The position of the disulfide bonds in human plasma α2HS-glycoprotein and the repeating double disulfide bonds in the domain structure

The positions of the inter- and intra-chain disulfide bonds of human plasma α₂HS-glycoprotein were determined. α₂HS-glycoprotein was digested with acid proteinase and then with thermolysin. The disulfide bonds containing peptides were separated by reversed-phase HPLC and detected by SBD-F (7-fluorobenzo-2-oxa-1,3-diasole-4-sulfonic acid ammonium salt) method. One inter-disulfide bond containing peptide and five intra-disulfide bond containing peptides (A-chain) were purified and identified as Cys-18 (B-chain)-Cys-14 (A-chain), Cys-71-Cys-82, Cys-96-Cys-114, Cys-128-Cys-131, Cys-190-Cys-201 and Cys-212-Cys-229, respectively. The location of the intra-disulfide bonds revealed that the A-chain of α₂HS-glycoprotein is composed of three domains. Two domains were shown to possess intramolecular homology judging from the total chain length of the domains, size of the loops formed by the SS bonds, the location of two disulfide loops near the C-terminal end of domains A and B, the distance between two SS bonds of each domain, the amino acid sequence homology between these two domains (22.6%), number of amino acid residues between the second SS loops and the end of domains A and B, and the positions of the ordered structures.     

Purification of the STB enterotoxin of Escherichia coli and the role of selected amino acids on its secretion, stability and toxicity

The methanol-insoluble heat-stable enterotoxin of Escherichia coli (STB) was purified and characterized by automated Edman degradation and tryptic peptide analysis. The amino-terminal residue, Ser-24, confirmed that the first 23 amino acids inferred from the gene sequence were removed during translocation through the E. coli inner membrane. Tryptic peptide analysis coupled with automated Edman degradation revealed that disulphide bonds are formed between residues Cys-33 and Cys-71 and between Cys-44 and Cys-59. Oligonucleotide-directed mutagenesis performed on the STB gene demonstrated that disulphide bond formation does not precede translocation of the polypeptide through the inner membrane and that disulphide bridge formation is a periplasmic event; apparently, elimination of either of two disulphides of STB renders the molecule susceptible to periplasmic proteolysis. In addition, a loop defined by the Cys-44-Cys-59 bond contains at least two amino acids (Arg-52 and Asp-53) required for STB toxic activity.     

The positions of the disulfide bonds and the glycosylation site in a lectin of the acorn barnacle Megabalanus rosa

The positions of the interchain and intrachain disulfide bonds and the glycosylation site in a lectin of the acorn barnacle Megabalanus rosa were determined. The lectin (Mr 140,000) is composed of the same subunit (Mr 22,000) which is cross-linked by disulfide bonds to form a dimer. Intact lectin yielded two fragments, CB1 and CB2, by cleavage with cyanogen bromide. One intrachain and two interchain disulfide bonds were identified as Cys-53-Cys-61, Cys-14-Cys-50' and Cys-50-Cys-14', respectively, by enzymatic digestion and Edman degradation of CB1. Two intrachain disulfide bonds were determined as Cys-78-Cys-168 and Cys-144-Cys-160 by enzymatic digestion of CB2. The two intrachain disulfide bonds are well conserved through all invertebrate lectins and calcium-dependent animal lectins. S-Carboxamidomethylated lectin was digested with Staphylococcus aureus V8 proteinase and separated by reversed-phase HPLC. Glycopeptides were detected by the 4-N,N-dimethylamino-4'-azobenzene sulfonyl hyrazide method. Sequence analyses of the glycopeptides showed that a carbohydrate chain attached to Asn-39.     

Rat gro/melanoma growth-stimulating activity. Assessment of the structure responsible for chemotactic activity by use of its fragments prepared by proteolysis and chemical synthesis.

Rat gro/melanoma growth-stimulating activity is a dimer composed of two identical subunits. Each subunit consists of 72 amino-acid residues and contains two disulfide bridges. In order to obtain information on the structure responsible for chemotactic activity, various fragments of gro were prepared and tested for their ability to induce chemotaxis. None of the fragments corresponding to residues 1-6, 1-21, 12-31, 36-50 or 52-72 was active as a chemoattractant. Reduced and carboxymethylated gro as well as the tryptic peptide consisting of three peptides, residues 9-21, 28-45 were and 49-61, linked by two disulfide bonds Cys-9-Cys-35 and Cys-11-Cys-51, were inactive. Also, these, peptides did not inhibit the chemotactic activity of gro. Rat gro lacking the N-terminal 6 residues had a reduced activity and the one lacking the C-terminal Lys was as active as intact gro. Therefore, an almost entire portion of the molecule including disulfide cross-links is required for chemotactic activity.     

Locations of the six disulphide bonds in a variant surface glycoprotein (VSG 117) from Trypanosoma brucei.

The locations of the six disulphide bonds and the single free cysteine residue in a variant surface glycoprotein, VSG 117, from the African trypanosome Trypanosoma brucei have been determined to be Cys-14--Cys-140, Cys-121--Cys-182, Cys-389--Cys-404, Cys-398--417, Cys-447--Cys-461 and Cys-455--Cys-468. Cys-244 bears the single thiol group, which is unreactive towards 2-nitro-5-thiocyanobenzoate in the native molecule and is probably buried. Biosynthetically incorporated [35S]cysteine aided the location of the disulphide bonds. Two proteinase-resistant glycosylated domains, each containing two disulphide bonds, were identified in the C-terminal region of the glycoprotein. Details of purification of [35S]cysteine-containing peptides, and Tables of amino acid analyses, are presented in Supplementary Publication SUP 50119 (32 pages), which has been deposited with the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1981) 193,5.     

Unique disulfide bonds in epidermal growth factor (EGF) domains of β3 affect structure and function of αIIbβ3 and αvβ3 integrins in different manner

The β3 subunit of αIIbβ3 and αvβ3 integrins contains four epidermal growth factor (EGF)-like domains. Each domain harbors four disulfide bonds of which one is unique for integrins. We previously discerned a regulatory role of the EGF-4 Cys-560-Cys-583 unique bond for αIIbβ3 activation. In this study we further investigated the role of all four integrin unique bonds in both αIIbβ3 and αvβ3. We created β3 mutants harboring serine substitutions of each or both cysteines that disrupt the four unique bonds (Cys-437-Cys-457 in EGF-1, Cys-473-Cys-503 in EGF-2, Cys-523-Cys-544 in EGF-3, and Cys-560-Cys-583 in EGF-4) and transfected them into baby hamster kidney cells together with normal αv or αIIb. Flow cytometry was used to measure surface expression of αIIbβ3 and αvβ3 and their activity state by soluble fibrinogen binding. Most cysteine substitutions caused similarly reduced surface expression of both receptors. Disrupting all four unique disulfide bonds by single cysteine substitutions resulted in variable constitutive activation of αIIbβ3 and αvβ3. In contrast, whereas double C437S/C457S and C473S/C503S mutations yielded constitutively active αIIbβ3 and αvβ3, the C560S/C583S mutation did not, and the C523S/C544S mutation only yielded constitutively active αIIbβ3. Activation of C523S/C544S αvβ3 mutant by activating antibody and dithiothreitol was also impaired. Molecular dynamics of C523S/C544S β3 in αIIbβ3 but not in αvβ3 displayed an altered stable conformation. Our findings indicate that unique disulfide bonds in β3 differently affect the function of αIIbβ3 and αvβ3 and suggest a free sulfhydryl-dependent regulatory role for Cys-560-Cys-583 in both αIIbβ3 and αvβ3 and for Cys-523-Cys-544 only in αvβ3.     

The crystallographically determined structures of atypical strained disulfides engineered into subtilisin

The geometries of two disulfide bridges genetically engineered into subtilisin have been characterized by x-ray crystallography to determine the structural and energetic constraints involved in introducing disulfide bonds into proteins. Both disulfide bridges (Cys-24-Cys-87 and Cys-22-Cys-87) exhibit atypical sets of dihedral angles compared to those for other reported disulfide structures in proteins. The geometric trends for naturally occurring disulfides in protein crystal structures are examined. Comparison of the disulfide-containing mutant protein structures with the wild-type structure shows that, in both cases, disulfide incorporation is accommodated by relatively minor changes in local main-chain conformation. The Cys-22-Cys-87 disulfide has two high energy dihedral angles (X2 = 121 degrees, X2' = 143 degrees). Both disulfides produce short non-bonded contacts with the main-chain.     

Specific cysteines in beta3 are involved in disulfide bond exchange-dependent and -independent activation of alphaIIbbeta3

Disulfide bond exchange among cysteine residues in epidermal growth factor (EGF)-like domains of beta3 was suggested to be involved in activation of alphaIIbbeta3. To investigate the role of specific beta3 cysteines in alphaIIbbeta3 expression and activation, we expressed in baby hamster kidney cells normal alphaIIb with normal beta3 or beta3 with single or double cysteine substitutions of nine disulfide bonds in EGF-3, EGF-4, and beta-tail domains and assessed alphaIIbbeta3 surface expression and activation state by flow cytometry using P2 or PAC-1 antibodies, respectively. Most mutants displayed reduced surface expression of alphaIIbbeta3. Disruptions of disulfide bonds in EGF-3 yielded constitutively active alphaIIbbeta3, implying that these bonds stabilize the inactive alphaIIbbeta3 conformer. Mutants of the Cys-567-Cys-581 bond in EGF-4 were inactive even after exposure to alphaIIbbeta3-activating antibodies, indicating that this bond is necessary for activating alphaIIbbeta3. Disrupting Cys-560-Cys-583 in the EGF-3/EGF-4 or Cys-608-Cys-655 in beta-tail domain resulted in alphaIIbbeta3 activation only when Cys-560 or Cys-655 of each pair was mutated but not when their partners (Cys-583, Cys-608) or both cysteines were mutated, suggesting that free sulfhydryls of Cys-583 and Cys-608 participate in alphaIIbbeta3 activation by a disulfide bond exchange-dependent mechanism. The free sulfhydryl blocker dithiobisnitrobenzoic acid inhibited 70% of anti-LIBS6 antibody-induced activation of wild-type alphaIIbbeta3 and had a smaller effect on mutants, implicating disulfide bond exchange-dependent and -independent mechanisms in alphaIIbbeta3 activation. These data suggest that different disulfide bonds in beta3 EGF and beta-tail domains play variable structural and regulatory roles in alphaIIbbeta3.     

Peptide fractionation by high-performance liquid chromatography on an Asahipak GS-320 column: application to determination of the disulfide pairings in ribonuclease F1

High-performance liquid chromatography on an Asahipak GS-320 column using isocratic elution with 0.1 M acetic acid has proven effective for fractionation of peptides of molecular weights lower than 3000. This technique enabled the separation of the peptides derived from digestion of native ribonuclease F1 by trypsin and chymotrypsin in combination with conventional gel filtration through Sephadex G-25 and reversed-phase HPLC. Amino acid analysis of the cystine-containing peptides thus obtained revealed the disulfide linkages Cys-6-Cys-102 and Cys-24-Cys-84 in this protein. The behavior of a number of peptides in the HPLC on an Asahipak GS-320 column is described and the separation mechanism is discussed.     

The single-disulphide intermediates in the refolding of reduced pancreatic trypsin inhibitor

The intermediate species with one disulphide bond in the renaturation of reduced pancreatic trypsin inhibitor have been trapped, isolated, and the Cys residues involved in the disulphide bonds determined. Approximately half the intermediate species had the disulphide bond between Cys-30 and 51, a disulphide bond also present in the native inhibitor. The next most predominant species, representing one-quarter of the total, had a disulphide bond between Cys-5 and 30, and two more minor species involving Cys-30 and 55 and Cys-5 and 51 were detected; these disulphide bonds are not present in the native inhibitor.The nature of the disulphide bonds present are concluded to reflect primarily the conformational forces acting at this stage of folding, which may be primarily interactions between segments with propensities for secondary structure, either helices or β-sheet. The general importance of such interactions in protein folding is discussed.     

Different roles of the two disulfide bonds of the cysteine proteinase inhibitor, chicken cystatin, for the conformation of the active protein.

The Cys-71-Cys-81 disulfide bond of the cysteine proteinase inhibitor, chicken cystatin, was specifically reduced by thioredoxin or low concentrations of dithiothreitol. This cleavage, followed by S-carbamoylmethylation, induced a conformational change of the protein, as evidenced by changes in isoelectric point and circular dichroism spectra and by an increased susceptibility to digestion by nontarget proteinases. The proteinase binding ability and the immunological properties of the inhibitor, however, were not detectably altered, indicating that the conformational change was limited to the region around the disrupted bond. In contrast, reduction of both disulfide bonds of cystatin by higher concentrations of dithiothreitol and subsequent alkylation led to the slow conversion of the inhibitor into two forms lacking proteinase binding ability, indicative of more extensive conformational changes. Together, these results suggest that the less accessible Cys-95-Cys-115 disulfide bond of chicken cystatin, but not the more accessible Cys-71-Cys-81 bond, is of importance for maintaining the conformation of the inhibitor required for binding of target proteinases.     

Role of vicinal cysteine pairs in metalloid sensing by the ArsD As(III)-responsive repressor

ArsD is a 120-residue repressor that regulates expression of the arsRDABC arsenical resistance operon of plasmid R773 in Escherichia coli. ArsD is released from arsRDABC promoter DNA by binding of the compounds with the metalloids As(III) or Sb(III). ArsD has three vicinal cysteine pairs, Cys-12 and Cys-13, Cys-112 and Cys-113 and Cys-119 and Cys-120. In this study, the role of these three cysteine pairs was investigated. Mutation or deletion of Cys-119-Cys-120 had no effect on repression or metalloid responsiveness in vivo or in vitro. Mutagenesis of either the Cys-12-Cys-13 pair or the Cys-112-Cys-113 pair had no effect on repression but produced loss of inducibility, suggesting that both Cys-12-Cys-13 and Cys-112-Cys-113 may be required for As(III) or Sb(III) responsiveness. Assays of binding of wild-type and mutant ArsDs by As(III) affinity chromatography showed that each of the three vicinal cysteine pairs is capable of binding As(III) independently. The effect of As(III) or Sb(III) on intrinsic protein fluorescence was used to examine the properties of individual cysteine pairs. The fluorescence of Trp-97 was shown to be quenched by the addition of Sb(III) or As(III). The vicinal Cys-112-Cys-113 pair was required for the majority of the metalloid-dependent quenching of Trp-97 fluorescence. The data are consistent with a model in which Cys-12-Cys-13 and Cys-112-Cys-113 form independent As(III) binding sites, both of which are required for in vivo ArsD function.     

Carrier subunit of plasma membrane transporter is required for oxidative folding of its helper subunit

We study the amino acid transport system b(0,+) as a model for folding, assembly, and early traffic of membrane protein complexes. System b(0,+) is made of two disulfide-linked membrane subunits: the carrier, b(0,+) amino acid transporter (b(0,+)AT), a polytopic protein, and the helper, related to b(0,+) amino acid transporter (rBAT), a type II glycoprotein. rBAT ectodomain mutants display folding/trafficking defects that lead to type I cystinuria. Here we show that, in the presence of b(0,+)AT, three disulfides were formed in the rBAT ectodomain. Disulfides Cys-242-Cys-273 and Cys-571-Cys-666 were essential for biogenesis. Cys-673-Cys-685 was dispensable, but the single mutants C673S, and C685S showed compromised stability and trafficking. Cys-242-Cys-273 likely was the first disulfide to form, and unpaired Cys-242 or Cys-273 disrupted oxidative folding. Strikingly, unassembled rBAT was found as an ensemble of different redox species, mainly monomeric. The ensemble did not change upon inhibition of rBAT degradation. Overall, these results indicated a b(0,+)AT-dependent oxidative folding of the rBAT ectodomain, with the initial and probably cotranslational formation of Cys-242-Cys-273, followed by the oxidation of Cys-571-Cys-666 and Cys-673-Cys-685, that was completed posttranslationally.     

Determination of disulfide array and subunit structure of taste-modifying protein, miraculin

The taste-modifying protein, miraculin (Theerasilp, S. et al. (1989) J. Biol. Chem. 264, 6655-6659) has seven cysteine residues in a molecule composed of 191 amino acid residues. The formation of three intrachain disulfide bridges at Cys-47-Cys-92, Cys-148-Cys-159 and Cys-152-Cys-155 and one interchain disulfide bridge at Cys-138 was determined by amino acid sequencing and composition analysis of cystine-containing peptides isolated by HPLC. The presence of an interchain disulfide bridge was also supported by the fact that the cystine peptide containing Cys-138 showed a negative color test for the free sulfhydryl group and a positive test after reduction with dithiothreitol. The molecular mass of non-reduced miraculin (43 kDa) in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) was nearly twice the calculated molecular mass based on the amino acid sequence and the carbohydrate content of reduced miraculin (25 kDa). The molecular mass of native miraculin determined by low-angle laser light scattering was 90 kDa. Application of a crude extract of miraculin to a Sephadex G-75 column indicated that the taste-modifying activity appears at 52 kDa. It was concluded that native miraculin in pure form is a tetramer of the 25 kDa-peptide and native miraculin in crude state or denatured, non-reduced miraculin in pure form is a dimer of the peptide. Both tetramer miraculin and native dimer miraculin in crude state had the taste-modifying activity.     

Assignment of the disulphide bonds in the sweet-tasting protein thaumatin I

The disulphide linkages of the 16 half-cystine residues in the sweet-tasting protein thaumatin have been investigated by enzymatic hydrolysis of the intact molecule. The peptides obtained after proteolytic cleavage with trypsin and pepsin, and in one case with chymotrypsin have been purified by gel filtration, high-performance liquid chromatography and peptide mapping by paper high-voltage electrophoresis in one direction and paper chromatography in the second dimension. Disulphide bonds appeared to be formed by cysteine residues in positions 9-204, 56-66, 71-77, 121-193, 126-177, 134-149, 145-158 and 159-164. The labile disulphide bond responsible for the enzymatic properties of the sweet tasting protein thaumatin appeared to be between Cys-145 and Cys-158.     

Purification and characterization of Escherichia coli heat-stable enterotoxin II.

Escherichia coli heat-stable enterotoxin II (STII) was purified to homogeneity by successive column chromatographies from the culture supernatant of a strain harboring the plasmid encoding the STII gene. The purified STII evoked a secretory response in the suckling mouse assay and ligated rat intestinal loop assay in the presence of protease inhibitor, but the response was not observed in the absence of the inhibitor. Analyses of the peptide by the Edman degradation method and fast atom bombardment mass spectrometry revealed that purified STII is composed of 48 amino acid residues and that its amino acid sequence was identical to the 48 carboxy-terminal amino acids of STII predicted from the DNA sequence (C. H. Lee, S. L. Mosely, H. W. Moon, S. C. Whipp, C. L. Gyles, and M. So, Infect. Immun. 42:264-268, 1983). STII has four cysteine residues which form two intramolecular disulfide bonds. Two disulfide bonds were determined to be formed between Cys-10-Cys-48 and Cys-21-Cys-36 by analyzing tryptic hydrolysates of STII.     

Identification of in vivo disulfide conformation of TRPA1 ion channel

TRPA1 (transient receptor potential ankyrin 1) is an ion channel expressed in the termini of sensory neurons and is activated in response to a broad array of noxious exogenous and endogenous thiol-reactive compounds, making it a crucial player in chemical nociception. A number of conserved cysteine residues on the N-terminal domain of the channel have been identified as critical for sensing these electrophilic pungent chemicals, and our recent EM structure with modeled domains predicts that these cysteines form a ligand-binding pocket, allowing for the possibility of disulfide bonding between the cysteine residues. Here, we present a comprehensive mass spectrometry investigation of the in vivo disulfide bonding conformation and in vitro reactivity of 30 of the 31 cysteine residues in the TRPA1 ion channel. Four disulfide bonds were detected in the in vivo TRPA1 structure: Cys-666-Cys-622, Cys-666-Cys-463, Cys-622-Cys-609, and Cys-666-Cys-193. All of the cysteines detected were reactive to N-methylmaleimide (NMM) in vitro, with varying degrees of labeling efficiency. Comparison of the ratio of the labeling efficiency at 300 μM versus 2 mM NMM identified a number of cysteine residues that were outliers from the mean labeling ratio, suggesting that protein conformation changes rendered these cysteines either more or less protected from labeling at the higher NMM concentrations. These results indicate that the activation mechanism of TRPA1 may involve N-terminal conformation changes and disulfide bonding between critical cysteine residues.     

The thiol groups of mouse immunoglobulin A. Incomplete formation of the C alpha 1-domain disulphide bridge.

The BALB/c IgA (immunoglobulin A) myeloma protein M167 contained on average 5.7 free SH groups per IgA dimer. These groups were preponderantly on the heavy chains and comprised two distinct populations: 3.3 exposed SH groups per dimer in the Fc region, and 2.4 buried SH groups per dimer in the Fd region, detectable o only after denaturation. To locate the cysteine residues involved, labelled peptides were purified from thermolysin digests of radioalkylated IgA by high-performance liquid chromatography. From the amino acid compositions of the peptides, the exposed thiol groups were assigned to Cys-307 in the C alpha 2 domain, which thus existed in the reduced form to an extent exceeding 80%. This residue may allow attachment of secretory component to dimer IgA in the mouse to proceed via thiol-disulphide exchange. The buried thiol groups were assigned to Cys-150 and Cys-208, in the C alpha 1 domain, each being in the reduced form to the extent of approx. 30%. This pair of residues would normally give rise to the characteristic intradomain disulphide bridge. It appears that disulphide formation is not a crucial event during folding of the C alpha 1 domain in IgA biosynthesis. The sequence in the region 140-151 was re-investigated, and residue 142 was shown to be serine, not cysteine, helping explain the lack of heavy-chain-light chain bonding in BALB/c mouse IgA. A disulphide-bond model for mouse IgA is proposed on the basis of these assignments and other features of the mouse alpha-chain sequence.     

Identification of a segment of DsbB essential for its respiration-coupled oxidation

In the Escherichia coli protein disulphide bond formation pathway, membrane-bound DsbB oxidizes periplasmic DsbA, the disulphide bond-introducing enzyme. The Cys-41-Val-Leu-Cys-44 motif in the first periplasmic domain of DsbB is kept strongly oxidized by the respiratory function of the cell. We now show that the characteristic dithiothreitol resistance of the Cys-41-Cys-44 bond was retained even when the flanked Val-Leu combination was replaced by XX sequences from other oxidoreductases. Results of insertion mutagenesis showed that only the insertions (1-31 amino acids) in the region C-terminally adjacent to the CXXC motif impaired the oxidized state of DsbB. Deletion of a single amino acid from this region also rendered DsbB reduced and inactive. However, single amino acid substitutions of the four residues flanked by CXXC and the transmembrane segment did not abolish the oxidation of DsbB. These results suggest that some physical property, such as distance of the CXXC motif from the membrane, is important for the respiration-coupled oxidation of DsbB.     

Rat brain Thy-1 glycoprotein. The amino acid sequence, disulphide bonds and an unusual hydrophobic region.

The full sequence of the Thy-1 membrane glycoprotein of rat brain is reported. The sequence was determined from tryptic and V-8 proteinase peptides and consisted of 111 amino acids. The amino terminus was blocked and consisted of a pyroglutamic acid residue. The molecule contained two disulphide bonds, namely Cys-9--Cys-111 and Cys-19--Cys-85. Three N-linked amino sugars were located at Asn-23, Asn-74 and Asn-98. In each case the sequence on the C-terminal side of the attachment point was Asn-Xaa-Thr as would be expected for N-linkage. The C-terminal peptides were unusual, in that they were either obtained in a highly aggregated form, or could only be purified after binding to Brij 96 micelles. Thus they appeared to have hydrophobic properties, yet did not contain any extended sequence of hydrophobic amino acids. Other unusual features of the C-terminal peptides were the presence of unidentified ninhydrin-positive material and of glucosamine and galactosamine. The C-terminal residue has not been directly identified but Cys-111 is the last conventional amino acid. It is suggested that the hydrophobic properties of the C-terminal peptides may be due to the linkage of lipid. The sequence of the Thy-1 glycoprotein showed homologies with immunoglobulin domains. This relationship is examined in detail in the paper following [Cohen et al. (1981) Biochem. J. 193, 000--000].