Functional studies on the secreted form of human C4 (C4s), two incompletely processed two-subunit C4 molecules (beta - alpha + gamma and beta + alpha - gamma), and pro-C4

The functional properties of the secreted form of C4 (C4s), which has a Mr approximately 5000 greater than the predominant C4 molecule found in plasma (C4p), two incompletely processed two-chain C4 molecules (beta - alpha + gamma and beta + alpha - gamma), and the extracellular C4 precursor (designated pro-C4(E)] were evaluated. All four molecules are secreted in parallel by a human hepatoma-derived cell line (Hep G2). Secretion of hemolytically active C4 is linear up to approximately 12 hr, peaks at 24 hr, and then progressively decreases over the next 48 hr. This loss of C4s functional activity parallels the proteolytic conversion of C4s to C4bs. To compare the hemolytic efficiencies of C4s and C4p, a solid-phase competitive radioimmunoassay was developed to permit measurement of the small quantities of C4 antigen in these cultures. The hemolytic efficiencies of C4s and C4p were similar. These results indicate that extracellular processing of C4s to C4p does not modulate the hemolytic activity of the molecule. Consistent with their ability to bind methylamine, both the alpha s-chain and the alpha - gamma subunit undergo denaturation-induced autolysis. The extracellular and intracellular pro-C4 molecules are also sensitive to autolytic cleavage. Interestingly, the beta - alpha subunit is resistant to autolysis. In experiments in which C4s and C4p were cleaved by C1-s to C4bs, C4(beta - alpha + gamma), C4(beta + alpha - gamma), and pro-C4(E) were resistant to C1-s cleavage and thus hemolytically inactive relative to C4s. These data indicate that processing of C4 to a three-chain structure is required to provide the proper conformation for efficient activation by C1.     

Mechanism-based fragmentation of coenzyme A transferase. Comparison of alpha 2-macroglobulin and coenzyme A transferase thiol ester reactions

The plasma proteins, alpha 2-macroglobulin and complement components 3 and 4, contain an internal thiol ester involving a glutamyl and cysteinyl residue. The thiol ester is susceptible to cyclization at greater than 37 degrees C and forms an unstable 5-oxyproline intermediate. The latter can be hydrolyzed to produce two peptide fragments. We propose that enzymes having activated glutamyl residues as part of their catalytic mechanisms may undergo an analogous cyclization and peptidyl cleavage. As a model, we have investigated pig heart succinyl-CoA:3-keto acid transferase. When the CoA-enzyme thiolester intermediate is heated at pH 7.4 and 70 degrees C for 1 h, approximately 60% of the Mr = 60,000 subunits are cleaved to give Mr = 40,000 and 20,000 fragments. We have shown that formation of the enzyme thiolester is an obligate precursor for the protein fragmentation. However, the reaction was incomplete with a maximum of approximately 65% cleavage at times greater than 60 min. These results suggest that there is a competing, deactivation reaction; namely, the thiol ester and oxyproline intermediates are hydrolyzed to regenerate the active site glutamic acid. Although the maximum rate of cleavage is at 70 degrees C, approximately 15% autolysis also occurs at 37 degrees C. The Mr = 40,000 fragment had the same amino terminal sequence as the Mr = 60,000 subunit, (Trp-Lys-Phe-Tyr-Thr-Asp-Ala-Val-Glu-Ala-). No amino terminal could be detected for the Mr = 20,000 fragment, even after digesting the fragment with pyroglutaminase. Peptide maps of the fragments and the uncleaved subunit indicate that the fragments are generated in parallel. The size of the fragments puts the active site about two-thirds of the way from the amino terminal of the protein.     

Heat-induced thiol-disulfide interchange reaction on the third component of human complement, C3

The third component of human complement, C3 is composed of two disulfide-bridged polypeptide chains of Mr 120,000 (alpha chain) and Mr 70,000 (beta chain). C3 has a thioester bond that serves as a binding site for targets when C3 is activated. Heat treatment of C3 induces autolytic peptide bond cleavage at the thioester site in the alpha chain as well as rupture of the thioester bond. The alpha chain fragments are linked to each other and beta chain via disulfide bonds. This study, however, documented that prolonged heating gave rise to liberation of several fragments including beta and the larger fragment of alpha chain. Using a fluorescent thiol reagent and [14C]iodoacetamide, we analyzed thiol residues present on each fragment, and elucidated that the thiol residue exposed by rupture of the thioester bond shifts in turn to another fragment resulting in the liberation of the fragments. The results were compatible with those on C4, and suggested that the generated thiol residue induces thiol-disulfide interchange reaction. On heating of plasma, fragments of C3 were not released, while the cleavage of the alpha chain occurred more effectively. The heated C3 (56 degrees C, 15 min) became insusceptible to C3b inactivator (I) and factor H, suggesting that additional conformational change is accompanied with cleavage of the thioester bond.     

Identification of major autolytic cleavage sites in the regulatory subunit of vascular calpain II. A comparison of partial amino-terminal sequences to deduced sequence from complementary DNA

Purified calpain II from vascular smooth muscle is a heterodimer consisting of catalytic (Mr = 76,000) and regulatory (Mr = 30,000) subunits. In the presence of Ca2+, the regulatory subunit undergoes stepwise autolysis resulting in enzyme activation. By slowing autoproteolysis, we identified major autolytic intermediates of the regulatory subunit. Gas-phase sequencing of the regulatory subunit and its autolytic fragments revealed that the NH2-terminus of the Mr = 30,000 form was blocked, whereas each fragment yielded a unique amino acid sequence, suggesting that autolysis proceeds in an NH2- to COOH-terminal direction. By comparison of actual amino acid sequences of autolytic cleavage intermediates to the full sequence deduced from cDNA, we have identified the major autolytic cleavage sites. Three different peptide bonds were cleaved, with neutral amino acids predominating on both sides of the peptide bond hydrolyzed. Importantly, leucine or isoleucine was identified in the second position upstream from the cleavage site in all three autolytic sequences. The presence of an upstream leucine residue in the autolytic cleavage sequence is reminiscent of the structure of potent microbial and synthetic peptide inhibitors of calpain.     

Hydrogen peroxide stabilizes the steroid-binding state of rat liver glucocorticoid receptors by promoting disulfide bond formation

Hydrogen peroxide and diamide inactivate the steroid-binding capacity of unoccupied glucocorticoid receptors in rat liver cytosol at 0 degrees C, and steroid-binding capacity is reactivated with dithiothreitol. Treatment of cytosol with peroxide or sodium molybdate, but not diamide, inhibits the irreversible inactivation (i.e., inactivation not reversed by dithiothreitol) of steroid-binding capacity that occurs when cytosol is incubated at 25 degrees C. Pretreatment of cytosol with the thiol derivatizing agent methyl methanethiosulfonate at 0 degrees C prevents the ability of peroxide, but not molybdate, to stabilize binding capacity at 25 degrees C. As derivatization of thiol groups prevents peroxide stabilization of steroid-binding capacity and as treatment with dithiothreitol reverses the effect, we propose that peroxide acts by promoting the formation of new disulfide linkages. The receptor in our rat liver cytosol preparations is present as three major degradation products of Mr 40,000, 52,000, and 72,000 in addition to the Mr 94,000 intact receptor. Like the intact receptor, these three forms exist in the presence of molybdate as an 8-9S complex, they bind glucocorticoid in a specific manner, and they copurify with the intact Mr 94,000 receptor on sequential phosphocellulose and DNA-cellulose chromatography. Despite the existence of receptor cleavage products, it is clear that peroxide does not stabilize steroid-binding capacity by inhibiting receptor cleavage.     

Altered levels and protein kinase C-mediated phosphorylation of substrates in normal and transformed mouse lung epithelial cells.

Protein phosphorylation and protein kinase C (PKC) levels were analyzed in intact cultures of spontaneously transformed, chemically transformed, and untransformed mouse pulmonary epithelial cell lines. It was found that although the transformed cell lines contained about 80% less protein kinase C, measured as total enzyme activity or binding of [3H]phorbol ester, phosphorylation events after phorbol ester treatment could still be easily detected. A commonly described Mr 80-kDa protein kinase C substrate (p80, 80 K, MARKS) was identified using 2D-PAGE, following phosphorylation in intact cells, and found to have reduced availability for phosphorylation in the transformed cell lines C4SE9, C1SA5 and NULB5 in comparison to the untransformed C4E10 and C1C10. Available levels of p80 were further analyzed in heat-denatured extracts from all cell lines using partially purified bovine brain PKC and correlated well with changes seen in intact cells. It was also noted that all transformed cell lines contained large amounts of a family of phosphoproteins of Mr 55-65 kDa, that could not be detected in the untransformed cell lines and whose phosphorylation state was increased by protein kinase C activation. This protein was found to be located in the nucleus. Hence, spontaneously and chemically transformed mouse pulmonary epithelial cells exhibit reduced levels of PKC, along with an altered pattern of PKC-mediated phosphorylation.     

Calcium depletion blocks proteolytic cleavages of plasma protein precursors which occur at the Golgi and/or trans-Golgi network. Possible involvement of Ca(2+)-dependent Golgi endoproteases.

The effects of calcium depletion on the proteolytic cleavage and secretion of plasma protein precursors were investigated in primary cultured rat hepatocytes and HepG2 cells. When the cells were incubated with A23187, the calcium-specific ionophore, in a medium lacking CaCl2, precursors of serum albumin and the third and fourth components of complement, C3 and C4, respectively, were found to be released into the medium. The addition of ionomycin or EGTA to the medium inhibited the processing of pro-C3 as well. Blocking the secretory pathway either at the mixed endoplasmic reticulum/Golgi in the presence of brefeldin A or at the endoplasmic reticulum/tubular-vesicular structure at a reduced temperature caused accumulation of pro-C3 within hepatocytes or HepG2 cells, indicating that the cleavage of the precursor occurs at a later stage of the secretory pathway. Once the blockade was released by incubating the cells either in the brefeldin A-free medium or at 37 degrees C, the secretion of plasma proteins resumed, irrespective of the presence of A23187. However, the processing of pro-C3 was almost completely inhibited in the presence of A23187, with only the precursor being released into the medium, implying that a decline in Ca2+ levels within the cell modulates the activity of a Golgi endoprotease responsible for the cleavage of pro-C3. When incubated with isolated Golgi membranes, pro-C3 secreted from Ca(2+)-depleted cells was cleaved in vitro into their subunits in the presence of Ca2+ but not in its absence, pointing to the involvement of a Ca(2+)-dependent Golgi endoprotease in the processing of pro-C3. These results collectively suggest that calcium depletion blocks the proteolytic cleavages of plasma protein precursors presumably by exhausting a Ca2+ pool available to the Ca(2+)-dependent processing enzyme(s) located at the Golgi and/or trans-Golgi network.     

Autolytic fragmentation of complement components C3 and C4 under denaturing conditions, a property shared with alpha 2-macroglobulin.

The alpha polypeptide chain of the complement protein C3 splits into two fragments of 74 000 and 46 000 apparent mol.wt. under certain conditions used to prepare the protein for SDS (sodium dodecyl sulphate)/polyacrylamide-gel electrophoresis. The cleavage reaction occurs over a wide range of temperatures and from pH 4.6 to 10.6 in the presence of denaturants such as urea, SDS and guanidine hydrochloride. It is also induced by heat-denaturation of C3 in the absence of chemical denaturants. The reaction occurs only with haemolytically active C3, and is not observed with hydroxylamine-inactivated C3 or with C3b. A similar cleavage of the alpha-chain of complement component C4 occurs under the same conditions, forming fragments of 53 000 and 41 000 apparent mol.wt. This reaction is again specific for haemolytically active C4, and does not occur with C4b or hydroxylamine-inactivated C4. The complement component C5, although structurally similar to C3 and C4, does not undergo a reaction of this type. The characteristics of the denaturation-induced cleavage of C3 and C4 match those described for the 'heat-induced' cleavage of alpha 2-macroglobulin [Harpel, Hayes & Hugli (1979) J. Biol. Chem. 254, 8669-8678]. Cleavage of alpha 2-macroglobulin is also specific for the active form of the protein, and does not occur with chemically inactivated or proteinase-cleaved forms. The unusual conditions and specificity of the peptide-bond cleavage in all three proteins suggest that it is an autolytic process rather than being the result of trace proteinase contamination. The active forms of C3, C4 and alpha 2-macroglobulin have the transient ability to form covalent bonds after activation. The autolytic cleavage reaction is likely to be related to the covalent-bond-forming reactions of these proteins.     

Isolation and characterization of an alpha-macroglobulin from the gastropod mollusc Helix pomatia with tetrameric structure and preserved activity after methylamine treatment

A proteinase inhibitor with M(r) 697000 and 20.3% (w/w) carbohydrate was isolated from the haemolymph of the snail Helix pomatia and characterized. It was shown to have a tetrameric structure with subunits disulphide linked by two. It inhibited the activity of several types of proteinases against large substrates but not that of trypsin against N-alpha-benzoyl-DL-arginine-4-nitroanilide. This indicated a nonspecific and steric hindrance mode of inhibition. The ratio of trypsin molecules inactivated per inhibitor amounted to 1.5. This interaction led to a cleavage of the subunits into two equal fragments and to a slow to fast conformational change of the whole molecules. Experiments with 125I-labelled trypsin indicated that the proteinase had become covalently linked to one of the fragments. Heating of the inhibitor led to autolytic cleavage products but not when methylamine treated. Thiol titration after trypsin or methylamine treatment indicated the presence of one thiol ester bond per subunit. These facts are all indicative of an alpha-macroglobulin type of inhibitor. However, unlike for most of them the methylamine treatment did not induce a conformational change nor suppress its proteinase inhibitory activity. Moreover, invertebrate alpha-macroglobulins are mostly dimeric in structure but tetramers likewise do occur in Biomphalaria glabrata.     

Identification of the insulin receptor in undifferentiated and differentiated NB-15 mouse neuroblastoma cells by affinity labeling

Plasma membranes prepared from clonal NB-15 mouse neuroblastoma cells were sequentially incubated with 125I-labeled insulin (10 nM) and the bifunctional cross-linking agent disuccinimidyl suberate. This treatment resulted in the cross-linking of 125I-labeled insulin to a polypeptide that gave an apparent Mr of 135 000 on a sodium dodecyl sulfate-polyacrylamide gel electrophoresed in the presence of 10% beta-mercaptoethanol. Affinity labeling of this polypeptide was inhibited by the presence of 5 microM unlabeled insulin, but not by 1 microM unlabeled nerve growth factor. Using the same affinity labeling technique, 125I-labeled nerve growth factor (1 nM) did not label any polypeptide appreciably in the plasma membranes of NB-15 cells but labeled an Mr 145 000 and an Mr 115 000 species in PC-12 rat pheochromocytoma cells. The number of insulin binding sites per cell in the intact differentiated NB-15 mouse neuroblastoma cells was approx. 6-fold greater than that in the undifferentiated NB-15 mouse neuroblastoma cells as measured by specific binding assay, suggesting an increase of the number of insulin receptors in NB-15 mouse neuroblastoma cells during differentiation.     

Light chain dependent effects of actin binding on the S-1/S-2 swivel in myosin

The S-1/S-2 swivel in myosin provides a flexible link between the head and tail portions of the molecule. We have investigated the properties of the swivel by employing limited proteolysis methods. Our results indicate that the binding of actin to heavy meromyosin inhibits both the chymotryptic and papain cleavage of the S-1/S-2 swivel, and that this effect is dependent on the presence of intact LC-2 light chains. Actin did not slow digestions carried out using heavy meromyosin previously treated with proteases to nick the LC-2 chains to 17,000 or 14,000 Mr fragments. Although the integrity of the LC-2 light chain appears to be required to transmit the effects of actin binding from the myosin head to the S-1/S-2 swivel, the binding of Ca2+ to the 17,000 Mr LC-2 fragment can still affect the chemical reactivity of SH1 thiol groups. Both chymotryptic and papain digestions of heavy meromyosin containing intact or fragmented LC-2 light chain show substantial temperature sensitivity between 5 degrees C and 35 degrees C. Calculated apparent activation energies for this process indicate that the S-1/S-2 swivel in myosin can undergo temperature-dependent structural changes independently of the state of the LC-2 light chain. Thus, both actin binding and temperature variations can induce structural transitions in the S-1/S-2 swivel.     

Subunit structure and dynamics of the insulin receptor

A model for the minimum subunit composition and stiochiometry of the physiologically relevant insulin receptor has been deduced based on results obtained by affinity labeling of this receptor in a variety of cell types and species. We propose that the receptor is a symmetrical disulfide-linked heterotetramer composed of two alpha (apparent Mr = 125,000) and two beta (apparent Mr = 90,000) glycoprotein subunits in the configuration (beta-S-S-alpha)-S-S-(alpha-S-S-beta). The disulfide or disulfides linking the two (alpha-S-S-beta) halves (class I disulfides) exhibit greater sensitivity to reduction by exogenous reductants than those linking the alpha and beta subunits (class II disulfides). When the class I disulfides are reduced by addition of diothiothreitol to intact cells, the receptor retains its ability to bind insulin and to effect a biological response. The beta subunit contains a site at about the center of its amino acid sequence that is extremely sensitive to proteolytic cleavage by elastaselike proteases, yielding a beta 1 fragment (Mr = 45,000) that remains disulfide linked to the receptor complex and a free beta 2 fragment. Binding of insulin to the receptor complex appears to result in the formation or stabilization of a new receptor conformation as evidenced by an altered susceptibility of the alpha subunit to exogenous trypsin. A receptor structure with high affinity for insulinlike growth factor (IGF) I and low affinity for insulin in fibroblast and placental membranes has also been affinity labeled. It exhibits the same structural features found for the insulin receptor, including two classes of disulfide bridges and beta subunits highly sensitive to proteolytic cleavage. These recent observations identifying the presence of distinct insulin and IGF-I receptors that share similar complex structures suggest that these hormones may also share common mechanisms of transmembrane signaling.     

Reactivity of the cysteine residues in the protein splicing active center of the Mycobacterium tuberculosis RecA intein

Protein splicing involves the self-catalyzed excision of an intervening polypeptide segment, an intein, from a precursor protein. The first two steps in the protein splicing process lead to the formation of ester intermediates through nucleophilic attacks by the side chains of cysteine, serine, or threonine residues adjacent to the splice junctions. Since both nucleophilic residues in the Mycobacterium tuberculosis RecA intein are cysteine, their reactivities could be compared by sulfhydryl group titration. This was accomplished by using fusion proteins containing a truncated RecA intein modified by mutation to prevent protein splicing, in which the cysteines at the splice junctions were the only sulfhydryl groups. The ability to undergo hydroxylamine-induced cleavage at the upstream splice junction showed that the modified intein was not impaired in the ability to form ester intermediates. Sulfhydryl titration with iodoacetamide, monitored by quantitating the residual thiols after reaction with a maleimide derivative of biotin, revealed a striking difference in the apparent pK(a) values of the cysteines at the two splice junctions. The apparent pK(a) of the cysteine at the upstream splice junction, which initiates the N-S acyl rearrangement leading to the linear ester intermediate, was approximately 8.2, whereas that of the cysteine residue at the downstream splice junction, which initiates the transesterification reaction converting the linear ester to the branched ester intermediate, was about 5.8. This suggests that the transesterification step is facilitated by an unusually low pK(a) of the attacking thiol group. Comparison of the rates of cleavage of the linear ester intermediates derived from the M. tuberculosis RecA and the Saccharomyces cerevisiae VMA inteins by dithiothreitol and hydroxylamine revealed that the former reacted relatively more slowly with dithiothreitol, suggesting that the RecA intein has diverged in the course of evolution to react preferentially with thiolate anions and thus lacks the basic groups that may facilitate nucleophilic attack by thiols in other inteins.     

Catalytic and structural properties of trypsin-treated 4-aminobutyrate aminotransferase

4-Aminobutyrate aminotransferase (EC 2.6.1.19, 4 aminobutyrate:2-oxoglutarate aminotransferase) is cleaved by trypsin, yielding an enzymatically active species which can be separated from the split peptides by gel filtration. The shortened enzyme derivative gives one band (Mr = 95,000) on polyacrylamide gradient gel electrophoresis. Changes in protein conformation induced by tryptic digestion were studied by fluorescence spectroscopy. The native enzyme tagged with the chromophore fluorescein yields a rotational relaxation time of 106 ns, whereas the trypsin-digested enzyme gives a rotational relaxation time of 33 ns. The decrease in rotational relaxation time is attributed to flexibility of the polypeptide chain with enhanced rotational freedom of the probe covalently linked to one thiol group. The reactivity of sulfhydryl groups toward 5,5'-dithiobis(2-nitrobenzoic acid) is also affected by trypsin cleavage. More--SH groups (2.6/dimer) become reactive toward 5,5'-dithiobis(2-nitrobenzoic acid) as a result of trypsin digestion. Local conformational fluctuations are induced as a result of tryptic cleavage, but the catalytic sites remain intact. The peptides released from 4-aminobutyrate aminotransferase were characterized by fingerprint analysis and their amino acid composition determined.     

Demonstration of insulin-like growth factor (IGF-I and -II) receptors and binding protein in human breast cancer cell lines

The insulin like growth factors (IGFs), potent mitogens for a variety of normal and transformed cells, have been reported to be secreted by several human breast cancer cell lines (BC). We have investigated the binding characteristics of IGF-I and -II in four human BC: MCF-7, T-47D, MDA 231 and Hs578T. Binding studies in microsomal membrane preparations detected high specific binding for both IGF in all four BC studied. Cross-linking with 125I-IGF-I, followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) under reduced conditions, revealed the presence of an alpha subunit of apparent Mr = 130,000 in MCF-7, T-47D and MDA 213 cells. When 125I-IGF-II was cross-linked, a major band of apparent Mr = 260,000 was seen in all BC. This band was inhibited by IGF-II, but not by insulin. Cross-linking of 125I-IGF-I to conditioned media from BC demonstrated the presence of three binding proteins of apparent Mr = 45,000, 36,000 and 29,000 in all BC but T-47D, in which the 36,000 band was not seen. These data demonstrate that BC possess classical receptors for both IGF-I and -II and, furthermore, that BC produce specific binding proteins for these growth factors.     

Fragmentation of fibronectin by inherent autolytic and matrix metalloproteinase activities

Fibronectin (FN) purified by gelatin affinity chromatography is unstable and undergoes fragmentation. The cleavage has been ascribed to inherent autolytic protease activities as well as co-purified matrix metalloproteinases (MMP). Understanding the mechanism by which the proteolysis of FN occurs is important, because the FN fragments have biological activities that differ from those of intact FN. Having excluded contributions of other plasma-derived proteases, the present experiments demonstrated that cleavage of FN by MMP-2 to distinct fragments occurred in synergy with inherent FN activities. Limited heat treatment of FN at 56 °C for 30 min inactivated the inherent protease activities sharply reducing autolysis of FN in a manner similar to that seen in the presence of serine proteinase inhibitors. Heat treatment did not alter cell attachment to FN, but significantly increased the susceptibility of FN to enzymatic cleavage by MMP-2. The carboxyl-terminal hemopexin-like domain (PEX) of MMP-2 was shown to possess critical exodomain properties required for the interactions of MMP-2 with FN, and FN was cleaved at a significantly reduced rate by an MMP-2 variant with deletion of PEX. Verifying the specificity of interactions, isolated PEX competed FN cleavage by MMP-2 in a concentration-dependent manner. These results have further elucidated the synergistic contributions of inherent autolytic serine protease-like activities and MMP-2 to fragmentation of FN and provide the rationale and basis for modified preparation and handling of FN used in biological research.     

Differences in the proteinase inhibition mechanism of human alpha 2-macroglobulin and pregnancy zone protein.

Different conformational states of human alpha 2-macroglobulin (alpha 2M) and pregnancy zone protein (PZP) were investigated following modifications of the functional sites, i.e. the 'bait' regions and the thiol esters, by use of chymotrypsin, methylamine and dinitrophenylthiocyanate. Gel electrophoresis, mAb (7H11D6 and alpha 1:1) and in vivo plasma clearance were used to describe different molecular states in the proteinase inhibitors. In alpha 2M, in which the thiol ester is broken by binding of methylamine and the 'trap' is closed, cyanylation of the liberated thiol group from the thiol ester modulates reopening of the 'trap' and the 'bait' regions become available for cleavage again. The trapping of proteinases in the cyanylated derivative indicates that the trap functions as in native alpha 2M. In contrast, cyanylation has no effect on proteinase-treated alpha 2M. As demonstrated by binding to mAb, the methylamine and dinitrophenylthiocyanate-treated alpha 2M exposes the receptor-recognition site, but the derivative is not cleared from the circulation in mice. The trap is not functional in PZP. In native PZP and PZP treated with methylamine, the conformational states seem similar. The receptor-recognition sites are not exposed and removal from the circulation in vivo is not seen for these as for the PZP-chymotrypsin complex. Tetramers are only formed when proteinases can be covalently bound to the PZP. Conformational changes are not detected in PZP derivatives in which the thiol ester is treated with methylamine and dinitrophenylthiocyanate. The results suggest that the conformational changes in alpha 2M are generated by mechanisms different to these in PZP. The key structure gearing the conformational changes in alpha 2M is the thiol ester, by which the events 'trapping' and exposure of the receptor-recognition site can be separated. In PZP, the crucial step for the conformational changes is the cleavage of the 'bait' region, since cleavage of the thiol ester does not lead to any detectable conformational changes by the methods used.     

Redox modulation of integrin [correction of integin] alpha IIb beta 3 involves a novel allosteric regulation of its thiol isomerase activity

The molecular mechanisms involved in regulating the activation-dependent conformational switch in integrins are not known although recent evidence suggests that integrins are a direct target for redox modulation. We have identified an endogenous integrin thiol isomerase activity that may be responsible for regulating integrin activation states. The purpose of this study was to examine the effects of redox conditions elicited by nitric oxide and glutathione on the thiol isomerase activity of the platelet integrin alphaIIbbeta3 and also on the activation status of this integrin in intact platelets. The universal integrin activator, Mn2+, stimulates the thiol isomerase activity in purified alphaIIbbeta3. Kinetic analysis reveals that alphaIIbbeta3 is an allosteric enzyme which displays positive cooperativity in the presence of Mn2+ with an apparent Hill coefficient of 1.9. Also, addition of Mn2+ to platelets results solely in activation of the integrin as demonstrated by the binding of the antibody PAC-1. The addition of the nitric oxide donors SNP, SIN-1, and SNOAC in combination with glutathione can directly reverse the activation state of the platelet integrin induced by Mn2+. These compounds have no effect on platelet secretory responses indicating a direct effect on the integrin. In the presence of nitric oxide and glutathione, the enzymatic activity of alphaIIbbeta3 also displays positive cooperativity (apparent Hill coefficient of 1.9), and a significant increase in the saturability of the enzyme was observed. Thus, redox agents simultaneously modulate the thiol isomerase activity of purified alphaIIbbeta3 and its active conformation in intact platelets, suggesting a molecular mechanism for integrin regulation.     

Location of the interchain disulfide bonds of the fourth component of human complement (C4): evidence based on the liberation of fragments secondary to thiol-disulfide interchange reactions

Treatment of human C4 with chemical denaturants and heat produces rapid, autolytic peptide bond cleavage of the alpha-chain. These alpha-chain fragments are linked to the parent C4 molecule through disulfide bonds. On more prolonged heating, however, there is liberation of several peptides, including the beta-chain, the gamma-chain, and a C-terminal alpha-chain fragment. This reaction is inhibited by iodoacetamide. By using a fluorescent thiol reagent and 14C-iodoacetamide, the thiol group present on each peptide was analyzed. The results suggest that the thiol residue exposed by cleavage of the thioester bond induces thiol-disulfide interchange reactions to liberate the peptides. Based on the identification of fragments liberated, the kinetics of their appearance, their sulfhydryl content, and the reported primary structure of human C4, a model of the interchain disulfide bonds is proposed in which the amino terminal portion of the alpha-chain is disulfide-linked to both the beta- and gamma-chains, whereas the carboxyl terminal portion of the alpha-chain is disulfide-linked to only the gamma-chain.