C1 inhibitor serpin domain structure reveals the likely mechanism of heparin potentiation and conformational disease

C1 inhibitor, a member of the serpin family, is a major down-regulator of inflammatory processes in blood. Genetic deficiency of C1 inhibitor results in hereditary angioedema, a dominantly inheritable, potentially lethal disease. Here we report the first crystal structure of the serpin domain of human C1 inhibitor, representing a previously unreported latent form, which explains functional consequences of several naturally occurring mutations, two of which are discussed in detail. The presented structure displays a novel conformation with a seven-stranded beta-sheet A. The unique conformation of the C-terminal six residues suggests its potential role as a barrier in the active-latent transition. On the basis of surface charge pattern, heparin affinity measurements, and docking of a heparin disaccharide, a heparin binding site is proposed in the contact area of the serpin-proteinase encounter complex. We show how polyanions change the activity of the C1 inhibitor by a novel "sandwich" mechanism, explaining earlier reaction kinetic and mutagenesis studies. These results may help to improve therapeutic C1 inhibitor preparations used in the treatment of hereditary angioedema, organ transplant rejection, and heart attack.     

Trapping a 96 degrees domain rotation in two distinct conformations by engineered disulfide bridges

Engineering disulfide bridges is a common technique to lock a protein movement in a defined conformational state. We have designed two double mutants of Escherichia coli 5'-nucleotidase to trap the enzyme in both an open (S228C, P513C) and a closed (P90C, L424C) conformation by the formation of disulfide bridges. The mutant proteins have been expressed, purified, and crystallized, to structurally characterize the designed variants. The S228C, P513C is a double mutant crystallized in two different crystal forms with three independent conformers, which differ from each other by a rotation of up to 12 degrees of the C-terminal domain with respect to the N-terminal domain. This finding, as well as an analysis of the domain motion in the crystal, indicates that the enzyme still exhibits considerable residual domain flexibility. In the double mutant that was designed to trap the enzyme in the closed conformation, the structure analysis reveals an unexpected intermediate conformation along the 96 degrees rotation trajectory between the open and closed enzyme forms. A comparison of the five independent conformers analyzed in this study shows that the domain movement of the variant enzymes is characterized by a sliding movement of the residues of the domain interface along the interface, which is in contrast to a classical closure motion where the residues of the domain interface move perpendicular to the interface.     

Analysis of the alphaB-crystallin domain responsible for inhibiting tubulin aggregation

The cytoskeleton has a unique property such that changes of conformation result in polymerization into a filamentous form. alphaB-Crystallin, a small heat shock protein (sHsp), has chaperone activities for various substrates, including proteins constituting the cytoskeleton, such as actin; intermediate filament; and tubulin. However, it is not clear whether the "alpha-crystallin domain" common to sHsps also has chaperone activity for the protein cytoskeleton. To investigate the possibility that the C-terminal alpha-crystallin domain of alpha-crystallin has the aggregation-preventing ability for tubulin, we constructed an N-terminal domain deletion mutant of alphaB-crystallin. We characterized its structural properties and chaperone activities. Far-ultraviolet (UV) circular dichroism measurements showed that secondary structure in the alpha-crystallin domain of the deletion mutant is maintained. Ultracentrifuge analysis of molecular masses indicated that the deletion mutant formed smaller oligomers than did the full-length protein. Chaperone activity assays demonstrated that the N-terminal domain deletion mutant suppressed heat-induced aggregation of tubulin well. Comparison of chaperone activities for 2 other substrates (citrate synthase and alcohol dehydrogenase) showed that it was less effective in the suppression of their aggregation. These results show that alphaB-crystallin recognizes a variety of substrates and especially that alpha-crystallin domain binds free cytoskeletal proteins. We suggest that this feature would be advantageous in its functional role of holding or folding multiple proteins denatured simultaneously under stress conditions.     

The extreme C terminus of rat liver carnitine palmitoyltransferase I is not involved in malonyl-CoA sensitivity but in initial protein folding

We previously reported that the N-terminal domain (1-147 residues) of rat liver carnitine palmitoyltransferase I (L-CPTI) was essential for import into the outer mitochondrial membrane and for maintenance of a malonyl-CoA-sensitive conformation. Malonyl-CoA binding experiments using mitochondria of Saccharomyces cerevisiae strains expressing wild-type L-CPTI or previously constructed chimeric CPTs (Cohen, I., Kohl, C., McGarry, J.D., Girard, J., and Prip-Buus, C. (1998) J. Biol. Chem. 273, 29896-29904) indicated that the N-terminal domain was unable, independently of the C-terminal domain, to bind malonyl-CoA with a high affinity, suggesting that the modulation of malonyl-CoA sensitivity occurred through N/C intramolecular interactions. To assess the role of the C terminus in malonyl-CoA sensitivity, a series of C-terminal deletion mutants was generated. The kinetic properties of Delta772-773 and Delta767-773 deletion mutants were similar to those of L-CPTI, indicating that the last two highly conserved Lys residues in all known L-CPTI species were not functionally essential. By contrast, Delta743-773 deletion mutant was totally inactive and unfolded, as shown by its sensitivity to trypsin proteolysis. Because the C terminus of the native folded L-CPTI could be cleaved by trypsin without inducing protein unfolding, we concluded that the last 31 C-terminal residues constitute a secondary structural determinant essential for the initial protein folding of L-CPTI.     

The C-D interhelical domain of the serpin plasminogen activator inhibitor-type 2 is required for protection from TNF-alpha induced apoptosis

The serine proteinase inhibitor (serpin), plasminogen activator inhibitor type 2 (PAI-2), has been reported to inhibit tumor necrosis factor-alpha (TNF) induced apoptosis. In order to begin to understand the molecular basis for this protection, we have investigated the importance of a structural domain within the PAI-2 molecule, the C-D interhelical region, in mediating the protective effect. The C-D interhelical region is a 33 amino acid insertion which is unique among serpins and has been implicated in transglutaminase catalyzed cross-linking of PAI-2 to cell membranes. We have constructed a mutant of PAI-2 wherein 23 amino acids are deleted from the C-D interhelical region generating a structure predicted to be homologous to the closely related, but non-inhibitory serpin, chicken ovalbumin. The PAI-2Delta65/87 deletion mutant retained inhibitory activity against its known serine proteinase target, urokinase-type plasminogen activator (uPA); however expression of this mutant in HeLa cells failed to protect from TNF-induced apoptosis. Analyses of the cellular distribution of PAI-2 showed that intracellular PAI-2, and not secreted or cell-surface PAI-2, was likely responsible for the observed protection from TNF-induced apoptosis. No evidence was found for specific cross-linking of PAI-2 to the plasma membrane in either control or TNF/cycloheximide treated cells. The data demonstrate that the PAI-2 C-D interhelical domain is functionally important in PAI-2 protection from TNF induced apoptosis and suggest a novel function for the C-D interhelical domain in the protective mechanism.     

Partial activation of antithrombin without heparin through deletion of a unique sequence on the reactive site loop of the serpin.

Native antithrombin (AT) has an inactive reactive site loop conformation unless it is activated by a unique pentasaccharide fragment of heparin (H(5)). Structural data suggests that this may be due to preinsertion of two N-terminal residues of the reactive site loop of the serpin into the A-beta-sheet of the molecule. Relative to alpha(1)-antitrypsin, the reactive site loop of AT has three additional residues, Arg(399), Val(400), and Thr(401), at the C-terminal P' end of the loop. To determine whether a longer reactive site loop of AT is responsible for loop preinsertion in the native conformation, mutants of the serpin were expressed in which these residues were individually or in combination deleted. Kinetic analysis suggested that deletion of two residues, Val(400) and Thr(401), changed the solution equilibrium of the serpin in favor of the active conformation, thereby enhancing the inhibition of factor Xa by an order of magnitude independent of H(5). Interestingly, the reactivity of this mutant with thrombin was impaired by the same order of magnitude in the absence, but not in the presence of H(5). These results suggest that a longer reactive site loop in AT is responsible for its inactive native conformation toward factor Xa, while at same time AT requires this feature to regulate the activity of thrombin.     

Dissecting the role of the N-terminal domain of human immunodeficiency virus integrase by trans-complementation analysis

The human immunodeficiency virus (HIV) integrase protein (IN) catalyzes two reactions required to integrate HIV DNA into the human genome: 3' processing of the viral DNA ends and integration. IN has three domains, the N-terminal zinc-binding domain, the catalytic core, and the C-terminal SH3 domain. Previously, it was shown that IN proteins mutated in different domains could complement each other. We now report that this does not require any overlap between the two complementing proteins; an N-terminal domain, provided in trans, can restore IN activity of a mutant lacking this domain. Only the zinc-coordinating form of the N-terminal domain can efficiently restore IN activity of an N-terminal deletion mutant. This suggests that interaction between different domains of IN is needed for functional multimerization. We find that the N-terminal domain of feline immunodeficiency virus IN can support IN activity of an N-terminal deletion mutant of HIV type 2 IN. These cross-complementation experiments indicate that the N-terminal domain contributes to the recognition of specific viral DNA ends.     

Recombinant human C1-inhibitor produced in Pichia pastoris has the same inhibitory capacity as plasma C1-inhibitor

Therapeutic application of the serpin C1-inhibitor (C1-Inh) in inflammatory diseases like sepsis, acute myocardial infarction and vascular leakage syndrome seems promising, but large doses may be required. Therefore, a high-yield recombinant expression system for C1-Inh is very interesting. Earlier attempts to produce high levels of C1-Inh resulted in predominantly inactive C1-Inh. We describe the high yield expression of rhC1-Inh in Pichia pastoris, with 180 mg/l active C1-Inh at maximum. On average, 30 mg/l of 80-100% active C1-Inh was obtained. Progress curves were used to study the interaction with C1s, kallikrein, coagulation factor XIIa and XIa, and demonstrated that rhC1-Inh had the same inhibitory capacity as plasma C1-Inh. Structural integrity, as monitored via heat stability, was comparable despite differences in extent and nature of glycosylation. We conclude that the P. pastoris system is capable of high-level production of functionally and structurally intact human C1 inhibitor.     

Importance of the release of strand 1C to the polymerization mechanism of inhibitory serpins.

Serpin polymerization is the underlying cause of several diseases, including thromboembolism, emphysema, liver cirrhosis, and angioedema. Understanding the structure of the polymers and the mechanism of polymerization is necessary to support rational design of therapeutic agents. Here we show that polymerization of antithrombin is sensitive to the addition of synthetic peptides that interact with the structure. A 12-m34 peptide (homologous to P14-P3 of antithrombin reactive loop), representing the entire length of s4A, prevented polymerization totally. A 6-mer peptide (homologous to P14-P9 of antithrombin) not only allowed polymerization to occur, but induced it. This effect could be blocked by the addition of a 5-mer peptide with s1C sequence of antithrombin or by an unrelated peptide representing residues 26-31 of cholecystokinin. The s1C or cholecystokinin peptide alone was unable to form a complex with native antithrombin. Moreover, an active antitrypsin double mutant, Pro 361-->Cys, Ser 283-->Cys, was engineered for the purpose of forming a disulfide bond between s1C and s2C to prevent movement of s1C. This mutant was resistant to polymerization if the disulfide bridge was intact, but, under reducing conditions, it regained the potential to polymerize. We have also modeled long-chain serpin polymers with acceptable stereochemistry using two previously proposed loop-A-sheet and loop-C-sheet polymerization mechanisms and have shown both to be sterically feasible, as are "mixed" linear polymers. We therefore conclude that the release of strand 1C must be an element of the mechanism of serpin polymerization.     

Homology modeling and molecular dynamics simulations of the N-terminal domain of wheat high molecular weight glutenin subunit 10

High molecular weight glutenin subunits (HMW-GS) are of a particular interest because of their biomechanical properties, which are important in many food systems such as breadmaking. Using fold-recognition techniques, we identified a fold compatible with the N-terminal domain of HMW-GS Dy10. This fold corresponds to the one adopted by proteins belonging to the cereal inhibitor family. Starting from three known protein structures of this family as templates, we built three models for the N-terminal domain of HMW-GS Dy10. We analyzed these models, and we propose a number of hypotheses regarding the N-terminal domain properties that can be tested experimentally. In particular, we discuss two possible ways of interaction between the N-terminal domains of the y-type HMW glutenin subunits. The first way consists in the creation of interchain disulfide bridges. According to our models, we propose two plausible scenarios: (1) the existence of an intrachain disulfide bridge between cysteines 22 and 44, leaving the three other cysteines free of engaging in intermolecular bonds; and (2) the creation of two intrachain disulfide bridges (involving cysteines 22-44 and cysteines 10-55), leaving a single cysteine (45) for creating an intermolecular disulfide bridge. We discuss these scenarios in relation to contradictory experimental results. The second way, although less likely, is nevertheless worth considering. There might exist a possibility for the N-terminal domain of Dy10, Nt-Dy10, to create oligomers, because homologous cereal inhibitor proteins are known to exist as monomers, homodimers, and heterooligomers. We also discuss, in relation to the function of the cereal inhibitor proteins, the possibility that this N-terminal domain has retained similar inhibitory functions.     

Movement of the helical domain of the epsilon subunit is required for the activation of thermophilic F1-ATPase

The inhibitory effect of epsilon subunit in F(1)-ATPase from thermophilic Bacillus PS3 was examined focusing on the structure-function relationship. For this purpose, we designed a mutant for epsilon subunit similar to the one constructed by Schulenberg and Capaldi (Schulenberg, B., and Capaldi, R. A. (1999) J. Biol. Chem. 274, 28351-28355). We introduced two cysteine residues at the interface of N-terminal beta-sandwich domain (S48C) and C-terminal alpha-helical domain (N125C) of epsilon subunit. The alpha(3)beta(3)gammaepsilon complex containing the reduced form of this mutant epsilon subunit showed suppressed ATPase activity and gradual activation during the measurement. This activation pattern was similar to the complex with the wild type epsilon subunit. The conformation of the mutant epsilon subunit must be fixed and similar to the reported three-dimensional structure of the isolated epsilon subunit, when the intramolecular disulfide bridge was formed on this subunit by oxidation. This oxidized mutant epsilon subunit could form the alpha(3)beta(3)gammaepsilon complex but did not show any inhibitory effect. The complex was converted to the activated state, and the cross-link in the mutant epsilon subunit in the complex was efficiently formed in the presence of ATP-Mg, whereas no cross-link was observed without ATP-Mg, suggesting the conformation of the oxidized mutant epsilon subunit must be similar to that in the activated state complex. A non-hydrolyzable analog of ATP, 5'-adenylyl-beta,gamma-imidodiphosphate, could stimulate the formation of the cross-link on the epsilon subunit. Furthermore, the cross-link formation was stimulated by nucleotides even when this mutant epsilon subunit was assembled with a mutant alpha(3)beta(3)gamma complex lacking non-catalytic sites. These results indicate that binding of ATP to the catalytic sites induces a conformational change in the epsilon subunit and triggers transition of the complex from the suppressed state to the activated state.     

Disulfide bond assignment and identification of regions required for functional activity of oncostatin M

Oncostatin M is a polypeptide cytokine having unique structure and diverse biological activities, including the ability to inhibit growth of certain cultured tumor cells. Here we have determined the disulfide bonding pattern of recombinant oncostatin M and have used site-directed mutagenesis to identify regions of this molecule necessary for receptor binding and growth inhibitory activities. Two intramolecular disulfide bonds, C6-C127 and C49-C167, were identified in recombinant oncostatin M. Analysis of mutations at each of the five cysteines in oncostatin M indicated that mutants C49S and C167S were inactive (less than 1/10 wild type activity) in growth inhibitory assays and radioreceptor assays. Carboxyl-terminal deletion mutations terminating at S185 and beyond were active, but further shortening abolished activity in both assays. Two deletion mutants proximal to C49 (delta 22-36 and delta 44-47) and insertion mutant GAG77 also were inactive. One deletion mutant, delta 87-90, had significantly (approximately 3-fold) increased activities in both growth inhibitory assays and radioreceptor assays. A potential amphiphilic domain was identified beginning at C167 and extending toward the carboxyl terminus. Two mutants having altered hydrophobic residues within this domain (F176G and F184G) were inactive, suggesting that these residues are required for proper conformation of the receptor binding site. Taken together, these results indicate that biological activity of oncostatin M requires discontinuous regions of the molecule, including residues near the essential disulfide bond, C49-C167, and within a putative amphiphilic helix at the carboxyl terminus. Oncostatin M thus belongs to a growing family of cytokines whose interactions with their respective receptors are mediated in part by known or predicted carboxyl-terminal amphiphilic helices.     

Identification of factor Xa residues critical for interaction with protein Z-dependent protease inhibitor: both active site and exosite interactions are required for inhibition

Protein Z-dependent protease inhibitor (ZPI) is a plasma serpin, which can rapidly inactivate factor Xa (fXa) in the presence of protein Z (PZ), negatively charged phospholipids, and Ca2+. To investigate the mechanism by which ZPI inactivates fXa, we expressed the serpin in mammalian cells and characterized its reactivity with both wild-type and selected mutants of fXa that 1) contained substitutions in the autolysis loop and the heparin binding exosite, 2) lacked the first EGF-like domain (fXa-des-EGF-1), or 3) contained the Gla domain of protein C (fXa/PC-Gla). Inhibition studies in both the presence and absence of PZ revealed that Arg-143, Lys-147, and Arg-154 of the autolysis loop and Lys-96, Lys-169, and Lys-236 of the heparin binding exosite are required for recognition of ZPI, with Arg-143 being essential for the interaction. Similar studies with fXa-des-EGF-1 and fXa/PC-Gla suggested that protein-protein interaction with either the Gla or the EGF-1 domain may not play a dominant role in the PZ-dependent recognition of fXa by the serpin on phospholipid vesicles. Further studies showed that an inactive Ser-195 to Ala mutant of fXa effectively competes with wild-type fXa for binding to the non-serpin inhibitors tissue factor pathway inhibitor and recombinant tick anticoagulant peptide, but does not compete for binding to ZPI. This suggests that the catalytic residue of fXa is required for interaction with ZPI.     

Determinants of the trans-dominant negative effect of truncated forms of the CCR5 chemokine receptor

The human immunodeficiency virus, type 1 (HIV-1) entry process is triggered by interaction between the viral envelope and a seven membrane-spanning domain receptor at the cell surface, usually the CCR5 chemokine receptor. Different naturally occurring mutations in the CCR5 gene abolish receptor function, the most frequent being a 32-nucleotide deletion resulting in a truncated protein (Delta32) lacking the last three transmembrane domains (TM5-7). This mutant is retained in the endoplasmic reticulum and exerts a trans-dominant negative (TDN) effect on the wild type, preventing its exit from this compartment. This TDN effect is often considered as evidence for the oligomerization of CCR5 during transport to the cell surface. Here we use a genetic approach to define the structural determinants of the TDN effect of the Delta32 mutant. It was abolished by certain deletions and by mutations of cysteine residues preventing formation of a disulfide link between the first and second extracellular loops, suggesting that conformation of Delta32 is important for its interaction with CCR5. To circumvent this problem, we used chimeric forms of the Delta32 and wild type CCR5, consisting in substitutions with homologous domains from the mouse CCR5. All chimeric full-length receptors were expressed at the cell surface and were functional for interaction with HIV-1 or with a chemokine ligand, when assayed. The TDN effect was only observed if both the TM3 domain in CCR5 and the TM4 domain in Delta32 were from human origin, whereas the rest of the proteins could be from either origin. This suggests that the TDN effect involves some form of interaction between these transmembrane domains. Alternatively, but less likely to us, substitutions in TM4 could affect the conformation of CCR5 in the endoplasmic reticulum but not at the cell surface. However that may be, it seems that the TDN effect of the Delta32 mutant has no bearing to the issue of CCR5 dimerization and to its possible role in the processing of the receptor to the cell surface.     

A large hinge bending domain rotation is necessary for the catalytic function of Escherichia coli 5'-nucleotidase

Two variants of Escherichia coli 5'-nucleotidase with disulfide bridges that were engineered to link the two domains of the protein were used to demonstrate that a large domain rotation is required for the catalytic mechanism of the enzyme. Kinetic analysis demonstrates that the variant trapped in the open form is almost inactive but can be activated up to 250-fold by reduction of the disulfide bridge. The second variant can adopt a closed but also a half-open conformation despite the presence of the cystine linkage. As a result of this flexibility, the mutant is still active in its oxidized state, although it shows a more pronounced substrate inhibition than the wild-type protein. A theoretical model is proposed that allows estimation of the flexibility of the proteins in the presence of the disulfide domain cross-link. Despite the unexpected residual flexibility of the trapped mutants, the enzymes could be used as conformational reporters in CD spectroscopy, revealing that the wild-type protein exists predominantly in an open conformation in solution. The kinetic, spectroscopic, and theoretical data are brought together to discuss the domain rotation in terms of the kinetic functioning of E. coli 5'-nucleotidase.     

Role of the N-terminal domain of endoinulinase from Arthrobacter sp. S37 in regulation of enzyme catalysis

Endoinulinase from Arthrobacter sp. S37 (EnIA), a member of the glycoside hydrolase family 32, is unique in that, unlike other members of the family, it contains a 250-residue N-terminal domain including a "laminin-G like jelly-roll" fold. This unique N-terminal domain is here suggested to be involved in dimerization and catalysis. The essentially inactive nature of enzymes produced by N-terminal truncation (Delta15, Delta45, Delta70, and Delta250) supported the pivotal role of this unique domain in catalysis and the need for its structural integrity. Significant reductions in the enzyme efficiency (kcat/Km) were observed when mutations were introduced at highly conserved tryptophan residues (Trp75 and Trp141) in the laminin-G like jelly-roll fold, implying their involvement in catalysis. Results from size-exclusion chromatography of the native and chimeric enzymes in the presence and absence of the domain suggested that the N-terminal domain could mediate dimerization.     

Identification of the regulatory elements of the human von Willebrand factor for binding to platelet GPIb. Importance of structural integrity of the regions flanked by the CYS1272-CYS1458 disulfide bond

In vitro platelet glycoprotein Ib (GPIb) binding of the human von Willebrand factor (VWF) increases markedly by exogenous modulators such as ristocetin or botrocetin, and the binding does not occur in normal circulation. GPIb binding sites have been assigned in the VWF A1 domain, which consists of a disulfide loop Cys1272(509)-Cys1458(695) where amino acid residues are numbered from the starting methionine as +1. The previous numbering from the N-terminal Ser of the mature processed VWF is indicated in parentheses. In contrast, several gain-of-function mutations have been found in two regions comprised of the disulfide loop and its N- and C-terminal flanking regions. In this study, Cys1222(459)-Tyr1271(508), Gln1238(475)-Tyr1271(508), Glu1260(497)-Tyr1271(508), and Asp1459(696)-Asp1472(709) were sequentially deleted of full-length multimeric recombinant VWF. Deletions at either side resulted in normal GPIb binding, indicating that the flanking regions are not GPIb binding sites. However, the addition of a mutation at Arg1308(545) on each deletion mutant resulted in spontaneous GPIb binding without requiring modulators, suggesting that both regions are important for the inhibition of GPIb binding. Spontaneous binding was completely inhibited by monoclonal antibodies that recognize the GPIb binding sites. Interestingly, mutant proteins with N-terminal but not C-terminal deletions lost binding to monoclonal antibodies B328, B710, and 23C7, which selectively inhibit ristocetin-induced GPI binding. Their epitopes were found at His1268(505) or Asp1269(506). The crystallographic structure of the A1 domain suggests that GPIb binding is influenced by the molecular interface between the two regions and that the antibody binding to the interface inhibits binding.     

Defining the native disulfide topology in the somatomedin B domain of human vitronectin

The N-terminal 44 amino acid residues of the human plasma glycoprotein vitronectin, known as the somatomedin B (SMB) domain, mediates the interaction between vitronectin and plasminogen activator inhibitor 1 (PAI-1) in a variety of important biological processes. Despite the functional importance of the Cys-rich SMB domain, how its four disulfide bridges are arranged in the molecule remains highly controversial, as evidenced by three different disulfide connectivities reported by several laboratories. Using native chemical ligation and orthogonal protection of selected Cys residues, we chemically synthesized all three topological analogs of SMB with predefined disulfide connectivities corresponding to those previously published. In addition, we oxidatively folded a fully reduced SMB in aqueous solution, and prepared, by CNBr cleavage, the N-terminal segment of 51 amino acid residues of intact vitronectin purified from human blood. Proteolysis coupled with mass spectrometric analysis and functional characterization using a surface plasmon resonance based vitronectin-PAI-1-SMB competition assay allowed us to conclude that 1) only the Cys(5)-Cys(21), Cys(9)-Cys(39), Cys(19)-Cys(32), and Cys(25)-Cys(31) connectivity is present in native vitronectin; 2) only the native disulfide connectivity is functional; and 3) the native disulfide pairings can be readily formed during spontaneous (oxidative) folding of the SMB domain in vitro. Our results unequivocally define the native disulfide topology in the SMB domain of human vitronectin, providing biochemical as well as functional support to the structural findings on a recombinant SMB domain by Read and colleagues (Zhou, A., Huntington, J. A., Pannu, N. S., Carrell, R. W., and Read, R. J. (2003) Nat. Struct. Biol. 10, 541-544).