor activation by agonists and also suggest that conserved cysteine residues in an extracellular domain of the receptor may be involved in ligand binding.

Role of cysteine residues and of metal ions in the regulatory functioning of FNR, the transcriptional regulator of anaerobic respiration in Escherichia coli

FNR, the transcriptional regulator of gene expression of anaerobic respiration in Escherichia coli, contains a cluster of cysteine residues at the amino terminus which resembles the metal-binding domains of metal-binding proteins. It is possible, therefore, (i) that FNR binds metals with the cysteines as ligands and (ii) that this property is related to the regulatory function of FNR. These questions were investigated, with the following results. Approximately 2.4 of the 4 cysteine residues of FNR can be alkylated with iodoacetate in permeabilized aerobic or anaerobic bacteria without the addition of reducing agents. The time required for half-maximal labelling of the cysteines was 50 min in anaerobic bacteria and 6 min in aerobic bacteria. The difference in the reactivity was specific for the cysteines of FNR. These cysteine residues were also highly reactive in anaerobically grown bacteria, when the growth medium contained chelating agents such as 1,10-phenanthroline (15 microM). The effect of the chelating agents was reversed by an excess of divalent metal ions such as Fe(II) or Cu(II) in the medium. The presence of 1,10-phenanthroline (10 microM) also inhibits the expression of fumarate reductase, an FNR-dependent enzyme. These results suggest that FNR exists in two different forms which differ in terms of the reactivity of their cysteine residues to iodoacetate. The interconversion of both forms appears to be regulated by the availability of O2 and by the binding of metal ions. The two forms of FNR may be involved in the regulation of O2-dependent gene expression.

Is a certain amount of cysteine prerequisite to produce brain damage in neonatal rats?

In this study, various manipulations were used to determine if certain amounts of cysteine are essential to damage the neonatal rat brain. The information gathered from this study indicated that concentration of free cysteine may be 0.6 mumol/g of wet brain weight or more to cause the toxicity to produce the brain damage, and the results were discussed in the light that free cysteine might itself be the cause of brain damage.

Purification and partial characterization of a transcription-inhibitory peptide from Tetrahymena

The protozoa Tetrahymena excretes a small peptide complex with an Mr of about 5,000. The peptide inhibits transcription by reducing the activity of the RNA polymerase. We have purified and partially characterized the peptide complex. It contains two peptide chains of apparent Mr 2,300 and 2,600, respectively. Magnesium ions in connection with the SH groups of cysteine play a role in holding together the two chains of the intact complex.

The MAL63 gene of Saccharomyces encodes a cysteine-zinc finger protein

Inducible maltose fermentation by Saccharomyces carlesbergensis requires the product of the MAL63 gene of the MAL6 locus. It has been suggested that this gene product is an activator protein controlling the expression of the structural genes encoding the maltose fermentative enzymes perhaps by binding to DNA sequences upstream of these genes. We report the sequence of the MAL63 gene. A single open reading frame is seen capable of encoding a protein of 470 amino acid residues. The deduced sequence of this protein indicates that it is a cysteine-zinc finger protein supporting the hypothesis that the MAL63 gene product is a DNA binding protein.

Modified nucleosides and the chromatographic and aminoacylation behavior of tRNA(Ile) from Escherichia coli C6

Transfer RNA from Escherichia coli C6, a Met-, Cys-, relA- mutant, was previously shown to contain an altered tRNA(Ile) which accumulates during cysteine starvation (Harris, C.L., Lui, L., Sakallah, S. and DeVore, R. (1983) J. Biol. Chem. 258, 7676-7683). We now report the purification of this altered tRNA(Ile) and a comparison of its aminoacylation and chromatographic behavior and modified nucleoside content to that of tRNA(Ile) purified from cells of the same strain grown in the presence of cysteine. Sulfur-deficient tRNA(Ile) (from cysteine-starved cells) was found to have a 5-fold increased Vmax in aminoacylation compared to the normal isoacceptor. However, rates or extents of transfer of isoleucine from the [isoleucyl approximately AMP.Ile-tRNA synthetase] complex were identical with these two tRNAs. Nitrocellulose binding studies suggested that the sulfur-deficient tRNA(Ile) bound more efficiently to its synthetase compared to normal tRNA(Ile). Modified nucleoside analysis showed that these tRNAs contained identical amounts of all modified bases except for dihydrouridine and 4-thiouridine. Normal tRNA(Ile) contains 1 mol 4-thiouridine and dihydrouridine per mol tRNA, while cysteine-starved tRNA(Ile) contains 2 mol dihydrouridine per mol tRNA and is devoid of 4-thiouridine. Several lines of evidence are presented which show that 4-thiouridine can be removed or lost from normal tRNA(Ile) without a change in aminoacylation properties. Further, tRNA isolated from E. coli C6 grown with glutathione instead of cysteine has a normal content of 4-thiouridine, but its tRNA(Ile) has an increased rate of aminoacylation. We conclude that the low content of dihydrouridine in tRNA(Ile) from E. coli cells grown in cysteine-containing medium is most likely responsible for the slow aminoacylation kinetics observed with this tRNA. The possibility that specific dihydrouridine residues in this tRNA might be necessary in establishing the correct conformation of tRNA(Ile) for aminoacylation is discussed.

Characteristics of cysteine-requiring strains of Klebsiella isolated from urinary tract infections

Clinical and bacteriological findings in seven cases of urinary tract infection with cysteine-requiring strains of Klebsiella are described. The organisms were isolated from patients with long-standing urinary tract abnormalities and grew as small (c. 1 mm) colonies on MacConkey agar. The organisms failed to grow in a minimal medium supplemented with sodium sulphate but grew when the medium was supplemented with cysteine sulphinic acid, sodium sulphide or L-cysteine. The smallest amount of cysteine required for optimal growth in a chemically defined medium was 20 mg/L. Cysteine-requiring strains of Escherichia coli had previously been shown to require a similar amount of cysteine and to be unable to reduce sulphate to sulphite; this suggests a common influence in the selection of cysteine auxotrophs in vivo. However, the amino acid inhibited the growth of E. coli at concentrations which only slightly altered growth of the Klebsiella strains. Problems with the isolation, identification and sensitivity testing of cysteine-requiring Klebsiella were also observed and methods by which these may be minimised are suggested.

Analysis of glutathione-enhanced differentiation by microfilariae of Onchocerca lienalis (Filarioidea: Onchocercidae) in vitro

Reduced glutathione (GSH), but not its oxidized form (GSSG), stimulated development of Onchocerca lienalis microfilariae to the late first-larval stage in vitro. The degree and frequency of development was dose-related with a peak of activity at 15 mM, a concentration that is similar to known intracellular levels of GSH. To determine the mode(s) of action of this multifunctional compound, other reducing agents (L-cysteine, dithiothreitol), cysteine delivery agents (N-acetyl-L-cysteine, L-thiazolidine-4-carboxylic acid, L-2-oxothiazolidine-4-carboxylic acid), cysteine analogues (S-methyl-L-cysteine, D-glucose-L-cysteine, cysteine ethyl ester), free-component amino acids of GSH (glutamic acid, cysteine, and glycine), a specific metabolic inhibitor of gamma-glutamyl synthetase (buthionine sulfoximine), and an inhibitor of gamma-glutamyl transpeptidase (gamma-glutamyl glutamic acid) were also tested at concentrations of 0.01-50 mM in this system. N-acetyl-L-cysteine at 1-5 mM and D-glucose-L-cysteine at 2.5-10 mM significantly enhanced development. In contrast to those worms maintained in GSH-supplemented medium, microfilariae exposed to GSH for only the first 24 hr showed no enhancement by day 7 in culture. Neither buthionine sulfoximine nor gamma-glutamyl glutamic acid at 0.01-35 mM inhibited the effects of 15 mM GSH or 1 mM N-acetyl-L-cysteine. Results indicate that GSH or other cysteine analogues possessing a free sulfhydryl group must be present in the extranematodal environment to support microfilarial differentiation in vitro.

The contribution of cysteine residues to antigenicity and extent of processing of herpes simplex virus type 1 glycoprotein D

Glycoprotein D (gD) is an envelope component of herpes simplex virus types 1 (gD-1) and 2 (gD-2). The gD-1 polypeptide contains seven cysteine residues among its 369 amino acids; six are located on the N-terminal or luminal portion of the glycoprotein, and a seventh is located in the transmembrane region. Previous studies used a panel of monoclonal antibodies (MAbs) to define gD epitopes as continuous or discontinuous. Purified gD, denatured by reduction and alkylation, loses discontinuous epitopes, whereas continuous epitopes are retained. The contribution of disulfide bonds to maintenance of discontinuous epitopes is, therefore, significant. In the present study, our objective was to determine the contribution of individual cysteine residues to folding of gD-1 into its native conformation. Site-directed oligonucleotide mutagenesis was used to create seven mutants, each with a serine residue replacing a cysteine. The mutated genes were cloned into a eucaryotic expression vector and transfected into COS-1 cells, and the proteins were separated by nondenaturing polyacrylamide gel electrophoresis, followed by immunoblotting. Replacement of cysteine 7 (residue 333) had only a minimal effect on the antigenic properties of gD-1. In contrast, replacement of any one of the other six cysteine residues resulted in either a major reduction or a complete loss of binding of those MAbs that recognize discontinuous epitopes, with no effect on the binding of MAbs which recognize continuous epitopes. These mutations also had profound effects on the extent of oligosaccharide processing of gD-1. This was determined by digestion of the expressed proteins with various endoglycosidases, followed by electrophoresis and Western blotting (immunoblotting) to observe any mobility changes. Three mutant gD proteins which did not express discontinuous epitopes contained only high-mannose-type oligosaccharides, suggesting that processing had not proceeded beyond the precursor stage. Two mutant forms of gD exhibited reduced binding of MAbs to discontinuous epitopes. A small proportion of the molecules which accumulated at 48 h posttransfection contained complex oligosaccharides. One mutant exhibited reduced binding of MAbs to discontinuous epitopes, but was present at 48 h posttransfection only in the precursor form. The cysteine 7 mutant was processed to the same extent as wild-type gD. We conclude that the first six cysteine residues are critical to the correct folding, antigenic structure, and processing of gD-1, and we speculate that they form three disulfide-bonded pairs.