Methionine modification impairs the C5-cleavage function of cobra venom factor-dependent C3/C5 convertase

The complement-mediated lysis of guinea pig erythrocytes by cobra venom factor (CVF) decreased by 50-60% within 2 min of treatment with 5 mM sodium periodate at 0 degree C. This loss of activity paralleled modification of 3-4 Met; other amino acids and sugar residues of the oligosaccharide chains were not affected. Treatment with N-chlorosuccinimide or chloramine-T under conditions that specifically modified 3-4 readily-oxidizable Met also caused 50-60% loss of CVF activity. The secondary structure of CVF was not altered by these modifications. Methionine-modified CVF (MetCVF) supported the cleavage of factor B by factor D with equal efficiency as that of untreated CVF to form C3/C5 convertase (MetCVF,Bb) of the alternative pathway. MetCVF,Bb and CVF,Bb were indistinguishable with respect to C3 cleavage. However, the C5-cleavage ability of MetCVF,Bb was significantly lower than that of CVF,Bb. These results suggest the involvement of Met in CVF binding of C5.

Mutation of an active site residue of tryptophan synthase (beta-serine 377) alters cofactor chemistry

To better understand how an enzyme controls cofactor chemistry, we have changed a tryptophan synthase residue that interacts with the pyridine nitrogen of the pyridoxal phosphate cofactor from a neutral Ser (beta-Ser377) to a negatively charged Asp or Glu. The spectroscopic properties of the mutant enzymes are altered and become similar to those of tryptophanase and aspartate aminotransferase, enzymes in which an Asp residue interacts with the pyridine nitrogen of pyridoxal phosphate. The absorption spectrum of each mutant enzyme undergoes a pH-dependent change (pKa approximately 7.7) from a form with a protonated internal aldimine nitrogen (lambdamax = 416 nm) to a deprotonated form (lambdamax = 336 nm), whereas the absorption spectra of the wild type tryptophan synthase beta2 subunit and alpha2 beta2 complex are pH-independent. The reaction of the S377D alpha2 beta2 complex with L-serine, L-tryptophan, and other substrates results in the accumulation of pronounced absorption bands (lambdamax = 498-510 nm) ascribed to quinonoid intermediates. We propose that the engineered Asp or Glu residue changes the cofactor chemistry by stabilizing the protonated pyridine nitrogen of pyridoxal phosphate, reducing the pKa of the internal aldimine nitrogen and promoting formation of quinonoid intermediates.

Mutations in the contact region between the alpha and beta subunits of tryptophan synthase alter subunit interaction and intersubunit communication

Interaction between the alpha and beta subunits of tryptophan synthase leads to mutual stabilization of the active conformations and to coordinated control of the activities of the two subunits. To elucidate the roles of specific residues in the interaction site between the alpha and beta subunits, mutant alpha and beta subunits were constructed, and the effects of mutation on subunit interaction and intersubunit communication were determined. Mutation of either alpha subunit Asp56 (alphaD56A) or beta subunit Lys167 (betaK167T), residues that interact in some crystal structures of the tryptophan synthase alpha2beta2 complex, decreases the ability of the alpha subunit to activate the beta subunit and alters the reaction and substrate specificity of the beta subunit. Partial conformational repair is provided by alpha-glycerol 3-phosphate, a ligand that binds to the alpha subunit, or by Cs+ or NH4+, ligands that bind to the beta subunit. Mutation of beta subunit Arg175 (betaR175A), a residue that interacts with alpha subunit Pro57 in some structures, has much smaller effects on activity but results in a 15-fold increase in the apparent Kd for dissociation of the alpha and beta subunits. Replacement of the single tryptophan in the beta subunit by phenylalanine (W177F) has only small effects on activity but increases the apparent subunit dissociation constant approximately 10-fold. The most important conclusions of this investigation are that interaction between alphaAsp56 and betaLys167 is important for intersubunit communication and that mutual stabilization of the active conformations of the two subunits is impaired by mutation of either residue.

A review of the effects of manipulation of the cysteine residues of rat aryl sulfotransferase IV

Aryl sulfotransferase IV from rat liver has the broad substrate range that is characteristic of the enzymes of detoxication. With the standard assay substrates, 4-nitrophenol and 3'-phosphoadenosine 5'-phosphosulfate (PAPS), sulfation is optimum at pH 5.4 whereas the reaction is minimal in the physiological pH range. These properties preclude a physiological function for this cytosolic enzyme. Partial oxidation of the enzyme, however, results not only in an increase in the rate of sulfation but also in a shift of the pH optimum to the physiological pH range. The mechanism for this dependence on the redox environment involves oxidation at Cys66, the cysteine residue that is conserved throughout the phenol sulfotransferase family. As documented by mass spectroscopic methods, oxidation by GSSG leads to the formation of an internal disulfide between Cys66 and Cys232; for mutants at Cys232, the oxidation product is a mixed disulfide of Cys66 and glutathione. Both of these disulfide species activate the enzyme and allow it to function at a pH optimum in the physiological range. The activated enzyme differs from the reduced form by a more circumscribed substrate spectrum. All five mutants, in which each of the cysteines of the sulfotransferase subunit have been changed to serine, are catalytically active. Only Cys66 is required for the redox response.

Dominant negative activity of mutant thyroid hormone alpha1 receptors from patients with hepatocellular carcinoma

Complementary DNAs for two mutant thyroid hormone alpha1 receptors (TR alpha1) were isolated from hepatocellular carcinomas of two patients. Sequence analyses of the complementary DNAs showed a single Val390Ala and double Pro398Ser/Glu350Lys mutations in mutants H and L, respectively. We characterized their hormone-binding, DNA-binding, and dominant negative activities. Mutants H and L did not bind the hormone T3. Their DNA-binding activities were analyzed using three types of thyroid hormone response elements (TREs) in which the half-site binding motifs are arranged in an everted repeat (Lys), an inverted repeat (Pal), or a direct repeat separated by four nucleotides (DR4). Compared with wild-type TR alpha1 (w-TR alpha1), which bound these TREs with different homodimer/monomer ratios, binding of mutant L to the three TREs as homodimers was reduced by approximately 90%. However, binding of mutant H to these TREs was more complex. Although it bound normally to DR4 as homodimers, its binding to Lys as homodimers was reduced by approximately 80%. Surprisingly, its binding to Pal was markedly enhanced compared with w-TR alpha1. The binding of these two mutants to the three TREs as heterodimers with retinoid X receptors (RXR alpha and -beta) was not significantly affected. Consistent with the lack of T3-binding activity, both mutants had lost their trans-activation capacity. Mutants H and L exhibited dominant negative activity, but differed in their TRE dependency. The dominant negative potency of mutant H was in the rank order of Pal > DR4 > Lys, whereas no TRE dependency was observed for mutant L. The present study indicates that mutations of the TR alpha gene do occur in patients and that these novel TR alpha1 mutants provide a valuable tool to further understand the molecular basis of the dominant negative action of mutant TRs.

The gene regulating activity of thyroid hormone nuclear receptors is modulated by cell-type specific factors

To understand whether the transcriptional activity of thyroid hormone nuclear receptors (TRs) is modulated by cell-type specific factors, full length TR subtype alpha1 (TRalpha1) and beta1 (TRbeta1) cDNAs were cloned from human hepatoma cell lines: HA22T, SK-Hep-1 and HepG2. The cloned receptor bound to the thyroid hormone 3,3',5-triiodo-L-thyronine (T3) and the thyroid hormone response elements (TREs) similarly to those cloned from other tissues. They exhibited T3- and TRE-dependent transactivation activities, indicating these TRs were transcriptionally active. The lipogenic malic enzyme (ME), a T3-target gene in liver, was stimulated approximately 3- and 1.5-fold by T3 in HA22T and SK-Hep-1, respectively. The T3-stimulated ME gene expression was inhibited in HA22T, but stimulated in SK-Hep-1 cells by insulin. These results suggest that the gene regulating activity of TRs was modulated by cell-type specific factors. Furthermore, these cell-type specific factors could modulate the cross talk between TR- and insulin receptor-mediated pathways.

Effect of burn patient serum on fibroblast and keratinocyte cell morphology

The effect of burn patient serum on fibroblast and keratinocyte cell morphology in culture was investigated using the scanning electron microscope. Serum was taken from five patients with burn injuries ranging from 8 to 65 per cent TBSA (10-65 per cent full-thickness). One patient had superficial burns. Pooled serum from 23 volunteers was used as the control serum. The cells were seeded onto collagen-coated glass coverslips and incubated for 5 days with culture medium containing 10 per cent (v/v) control serum or patient serum taken during the early postburn period. Scanning electron micrographs demonstrated a reduction in fibroblast cell density with serum from patients with full-thickness burns. Furthermore, the spindle shape of the fibroblast cell was greatly exaggerated compared with control cultures. The integrity of the keratinocyte sheet was destroyed when keratinocyte cells were incubated with serum fron patients with full-thickness burns. Globular-like structures or membrane protrusions were present in concentrated areas on keratinocyte cells which were not present in control cultures. This study demonstrated the vulnerability of cutaneous cells to systemic factors present in the early postburn serum. The extent of the effect appears to be related to the presence of full-thickness injury. This effect may further explain the frequent aberrant wound healing response to burn injury.

Critical role of glutamine 252 in the hormone-dependent transcriptional activity of the thyroid hormone beta1 nuclear receptor

To understand the molecular basis of the ligand-dependent transcriptional activity of thyroid hormone nuclear receptors (TRs), we investigated the effect of mutation of glutamine 252 (Q252) on the function of human TR subtype beta1 (wTRbeta1). Q252 is conserved in TRs in all species and is located in a region of the hormone binding domain that has been shown to undergo 3,3',5-triiodo-L-thyronine (T3) induced conformational changes. Q252 was mutated to Gly (Q252G) or Asn (Q252N) and their immunoreactivity, hormone, and DNA binding activities were characterized. Mutants Q252G and Q252N bound to T3 with similar affinity as the wTRbeta1. However, they failed to interact with a monoclonal anti-wTRbeta1 antibody whose epitope is located in the region of amino acids 248-256, suggesting that mutation of Q252 to Gly or Asn resulted in local structural alteration in TRbeta1. In addition, mutation of Glu to Gly or Asn led to increases in their binding to the thyroid hormone response elements (TREs) as homodimers and as heterodimers with the retinoid X receptor. Mutants Q252G and Q252N were more effective as repressors in the absence of T3, while both had a 1.5-2-fold higher T3-dependent transcriptional activity mediated by three TREs than the wTRbeta1. The increases in the transcriptional activity were not due to an increase in the expression of the mutant receptor proteins because the in vivo expression level of the mutant receptor proteins was identical to that of the wTRbeta1. Our data indicate that the T3-dependent transcriptional activity is not entirely dependent on the T3 binding activity of the receptor. The interplay of ligand and DNA binding domains plays a pivotal role in the transcriptional activity of the TRs.

Control of activity through oxidative modification at the conserved residue Cys66 of aryl sulfotransferase IV

Oxidation at Cys66 of rat liver aryl suflotransferase IV alters the enzyme's catalytic activity, pH optima and substrate specificity. Although this is a cytosolic detoxification enzyme, the pH optimum for the standard assay substrate 4-nitrophenol is at pH 5.5; upon oxidation, the optimum changes to the physiological pH range. The principal effect of the change in pH optimum is activation, which is manifest by an increase in K'cat without any major influence on substrate binding. In contrast, with tyrosine methyl ester as a substrate, the enzyme's optimum activity occurs at pH 8.0; upon oxidation, it ceases to be a substrate at any pH. The presence of Cys66 was essential for activation to occur, thereby providing a putative reason underlying the conserved nature of this cysteine throughout the phenol sulfotransferase family. Mapping of disulfides by mass spectrometry showed the critical event to be the oxidation of Cys66 to form a disulfide with either Cys232 or glutathione, either one is effective. These results point to a mechanism for regulating the activity of a key enzyme in xenobiotic detoxication during cellular oxidative stress.

The differential hormone-dependent transcriptional activation of thyroid hormone receptor isoforms is mediated by interplay of their domains

Human thyroid hormone nuclear receptor isoforms (TRalpha1 and TRbeta1) express differentially in a tissue-specific and development-dependent manner. It is unclear whether these two isoforms have differential functions. We analyzed their interaction with a thyroid hormone response element with half-site binding motifs arranged in an everted repeat separated by six nucleotides (F2). Despite extensive sequence homologies, the two isoforms bound to F2 with different affinities and ratios of homodimer/monomer. Using F2-containing reporter gene, we found that the transcriptional activity of TRbeta1 was approximately 6-fold higher than that of TRalpha1. The lower activity of TRalpha1 was not due to differences in expression of the two isoforms because similar nuclear localization patterns were observed. To understand the structural determinants responsible for these differences, we constructed chimeric receptors in which hinge regions (domain D), hormone binding domains (domain E), and domains (D + E) were sequentially interchanged and their activities were compared. Chimeric TRs containing the domains D, E or (D + E) of TRbeta1 showed increased propensities to form homodimers and mediated higher transactivation activities than TRalpha1. Thus, differential transactivation activities of TR isoforms are mediated by interplay of their domains and could serve as an important regulatory mechanism to achieve diversity and specificity of pleiotropic T3 effect.

Differential sensitivity of thyroid hormone receptor isoform homodimers and mutant heterodimers to hormone-induced dissociation from deoxyribonucleic acid: its role in dominant negative action

General resistance to thyroid hormone is an inheritable disease with resistance of peripheral tissues to elevated levels of thyroid hormone. Genetic studies have shown that it is due to interference in the functions of wild-type thyroid hormone nuclear receptors (wTRs) via the dominant negative effect of mutant TRs (mTRs). The present study compared the heterodimerization of the two TR isoforms, TR beta1 and TR alpha1, with mutant TRs to understand if mTRs had isoform-dependent dominant negative action. Using electrophoresis gel mobility shift assay, we have demonstrated that mutant PV, S, ED, and OK form heterodimers with wTR alpha1 and deltaTR beta1 (in which the A/B domain of wTR beta1 has been deleted), on the F2-thyroid hormone response element (TRE). In the presence of T3, both homo- and heterodimer complexes are dissociated in a T3 concentration dependent manner. The ED50 for deltaTR beta1 homodimers was 3-fold higher than that of wTR alpha1 homodimers. ED50s for deltaTR beta1/mTR heterodimers were 10- to 40-fold higher than the corresponding wTR alpha1/mTR heterodimers. Mutant ED and OK homodimers were only partially dissociated at the highest T3 concentrations used (100 nM), whereas no dissociation could be detected for PV and S homodimers, indicating differential sensitivity of the F2-bound TR dimers to the T3-induced dissociation. We presented a model that indicates the dissociation of any particular TR dimer from F2 is determined by competition of T3 for both of its constituent TRs. By transfection assays, we showed that the potency of the dominant negative action of PV on TR alpha1 and TR beta1 inversely correlated with the sensitivity of the appropriate mTR/wTR heterodimer to T3-induced dissociation from F2. The differential dominant negative action of mutants on the two TR isoforms could play an important role in the heterogeneity of tissue-specific manifestations in patients with resistance to thyroid hormone.

A three-domain T cell receptor is biologically active and specifically stains cell surface MHC/peptide complexes

We have expressed in bacteria a single-chain T cell receptor (scTCR) with specificity for an HIV gp120-derived peptide bound to the murine MHC-I molecule, H-2Dd. This scTCR consists of V alpha covalently linked to the VbetaCbeta domains that was solubilized, refolded, and purified in high yield. Specific binding of the scTCR to MHC/peptide complexes was demonstrated by surface plasmon resonance, with a Kd of 2 to 8 x 10(-6) M. This scTCR specifically inhibited T cell activation, and stained cell surface MHC/peptide complexes as measured by cytofluorimetry. The preservation of binding specificity by such a three-domain scTCR suggests that this structure is sufficient for specific MHC/peptide recognition and that this strategy will be of general use as applied to other TCR.

A three-domain T cell receptor is biologically active and specifically stains cell surface MHC/peptide complexes

We have expressed in bacteria a single-chain T cell receptor (scTCR) with specificity for an HIV gp120-derived peptide bound to the murine MHC-I molecule, H-2Dd. This scTCR consists of V alpha covalently linked to the VbetaCbeta domains that was solubilized, refolded, and purified in high yield. Specific binding of the scTCR to MHC/peptide complexes was demonstrated by surface plasmon resonance, with a Kd of 2 to 8 x 10(-6) M. This scTCR specifically inhibited T cell activation, and stained cell surface MHC/peptide complexes as measured by cytofluorimetry. The preservation of binding specificity by such a three-domain scTCR suggests that this structure is sufficient for specific MHC/peptide recognition and that this strategy will be of general use as applied to other TCR.

Two phenol sulfotransferase species from one cDNA: nature of the differences

A phenol sulfotransferase from rat liver (EC 2.8.2.9), expressed in Escherichia coli from a single cDNA, was purified as two separable but catalytically active proteins. The proteins appeared to be identical to each other and to the natural liver sulfotransferase by comparison of their amino acid constitution, amino-terminal end group, and interaction with a polyclonal antibody raised against the liver enzyme. Each of the recombinant forms, alpha and beta, catalyzed the sulfuryl group transfer from 4-nitrophenylsulfate to an acceptor phenol, a reaction in which 3'-phospho-adenosine 5'-phosphate (PAP) is a necessary intermediate. Only form beta, however, catalyzes the physiological transfer of a sulfuryl group from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to the free phenol. Evidence is presented that sulfotransferase alpha, but not beta, has 1 mol of PAP tightly bound per enzyme dimer. The ability to utilize PAPS as a sulfate donor could be altered: form alpha could be treated and purified as form beta to acquire the ability to use PAPS, whereas form beta was treated by extended incubation with PAP, lost its ability to use PAPS, and was purified as form alpha.

Detection and localization of synexin (Annexin VII) in Xenopus adult and embryonic tissues using an antibody to the Xenopus N-terminal PGQM repeat domain

Synexin (Annexin VII) is a widely distributed member of the annexin gene family which forms calcium channels and drives calcium-dependent membrane fusion. In Xenopus laevis, different synexins contain two to six tandem repeats of the tetra amino acid sequence PGQM in the unique N-terminal, with a distribution specific to adult tissues and embryonic stages. Immunogold studies using the PGQM-specific polyclonal antibody showed that synexin is localized in adult muscle to myosin-rich A-bands, Z-bands, and T-tubules, and in other adult tissues to nuclei and mitochondria and other formed elements. In oocytes, synexin was also found associated with yolk granules. The PGQM tandem repeats could represent interaction sites for other proteins, and we therefore synthesized a synthetic peptide containing the maximum six tandem repeats [NH2-(PGQM)6-Y-COOH] to test this hypothesis. We found that the peptide alone could specifically bind and crosslink to different proteins in a tissue-specific manner. In liver, it bound to a single 35-kDa protein. In muscle, it bound to four proteins (35, 45, 48, and 116 kDa). Therefore, we conclude that the PGQM domain is accessible to specific antibodies and that the PGQM repeat is sufficiently ordered to unambiguously identify specific binding proteins in different Xenopus tissues.

Mechanism of activation of the tryptophan synthase alpha2beta2 complex. Solvent effects of the co-substrate beta-mercaptoethanol

To characterize the conformational transitions that lead to activation of catalysis by the tryptophan synthase alpha2beta2 complex, we have determined the solvent effects of a co-substrate, beta-mercaptoethanol, and of a model nonsubstrate, ethanol, on the catalytic and spectroscopic properties of the enzyme. Our results show that ethanol and beta-mercaptoethanol both alter the equilibrium distribution of pyridoxal 5'-phosphate intermediates formed in the reactions of L-serine at the beta site in the alpha2beta2 complex. Addition of increasing concentrations of ethanol increases the proportion of the external aldimine of L-serine and decreases the proportion of the external aldimine of aminoacrylate. Low concentrations of the co-substrate beta-mercaptoethanol (Kd = approximately 13 mM) decrease the proportion of the external aldimine of aminoacrylate and induce formation of the quinonoid of S-hydroxyethyl-L-cysteine. Higher concentrations of beta-mercaptoethanol decrease the concentration of the quinonoid intermediate and increase the proportion of the external aldimine of L-serine. Data analysis shows that beta-mercaptoethanol and ethanol both interact or bind preferentially with the conformer of the enzyme that predominates when the aldimine of L-serine is formed and shift the equilibrium in favor of this conformer. We propose that a nonpolar region of the beta subunit, possibly the hydrophobic indole tunnel, becomes less exposed to solvent in the conformational transition that activates the alpha2beta2 complex.

Identification of naturally occurring dominant negative mutants of thyroid hormone alpha 1 and beta 1 receptors in a human hepatocellular carcinoma cell line

To understand the function of thyroid hormone nuclear receptors (TRs) in human hepatocellular carcinoma cells (HCC), we characterized the hormone binding and transactivational activity of TRs in a HCC cell line, J7. TR alpha 1 (J7-TR alpha 1) and TR beta 1 (J7-TR beta 1) complementary DNAs were cloned from this cell line, and the binding activity to the hormone response elements (TREs) and to the thyroid hormone, 3,3',5-triiodo-L-thyronine (T3) of the expressed TR proteins were evaluated. J7-TR alpha 1 and J7-TR beta 1 bound to TREs similarly as the TRs isolated from other tissues. However, J7-TR alpha 1 did not bind to T3, and J7-TR beta 1 bound to T3 with only about 10% the affinity of the wild-type TR beta 1. Sequencing of the complementary DNAs shows a single Met259Ile mutation in J7-TR alpha 1 and Met334Val in J7-TR beta 1. Using reporters containing TREs, we found that J7-TR alpha 1 and J7-TR beta 1 had virtually lost their transactivational activity. Moreover, these two mutants inhibited the transactivational activity of the wild-type TRs by a dominant negative effect not only on the transfected TRs, but also on endogenous TRs in other two HCC cell lines, SK-Hep-1 and HepG2. The potency of the dominant negative effect of these two mutants inversely correlated with the expression level of endogenous TRs. The present studies identified two novel naturally occurring TR mutants that have potent dominant negative action. The identification of both the alpha and beta dominant negative mutants in J7 made this cell line a useful model system to further understand the molecular mechanism of the dominant negative action of TR mutants.

A T cell receptor V alpha domain expressed in bacteria: does it dimerize in solution?

To evaluate the potential for dimerization through a particular T cell receptor (TCR) domain, we have cloned the cDNA encoding a TCR V alpha from a hybridoma with specificity for the human immunodeficiency virus (HIV) envelope glycoprotein 120-derived peptide P18-110 (RGPGRAFVTI) bound to the murine major histocompatibility complex (MHC) class I molecule, H-2Dd. This cDNA was then expressed in a bacterial vector, and protein, as inclusion bodies, was solubilized, refolded, and purified to homogeneity. Yield of the refolded material was from 10 to 50 mg per liter of bacterial culture, the protein was soluble at concentrations as high as 25 mg/ml, and it retained a high level of reactivity with an anti-V alpha 2 monoclonal antibody. This domain was monomeric both by size exclusion gel chromatography and by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Circular dichroism spectra indicated that the folded V alpha domain had secondary structure similar to that of single immunoglobulin or TCR domains, consisting largely of beta sheet. Conditions for crystallization were established, and at least two crystal geometries were observed: hexagonal bipyramids that failed to diffract beyond approximately 6 A, and orthorhombic crystals that diffracted to 2.5 A. The dimerization of the V alpha domain was investigated further by solution nuclear magnetic resonance spectroscopy, which indicated that dimeric and monomeric forms of the protein were about equally populated at a concentration of 1 mM. Thus, models of TCR-mediated T cell activation that invoke TCR dimerization must consider that some V alpha domains have little tendency to form homodimers or multimers.

Peptides corresponding to CD4-interacting regions of murine MHC class II molecules modulate immune responses of CD4+ T lymphocytes in vitro and in vivo

Immune responses mediated by CD4+ T cells depend on Ag-specific alpha beta TCRs that recognize the specific antigenic peptide presented by MHC class II molecules. Interactions between CD4 coreceptors and monomorphic regions of MHC class II molecules contribute to these responses. To examine whether immune reactions could be modulated by specifically interfering with CD4-MHC class II interactions, we have used, in various in vitro and in vivo assays, peptides that correspond to a region of MHC class II molecules previously shown to control interaction with CD4. Depending on the chemical nature and concentration of these peptides, they modulated Ag-specific responses of CD4+ T cells. At high concentrations, these peptides inhibited T cell responses in vitro. However, under conditions that can cause Ag-induced unresponsiveness, the peptides enhanced T cell responses. Also, primary in vivo immune responses to systemically administered soluble protein Ag, keyhole limpet hemocyanin, were enhanced when mice were treated with peptides corresponding to the CD4-interacting region of MHC class II molecules but not when treated with control peptides. Lymphokine profiles suggested that the peptides may favor the differentiation of Th1 cells, because lymphocytes from peptide-treated mice secreted more IL-2 and IFN-gamma than lymphocytes from nontreated or control-peptide-treated mice upon restimulation with Ag in vitro. These results demonstrate that MHC class II-derived peptides can directly interfere with interactions with CD4 and modulate T cell responses in vitro and in vivo.

Peptides corresponding to CD4-interacting regions of murine MHC class II molecules modulate immune responses of CD4+ T lymphocytes in vitro and in vivo

Immune responses mediated by CD4+ T cells depend on Ag-specific alpha beta TCRs that recognize the specific antigenic peptide presented by MHC class II molecules. Interactions between CD4 coreceptors and monomorphic regions of MHC class II molecules contribute to these responses. To examine whether immune reactions could be modulated by specifically interfering with CD4-MHC class II interactions, we have used, in various in vitro and in vivo assays, peptides that correspond to a region of MHC class II molecules previously shown to control interaction with CD4. Depending on the chemical nature and concentration of these peptides, they modulated Ag-specific responses of CD4+ T cells. At high concentrations, these peptides inhibited T cell responses in vitro. However, under conditions that can cause Ag-induced unresponsiveness, the peptides enhanced T cell responses. Also, primary in vivo immune responses to systemically administered soluble protein Ag, keyhole limpet hemocyanin, were enhanced when mice were treated with peptides corresponding to the CD4-interacting region of MHC class II molecules but not when treated with control peptides. Lymphokine profiles suggested that the peptides may favor the differentiation of Th1 cells, because lymphocytes from peptide-treated mice secreted more IL-2 and IFN-gamma than lymphocytes from nontreated or control-peptide-treated mice upon restimulation with Ag in vitro. These results demonstrate that MHC class II-derived peptides can directly interfere with interactions with CD4 and modulate T cell responses in vitro and in vivo.

Novel isoforms of synexin in Xenopus laevis: multiple tandem PGQM repeats distinguish mRNAs in specific adult tissues and embryonic stages

Synexin (annexin VII) is a calcium-dependent, phospholipid-binding and membrane fusion protein in the annexin gene family, which forms calcium channels and may play a role in exocytotic secretion. We report here the cloning and characterization of five novel isoforms of cDNAs encoding Xenopus synexin from brain, oocyte and stage 24 cDNA libraries. The most prevalent Xenopus synexin has 1976 bp of cDNA sequence, which contains a 1539 bp open reading frame of 512 amino acids encoding a 54 kDa protein. This Xenopus protein is 6 kDa larger than the previously reported human and mouse synexins with which it shares approx. 73% identity in the C-terminal region and approx. 44% identity in the N-terminal region. Further studies with PCR revealed the molecular basis of the substantial divergence in the Xenopus synexin's N-terminal domain. The domain equivalent to the mammalian tissue-specific cassette exon occurs at a different position and is variable in size and sequence. The most interesting observation relates to the occurrence of different forms of synexin due to the varying numbers of tandem PGQM repeats that are expressed differently in different adult tissues and embryonic stages. For these reasons we have labelled this set of unique isoforms annexin VIIb, referring to mammalian forms, which lack the PGQM tandem repeats, as annexin VIIa. In spite of these differences from annexin VIIa, the form of recombinant annexin VIIb with three PGQM repeats was found to be catalytically active. We interpret these results to indicate that the actual calcium and phospholipid binding sites are conserved in Xenopus, and that the variations observed between members of the synexin gene family in the regulatory domain clearly point towards the tissue- and stage-specific roles of individual members, possibly involving the exocytotic process.

Core sugar residues of the N-linked oligosaccharides of Russell's viper venom factor X-activator maintain functionally active polypeptide structure

We have previously showed that factor X activator of Russell's viper venom (RVV-X) contains six N-linked oligosaccharide chains: four in the heavy chain and one in each of the two light chains [Gowda, D.C., Jackson, C.M., Hensley, P., & Davidson, E.A. (1994) J. Biol. Chem. 269, 10644-10650]. In the present study, we have investigated the role of the carbohydrate moieties in the structure and functional activity of RVV-X. Sequential removal of sugar residues from the terminal ends by exoglycosidases, up to 50% of total carbohydrates, did not significantly alter the activity of RVV-X, demonstrating that the peripheral carbohydrate moieties are not involved in interactions with factor X. However, removal of whole oligosaccharide chains by N-glycanase caused an almost total loss of the ability of RVV-X to activate factor X to factor Xa. In parallel with these observations, circular dichroism spectroscopy showed that complete deglycosylation, but not the removal of peripheral sugars, caused a significant change in the secondary structure. Together, these data demonstrate that the oligosaccharide chains are necessary for the functional activity, and that the trimannosylchitobiose core residues are sufficient for the maintenance of the native polypeptide structure.

A thermally induced reversible conformational transition of the tryptophan synthase beta2 subunit probed by the spectroscopic properties of pyridoxal phosphate and by enzymatic activity

A reversible thermally induced conformational transition of the beta2 subunit of tryptophan synthase from Salmonella typhimurium has been detected by use of the pyridoxal 5'-phosphate coenzyme as a spectroscopic probe. Increasing the temperature converts the major form of pyridoxal 5'-phosphate bound to the beta2 subunit from a ketoenamine species with lambdamax at 410 nm to a enolimine species with lambdamax at 336 nm (Tm = approximately 43 degrees C) and results in loss of the circular dichroism signal at 410 nm and of fluorescence emission at 510 nm. The results indicate that increasing the temperature favors a conformer of the enzyme that binds pyridoxal 5'-phosphate in a more nonpolar environment and leads to loss of asymmetric pyridoxal 5'-phosphate binding. The internal aldimine between pyridoxal 5'-phosphate and the epsilon-amino group of lysine 87 is not disrupted by increased temperature because sodium borohydride treatment of the enzyme at either 15 or 60 degrees C results in covalent attachment of [4'-3H]pyridoxal 5'-phosphate. The thermal transition of the beta2 subunit below 60 degrees C produces reversible thermal inactivation (Ti = approximately 52 degrees C) and occurs at a much lower temperature than the major reversible unfolding at approximately 80 degrees C (Remeta, D. P., Miles, E. W., and Ginsburg, A. (1995) Pure Appl. Chem. 67, 1859-1866). Our new results indicate that the 410 nm absorbing species of pyridoxal 5'-phosphate is the catalytically active form of the cofactor in the beta2 subunit and that the low temperature reversible conformational transition disturbs the active site and causes loss of catalytic activity.

Understanding the molecular mechanism of dominant negative action of mutant thyroid hormone beta 1-receptors: the important role of the wild-type/mutant receptor heterodimer

The clinical manifestations of patients with resistance to thyroid hormone result from inhibition of the functions of wild-type thyroid hormone receptors (wTRs) by the dominant negative effect of mutant TR beta 1 receptors (mTR beta 1). One of the proposed mechanisms by which mTR beta 1 exerts its dominant negative action is via formation of the putative inactive wTR beta 1/mTR beta 1 heterodimer. However, the nature of the wTR beta 1/mTR beta 1 heterodimer is poorly understood. The present study characterizes the wTR beta 1/mTR beta 1 heterodimer by electrophoretic mobility shift assay. The mutant TR beta 1 used was PV, which contains a frame shift mutation in the C-terminal part of TR beta 1 and has less than 1% of the T3 binding affinity of the wTR beta 1. Because of the difficulty in resolving wTR beta 1 and mutant PV dimers, we used a truncated wTR beta 1 in which the A/B domain was deleted (delta TR beta 1) to demonstrate the formation of the heterodimer on thyroid hormone response elements (TREs) in which the half-site binding motifs are oriented in an inverted repeat (F2), a direct repeat separated by four nucleotides (DR4), or an inverted repeat (Pal). Deletion of the A/B domain had no effect on the binding of T3 and TREs to wTR beta 1. In the presence of equal amounts of delta TR beta 1 and PV, three types of molecular complexes. delta TR beta 1 homodimer, delta TR beta 1/PV heterodimer, and PV homodimer bound to each TRE in a ratio of approximately 1:2:1. The identities of these complexes were confirmed by their ability to be supershifted by anti-TR beta 1 and/or anti-PV antibodies. delta TR beta 1/PV heterodimer formation varied with different TREs. The ratio of apparent affinity constant (Ka) in the binding of delta TR beta 1/PV to TREs was F2:DR4:Pal = approximately 6:2:1. The effect of T3 on delta TR beta 1/PV heterodimer formation was TRE dependent. No T3-induced dissociation was observed for the delta TR beta 1/PV heterodimer when bound to F2 and Pal. In contrast, the delta TR beta 1/PV heterodimer bound to DR4 was dissociated by T3 with an ED50 of 3.9 +/- 0.9 nM. The T3-induced dissociation of delta TR beta 1 homodimer bound to F2, DR4, and Pal had ED50 values of 4.1 +/- 1.2, 1.3 +/- 0.3, and more than 100 nM, respectively. By transfection assays, the dominant negative action of PV was found to be TRE dependent with the rank order of F2 >> Pal > ME (a DR4-like TRE in the rat malic enzyme gene). Taken together, these results indicate a strong correlation between wTR beta 1/mTR beta 1 heterodimer formation and the dominant negative potency of PV. These results suggest that the wTR beta 1/mTR beta 1 heterodimer could play an important role in the dominant negative action of mTR beta 1.

On the one-sided action of amphotericin B on lipid bilayer membranes

The one-sided action of the polyene antibiotic, amphotericin B, on phospholipid bilayer membranes formed from synthetic phosphatidylcholines (DOPC and DPhPC) and sterols (ergosterol and cholesterol), has been investigated. We found formation of well-defined ionic channels for both sterols and not only for ergosterol-containing membranes (Bolard, J., P. Legrand, F. Heitz, and B. Cybulska. 1991. Biochemistry. 30:5707-5715). Characteristics of these channels were studied in the presence of different salts. It was found that the channels have comparable conductances but different lifetimes that are approximately 100-fold less in cholesterol-containing membranes than in ergosterol-containing ones. Channel blocking by tetraethylammonium (TEA) ions shows that TEA blockage of channels in the presence of cholesterol increases their lifetimes in analogy to the lengthening of lifetimes of protein channels blocked by local anesthetics (Neher, E., and J. H. Steinbach. 1978. J. Physiol. 277: 153-176). However, the effect of the blocker on single-channel conductance is very close for both sterols. The data support the classical model of amphotericin B pore formation from complexes initially lying on the membrane surface as nonconducting prepores. We explain the antibiotic's cytotoxic selectivity by differences in the lifetimes of the channels formed with different sterols and suggest that phosphatidylcholine-sterol membranes can be used as a tool for rapid estimation of polyene antibiotic cytotoxicity.