Effects of glycosylation on the folding and stability of human, recombinant and cleaved alpha 1-antitrypsin

Native and Ion Mobility Mass Spectrometry Characterization of Alpha 1 Antitrypsin Variants and Oligomers

In this chapter, we describe a method for analyzing both recombinant and plasma-derived alpha 1 antitrypsin and its oligomers by means of native ion mobility mass spectrometry. Our experimental workflow can be applied to other variants of alpha 1 antitrypsin and its oligomers as well as being used to probe their interactions with small molecules in the gas phase.

The conformational landscape of a serpin N-terminal subdomain facilitates folding and in-cell quality control

Many multi-domain proteins including the serpin family of serine protease inhibitors contain non-sequential domains composed of regions that are far apart in sequence. Because proteins are translated vectorially from N- to C-terminus, such domains pose a particular challenge: how to balance the conformational lability necessary to form productive interactions between early and late translated regions while avoiding aggregation. This balance is mediated by the protein sequence properties and the interactions of the folding protein with the cellular quality control machinery. For serpins, particularly α 1 -antitrypsin (AAT), mutations often lead to polymer accumulation in cells and consequent disease suggesting that the lability/aggregation balance is especially precarious. Therefore, we investigated the properties of progressively longer AAT N-terminal fragments in solution and in cells. The N-terminal subdomain, residues 1-190 (AAT190), is monomeric in solution and efficiently degraded in cells. More β -rich fragments, 1-290 and 1-323, form small oligomers in solution, but are still efficiently degraded, and even the polymerization promoting Siiyama (S53F) mutation did not significantly affect fragment degradation. In vitro, the AAT190 region is among the last regions incorporated into the final structure. Hydrogen-deuterium exchange mass spectrometry and enhanced sampling molecular dynamics simulations show that AAT190 has a broad, dynamic conformational ensemble that helps protect one particularly aggregation prone β -strand from solvent. These AAT190 dynamics result in transient exposure of sequences that are buried in folded, full-length AAT, which may provide important recognition sites for the cellular quality control machinery and facilitate degradation and, under favorable conditions, reduce the likelihood of polymerization.

Pre-Molten, Wet, and Dry Molten Globules en Route to the Functional State of Proteins

Transitions between the unfolded and native states of the ordered globular proteins are accompanied by the accumulation of several intermediates, such as pre-molten globules, wet molten globules, and dry molten globules. Structurally equivalent conformations can serve as native functional states of intrinsically disordered proteins. This overview captures the characteristics and importance of these molten globules in both structured and intrinsically disordered proteins. It also discusses examples of engineered molten globules. The formation of these intermediates under conditions of macromolecular crowding and their interactions with nanomaterials are also reviewed.

Recombinant Alpha-1 Antitrypsin as Dry Powder for Pulmonary Administration: A Formulative Proof of Concept

Alpha-1 antitrypsin (AAT) deficiency is a genetic disorder associated with pulmonary emphysema and bronchiectasis. Its management currently consists of weekly infusions of plasma-purified human AAT, which poses several issues regarding plasma supplies, possible pathogen transmission, purification costs, and parenteral administration. Here, we investigated an alternative administration strategy for augmentation therapy by combining recombinant expression of AAT in bacteria and the production of a respirable powder by spray drying. The same formulation approach was then applied to plasma-derived AAT for comparison. Purified, active, and endotoxin-free recombinant AAT was produced at high yields and formulated using L-leucine and mannitol as excipients after identifying compromise conditions for protein activity and good aerodynamic performances. An oxygen-free atmosphere, both during formulation and powder storage, slowed down methionine-specific oxidation and AAT inactivation. This work is the first peer-reviewed report of AAT formulated as a dry powder, which could represent an alternative to current treatments.

LMAN1-MCFD2 complex is a cargo receptor for the ER-Golgi transport of α1-antitrypsin

α1-antitrypsin (AAT) is a serine protease inhibitor synthesized in hepatocytes and protects the lung from damage by neutrophil elastase. AAT gene mutations result in AAT deficiency (AATD), which leads to lung and liver diseases. The AAT Z variant forms polymer within the endoplasmic reticulum (ER) of hepatocytes and results in reduction of AAT secretion and severe disease. Previous studies demonstrated a secretion defect of AAT in LMAN1 deficient cells, and mild decreases in AAT levels in male LMAN1 and MCFD2 deficient mice. LMAN1 is a transmembrane lectin that forms a complex with a small soluble protein MCFD2. The LMAN1-MCFD2 protein complex cycles between the ER and the Golgi. Here we report that LMAN1 and MCFD2 knockout (KO) HepG2 and HEK293T cells display reduced AAT secretion and elevated intracellular AAT levels due to a delayed ER-to-Golgi transport of AAT. Secretion defects in KO cells were rescued by wild-type LMAN1 or MCFD2, but not by mutant proteins. Elimination of the second glycosylation site of AAT abolished LMAN1 dependent secretion. Co-immunoprecipitation experiment in MCFD2 KO cells suggested that AAT interaction with LMAN1 is independent of MCFD2. Furthermore, our results suggest that secretion of the Z variant, both monomers and polymers, is also LMAN1-dependent. Results provide direct evidence supporting that the LMAN1-MCFD2 complex is a cargo receptor for the ER-to-Golgi transport of AAT and that interactions of LMAN1 with an N-glycan of AAT is critical for this process. These results have implications in production of recombinant AAT and in developing treatments for AATD patients.

Nascent chains can form co-translational folding intermediates that promote post-translational folding outcomes in a disease-causing protein

During biosynthesis, proteins can begin folding co-translationally to acquire their biologically-active structures. Folding, however, is an imperfect process and in many cases misfolding results in disease. Less is understood of how misfolding begins during biosynthesis. The human protein, alpha-1-antitrypsin (AAT) folds under kinetic control via a folding intermediate; its pathological variants readily form self-associated polymers at the site of synthesis, leading to alpha-1-antitrypsin deficiency. We observe that AAT nascent polypeptides stall during their biosynthesis, resulting in full-length nascent chains that remain bound to ribosome, forming a persistent ribosome-nascent chain complex (RNC) prior to release. We analyse the structure of these RNCs, which reveals compacted, partially-folded co-translational folding intermediates possessing molten-globule characteristics. We find that the highly-polymerogenic mutant, Z AAT, forms a distinct co-translational folding intermediate relative to wild-type. Its very modest structural differences suggests that the ribosome uniquely tempers the impact of deleterious mutations during nascent chain emergence. Following nascent chain release however, these co-translational folding intermediates guide post-translational folding outcomes thus suggesting that Z's misfolding is initiated from co-translational structure. Our findings demonstrate that co-translational folding intermediates drive how some proteins fold under kinetic control, and may thus also serve as tractable therapeutic targets for human disease.

The molecular species responsible for α1 -antitrypsin deficiency are suppressed by a small molecule chaperone

The formation of ordered Z (Glu342Lys) α1 -antitrypsin polymers in hepatocytes is central to liver disease in α1 -antitrypsin deficiency. In vitro experiments have identified an intermediate conformational state (M*) that precedes polymer formation, but this has yet to be identified in vivo. Moreover, the mechanism of polymer formation and their fate in cells have been incompletely characterised. We have used cell models of disease in conjunction with conformation-selective monoclonal antibodies and a small molecule inhibitor of polymerisation to define the dynamics of polymer formation, accumulation and secretion. Pulse-chase experiments demonstrate that Z α1 -antitrypsin accumulates as short-chain polymers that partition with soluble cellular components and are partially secreted by cells. These precede the formation of larger, insoluble polymers with a longer half-life (10.9 ± 1.7 h and 20.9 ± 7.4 h for soluble and insoluble polymers, respectively). The M* intermediate (or a by-product thereof) was identified in the cells by a conformation-specific monoclonal antibody. This was completely abrogated by treatment with the small molecule, which also blocked the formation of intracellular polymers. These data allow us to conclude that the M* conformation is central to polymerisation of Z α1 -antitrypsin in vivo; preventing its accumulation represents a tractable approach for pharmacological treatment of this condition; polymers are partially secreted; and polymers exist as two distinct populations in cells whose different dynamics have likely consequences for the aetiology of the disease.

High-resolution ex vivo NMR spectroscopy of human Z α1-antitrypsin

Genetic mutations predispose the serine protease inhibitor α1-antitrypsin to misfolding and polymerisation within hepatocytes, causing liver disease and chronic obstructive pulmonary disease. This misfolding occurs via a transiently populated intermediate state, but our structural understanding of this process is limited by the instability of recombinant α1-antitrypsin variants in solution. Here we apply NMR spectroscopy to patient-derived samples of α1-antitrypsin at natural isotopic abundance to investigate the consequences of disease-causing mutations, and observe widespread chemical shift perturbations for methyl groups in Z AAT (E342K). By comparison with perturbations induced by binding of a small-molecule inhibitor of misfolding we conclude that they arise from rapid exchange between the native conformation and a well-populated intermediate state. The observation that this intermediate is stabilised by inhibitor binding suggests a paradoxical approach to the targeted treatment of protein misfolding disorders, wherein the stabilisation of disease-associated states provides selectivity while inhibiting further transitions along misfolding pathways.

Probing the folding pathway of a consensus serpin using single tryptophan mutants

Conserpin is an engineered protein that represents the consensus of a sequence alignment of eukaryotic serpins: protease inhibitors typified by a metastable native state and a structurally well-conserved scaffold. Previously, this protein has been found to adopt a native inhibitory conformation, possess an atypical reversible folding pathway and exhibit pronounced resistance to inactivation. Here we have designed a version of conserpin, cAT, with the inhibitory specificity of α₁-antitrypsin, and generated single-tryptophan variants to probe its folding pathway in more detail. cAT exhibited similar thermal stability to the parental protein, an inactivation associated with oligomerisation rather a transition to the latent conformation, and a native state with pronounced kinetic stability. The tryptophan variants reveal the unfolding intermediate ensemble to consist of an intact helix H, a distorted helix F and ‘breach’ region structurally similar to that of a mesophilic serpin intermediate. A combination of intrinsic fluorescence, circular dichroism, and analytical gel filtration provide insight into a highly cooperative folding pathway with concerted changes in secondary and tertiary structure, which minimises the accumulation of two directly-observed aggregation-prone intermediate species. This functional conserpin variant represents a basis for further studies of the relationship between structure and stability in the serpin superfamily.

Lipid oxidation inactivates the anticoagulant function of protein Z-dependent protease inhibitor (ZPI)

Lipid oxidation due to oxidative stress plays an important role in the pathogenesis of inflammatory and thrombotic cardiovascular diseases. Several findings suggest that lipid peroxidation can alter the function of coagulation proteins and contribute to a hypercoagulable state, but the molecular mechanisms are unclear. Here, we report that oxidized phospholipids suppress the anticoagulant function of the serpin, protein Z-dependent protease inhibitor (ZPI), a specific inhibitor of membrane-associated factor Xa (FXa) that requires protein Z (PZ), phospholipid, and calcium as cofactors. We found that this suppression arises from a diminished ability of the oxidized membrane to function as a cofactor to promote ZPI inhibition of membrane-bound FXa, due fully or in part to the susceptibility of the bound ZPI-PZ complex to oxidative inactivation. Surprisingly, free ZPI was also susceptible to inactivation by oxidized membrane vesicles in the absence of calcium. Oxidized vesicles containing both phosphatidylserine and polyunsaturated fatty acids were required to promote inactivation of the ZPI-PZ complex or free ZPI, indicating that binding of the PZ-complexed or free ZPI to peroxide-modified phospholipid vesicles mediates the inactivation. Heparin protected the ZPI-PZ complex and free ZPI from inactivation, suggesting that blocking the heparin-binding site on ZPI interferes with ZPI binding to lipid or to PZ. This was confirmed by direct lipid-binding experiments. Native PAGE indicated that oxidization induced dissociation of the ZPI-PZ complex and increased the negative charge of ZPI. We conclude that compromised ZPI anticoagulant function could contribute to thrombus initiation and growth in oxidative stress-induced cardiovascular diseases.

Electrophoresis- and FRET-Based Measures of Serpin Polymerization

Many serpinopathies, including alpha-1 antitrypsin (A1AT) deficiency, are associated with the formation of unbranched polymer chains of mutant serpins. In vivo, this deficiency is the result of mutations that cause kinetic or thermodynamic destabilization of the molecule. However, polymerization can also be induced in vitro from mutant or wild-type serpins under destabilizing conditions. The characteristics of the resulting polymers are dependent upon induction conditions. Due to their relationship to disease, serpin polymers, mainly those formed from A1AT, have been widely studied. Here, we describe Förster resonance energy transfer (FRET) and gel-based approaches for their characterization.

Expression and Purification of Active Recombinant Human Alpha-1 Antitrypsin (AAT) from Escherichia coli

Well-established genetic manipulation procedures along with a fast doubling time, the ability to grow in inexpensive media, and easy scaleup make Escherichia coli (E. coli) a preferred recombinant protein expression platform. Human alpha-1 antitrypsin (AAT) and other serpins are easily expressed in E. coli despite their metastability and complicated topology. Serpins can be produced as soluble proteins or aggregates in inclusion bodies, and both forms can be purified to homogeneity. In this chapter, we describe an ion-exchange chromatography-based protocol that we have developed involving the use of two anion-exchange columns to purify untagged human AAT from E. coli. We also outline methods that can be used to determine the inhibitory activity of both AAT in cell lysates and purified AAT. Our protocol for the purification of bacterially expressed AAT yields pure and active protein at 6-7 mg/l culture.

Glyco-variant library of the versatile enzyme horseradish peroxidase

When the glycosylated plant enzyme horseradish peroxidase (HRP) is conjugated to specific antibodies, it presents a powerful tool for medical applications. The isolation and purification of this enzyme from plant is difficult and only gives low yields. However, HRP recombinantly produced in the yeast Pichia pastoris experiences hyperglycosylation, which impedes the use of this enzyme in medicine. Enzymatic and chemical deglycosylation are cost intensive and cumbersome and hitherto existing P. pastoris strain engineering approaches with the goal to avoid hyperglycosylation only resulted in physiologically impaired yeast strains not useful for protein production processes. Thus, the last resort to obtain less glycosylated recombinant HRP from P. pastoris is to engineer the enzyme itself. In the present study, we mutated all the eight N-glycosylation sites of HRP C1A. After determination of the most suitable mutation at each N-glycosylation site, we physiologically characterized the respective P. pastoris strains in the bioreactor and purified the produced HRP C1A glyco-variants. The biochemical characterization of the enzyme variants revealed great differences in catalytic activity and stability and allowed the combination of the most promising mutations to potentially give an unglycosylated, active HRP C1A variant useful for medical applications. Interestingly, site-directed mutagenesis proved to be a valuable strategy not only to reduce the overall glycan content of the recombinant enzyme but also to improve catalytic activity and stability. In the present study, we performed an integrated bioprocess covering strain generation, bioreactor cultivations, downstream processing and product characterization and present the biochemical data of the HRP glyco-library.

Comparability of a three-dimensional structure in biopharmaceuticals using spectroscopic methods

Protein structure depends on weak interactions and covalent bonds, like disulfide bridges, established according to the environmental conditions. Here, we present the validation of two spectroscopic methodologies for the measurement of free and unoxidized thiols, as an attribute of structural integrity, using 5,5'-dithionitrobenzoic acid (DTNB) and DyLight Maleimide (DLM) as derivatizing agents. These methods were used to compare Rituximab and Etanercept products from different manufacturers. Physicochemical comparability was demonstrated for Rituximab products as DTNB showed no statistical differences under native, denaturing, and denaturing-reducing conditions, with Student's t-test P values of 0.6233, 0.4022, and 0.1475, respectively. While for Etanercept products no statistical differences were observed under native (P = 0.0758) and denaturing conditions (P = 0.2450), denaturing-reducing conditions revealed cysteine contents of 98% and 101%, towards the theoretical value of 58, for the evaluated products from different Etanercept manufacturers. DLM supported equality between Rituximab products under native (P = 0.7499) and denaturing conditions (P = 0.8027), but showed statistical differences among Etanercept products under native conditions (P < 0.001). DLM suggested that Infinitam has fewer exposed thiols than Enbrel, although DTNB method, circular dichroism (CD), fluorescence (TCSPC), and activity (TNF α neutralization) showed no differences. Overall, this data revealed the capabilities and drawbacks of each thiol quantification technique and their correlation with protein structure.

Altered native stability is the dominant basis for susceptibility of α1-antitrypsin mutants to polymerization

Serpins are protease inhibitors whose most stable state is achieved upon transition of a central 5-stranded β-sheet to a 6-stranded form. Mutations, low pH, denaturants and elevated temperatures promote this transition, which can result in a growing polymer chain of inactive molecules. Different types of polymer are possible, but, experimentally only heat has been shown to generate polymers in vitro consistent with ex vivo pathological specimens. Many mutations that alter the rate of heat-induced polymerization have been described, but interpretation is problematic because discrimination is lacking between the effect of global changes in native stability and specific effects on structural mechanism. We show that the temperature midpoint (Tm) of thermal denaturation reflects the transition of α1-antitrypsin to the polymerization intermediate, and determine the relationship with fixed-temperature polymerization half-times (t0.5) in the presence of stabilizing additives [TMAO (trimethylamine N-oxide), sucrose and sodium sulfate], point mutations and disulfide bonds. Combined with a retrospective analysis of 31 mutants characterized in the literature, the results of the present study show that global changes to native state stability are the predominant basis for the effects of mutations and osmolytes on heat-induced polymerization, summarized by the equation: ln(t0.5,mutant/t0.5,wild-type)=0.34×ΔTm. It is deviations from this relationship that hold key information about the polymerization process.

Collapse of a long axis: single-molecule Förster resonance energy transfer and serpin equilibrium unfolding

The energy required for mechanical inhibition of target proteases is stored in the native structure of inhibitory serpins and accessed by serpin structural remodeling. The overall serpin fold is ellipsoidal with one long and two short axes. Most of the structural remodeling required for function occurs along the long axis, while expansion of the short axes is associated with misfolded, inactive forms. This suggests that ellipticity, as typified by the long axis, may be important for both function and folding. Placement of donor and acceptor fluorophores approximately along the long axis or one of the short axes allows single-pair Förster resonance energy transfer (spFRET) to report on both unfolding transitions and the time-averaged shape of different conformations. Equilibrium unfolding and refolding studies of the well-characterized inhibitory serpin α1-antitrypsin reveal that the long axis collapses in the folding intermediates while the monitored short axis expands. These energetically distinct intermediates are thus more spherical than the native state. Our spFRET studies agree with other equilibrium unfolding studies that found that the region around one of the β strands, s5A, which helps define the long axis and must move for functionally required loop insertion, unfolds at low denaturant concentrations. This supports a connection between functionally important structural lability and unfolding in the inhibitory serpins.

Single amino acid substitutions in recombinant plant-derived human α1-proteinase inhibitor confer enhanced stability and functional efficacy

BACKGROUND

Human α1-proteinase inhibitor (α1-PI) is the most abundant serine protease inhibitor in the blood and the heterologous expression of recombinant α1-PI has great potential for possible therapeutic applications. However, stability and functional efficacy of the recombinant protein expressed in alternate hosts are of major concern.

METHODS

Five variants of plant-expressed recombinant α1-PI protein were developed by incorporating single amino acid substitutions at specific sites, namely F51C, F51L, A70G, M358V and M374I. Purified recombinant α1-PI variants were analyzed for their expression, biological activity, oxidation-resistance, conformational and thermal stability by DAC-ELISA, porcine pancreatic elastase (PPE) inhibition assays, transverse urea gradient (TUG) gel electrophoresis, fluorescence spectroscopy and far-UV CD spectroscopy.

RESULTS

Urea-induced unfolding of recombinant α1-PI variants revealed that the F51C mutation shifted the mid-point of transition from 1.4M to 4.3M, thus increasing the conformational stability close to the human plasma form, followed by F51L, A70G and M374I variants. The variants also exhibited enhanced stability for heat denaturation, and the size-reducing substitution at Phe51 slowed down the deactivation rate ~5-fold at 54°C. The M358V mutation at the active site of the protein did not significantly affect the conformational or thermal stability of the recombinant α1-PI but provided enhanced resistance to oxidative inactivation.

CONCLUSIONS

Our results suggest that single amino acid substitutions resulted in improved stability and oxidation-resistance of the plant-derived recombinant α1-PI protein, without inflicting the inhibitory activity of the protein.

GENERAL SIGNIFICANCE

Our results demonstrate the significance of engineered modifications in plant-derived recombinant α1-PI protein molecule for further therapeutic development.

Production and purification of the multifunctional enzyme horseradish peroxidase

The oxidoreductase horseradish peroxidase (HRP) is used in numerous industrial and medical applications. In this review, we briefly describe this well-studied enzyme and focus on its promising use in targeted cancer treatment. In combination with a plant hormone, HRP can be used in specific enzyme-prodrug therapies. Despite this outstanding application, HRP has not found its way as a biopharmaceutical into targeted cancer therapy yet. The reasons therefore lie in the present low-yield production and cumbersome purification of this enzyme from its natural source. However, surface glycosylation renders the recombinant production of HRP difficult. Here, we compare different production hosts for HRP and summarize currently used production and purification strategies for this enzyme. We further present our own strategy of glycoengineering this powerful enzyme to allow recombinant high-yield production in Pichia pastoris and subsequent simple downstream processing.

Reactive centre loop mutants of α-1-antitrypsin reveal position-specific effects on intermediate formation along the polymerization pathway

The common severe Z mutation (E342K) of α1-antitrypsin forms intracellular polymers that are associated with liver cirrhosis. The native fold of this protein is well-established and models have been proposed from crystallographic and biophysical data for the stable inter-molecular configuration that terminates the polymerization pathway. Despite these molecular 'snapshots', the details of the transition between monomer and polymer remain only partially understood. We surveyed the RCL (reactive centre loop) of α1-antitrypsin to identify sites important for progression, through intermediate states, to polymer. Mutations at P14P12 and P4, but not P10P8 or P2P1', resulted in a decrease in detectable polymer in a cell model that recapitulates the intracellular polymerization of the Z variant, consistent with polymerization from a near-native conformation. We have developed a FRET (Förster resonance energy transfer)-based assay to monitor polymerization in small sample volumes. An in vitro assessment revealed the position-specific effects on the unimolecular and multimolecular phases of polymerization: the P14P12 region self-inserts early during activation, while the interaction between P6P4 and β-sheet A presents a kinetic barrier late in the polymerization pathway. Correspondingly, mutations at P6P4, but not P14P12, yield an increase in the overall apparent activation energy of association from ~360 to 550 kJ mol(-1).

The roles of helix I and strand 5A in the folding, function and misfolding of α1-antitrypsin

α₁-Antitrypsin, the archetypal member of the serpin superfamily, is a metastable protein prone to polymerization when exposed to stressors such as elevated temperature, low denaturant concentrations or through the presence of deleterious mutations which, in a physiological context, are often associated with disease. Experimental evidence suggests that α₁-Antitrypsin can polymerize via several alternative mechanisms in vitro. In these polymerization mechanisms different parts of the molecule are proposed to undergo conformational change. Both strand 5 and helix I are proposed to adopt different conformations when forming the various polymers, and possess a number of highly conserved residues however their role in the folding and misfolding of α₁-Antitrypsin has never been examined. We have therefore created a range of α₁Antitypsin variants in order to explore the role of these conserved residues in serpin folding, misfolding, stability and function. Our data suggest that key residues in helix I mediate efficient folding from the folding intermediate and residues in strand 5A ensure native state stability in order to prevent misfolding. Additionally, our data indicate that helix I is involved in the inhibitory process and that both structural elements undergo differing conformational rearrangements during unfolding and misfolding. These findings suggest that the ability of α₁-Antitrypsin to adopt different types of polymers under different denaturing conditions may be due to subtle conformational differences in the transiently populated structures adopted prior to the I and M* states.

Structural change in β-sheet A of Z α(1)-antitrypsin is responsible for accelerated polymerization and disease

The presence of the Z mutation (Glu342Lys) is responsible for more than 95% of α(1)-antitrypsin (α(1)AT) deficiency cases. It leads to increased polymerization of the serpin α(1)AT during its synthesis and in circulation. It has been proposed that the Z mutation results in a conformational change within the folded state of antitrypsin that enhances its polymerization. In order to localize the conformational change, we have created two single tryptophan mutants of Z α(1)AT and analyzed their fluorescence properties. α(1)AT contains two tryptophan residues that are located in distinct regions of the molecule: Trp194 at the top of β-sheet A and Trp238 on β-sheet B. We have replaced each tryptophan residue individually with a phenylalanine in order to study the local environment of the remaining tryptophan residue in both M and Z α(1)AT. A detailed fluorescence spectroscopic analysis of each mutant was carried out, and we detected differences in the emission spectrum, the Stern-Volmer constant for potassium iodide quenching and the anisotropy of only Trp194 in Z α(1)AT compared to M α(1)AT. Our data reveal that the Z mutation results in a conformational change at the top of β-sheet A but does not affect the structural integrity of β-sheet B.

Effects of glycosylation on the stability and flexibility of a metastable protein: the human serpin α(1)-antitrypsin

Protein glycosylation commonly stabilizes proteins thereby increasing protein half-lives and protecting against denaturation or proteolytic degradation. While generally beneficial, such stabilization is potentially disadvantageous in the case of inhibitory serpins. These protease inhibitors are metastable and a conformational transition to a more stable form is key to their function. Instability is therefore essential for these inhibitory serpins and mutagenesis has demonstrated that substantial stabilization results in compromised function. We have used optical spectroscopy and hydrogen/deuterium exchange and mass spectrometry to investigate the effects of glycosylation on the human serpin alpha-1 antitrypsin (α(1)-AT). Previous studies found that unglycosylated recombinant α(1)-AT populates a molten globule at low denaturant and that the ability to populate this state is correlated with efficient protease inhibition. Further, a high degree of conformational flexibility was found in several important regions. Guanidine hydrochloride denaturation monitored by circular dichroism indicates that plasma α(1)-AT, which is glycosylated at 3 sites, is substantially stabilized relative to the unglycosylated form. However, hydrogen exchange reveals complete loss of protection in plasma α(1)-AT above 1 M GuHCl, similar to what is seen for the recombinant form. Sugars therefore appear to stabilize the compact denatured state of α(1)-AT without significant stabilization of the folded state. Native state hydrogen exchange reveals minor perturbations to native flexibility, but high flexibility in key regions such as the f helix is conserved. β-strand 1c is stabilized in plasma α(1)-AT, which may confer increased resistance to forming pathogenic polymers. Overall, our results indicate that glycosylation of inhibitory serpins does not interfere with either native state flexibility or the native instability that is required for efficient function, though it may confer resistance to degradation by proteases and thus extend the half-life of circulating serpins.

Dynamic local unfolding in the serpin α-1 antitrypsin provides a mechanism for loop insertion and polymerization

The conformational plasticity of serine protease inhibitors (serpins) underlies both their activities as protease inhibitors and their susceptibility to pathogenic misfolding and aggregation. Here, we structurally characterize a sheet-opened state of the serpin α-1 antitrypsin (α₁AT) and show how local unfolding allows functionally essential strand insertion. Mutations in α₁AT that cause polymerization-induced serpinopathies map to the labile region, suggesting that the evolution of serpin function required sampling of high risk conformations on a dynamic energy landscape.

Targeting and post-translational processing of human alpha1-antichymotrypsin in BY-2 tobacco cultured cells

The post-translational processing of human alpha(1)-antichymotrypsin (AACT) in Bright Yellow-2 (BY-2) tobacco cells was assessed in relation to the cellular compartment targeted for accumulation. As determined by pulse-chase labelling experiments and immunofluorescence microscopy, AACT sent to the vacuole or the endoplasmic reticulum (ER) was found mainly in the culture medium, similar to a secreted form targeted to the apoplast. Unexpectedly, AACT expressed in the cytosol was found in the nucleus under a stable, non-glycosylated form, in contrast with secreted variants undergoing multiple post-translational modifications during their transit through the secretory pathway. All secreted forms of AACT were N-glycosylated, with the presence of complex glycans as observed naturally on human AACT. Proteolytic trimming was also observed for all secreted variants, both during their intracellular transit and after their secretion in the culture medium. Overall, the targeting of human AACT to different compartments of BY-2 tobacco cells led to the production of two protein products: (i) a stable, non-glycosylated protein accumulated in the nucleus; and (ii) a heterogeneous mixture of secreted variants resulting from post-translational N-glycosylation and proteolytic processing. Overall, these data suggest that AACT is sensitive to resident proteases in the ER, the Golgi and/or the apoplast, and that the production of intact AACT in the plant secretory pathway will require innovative approaches to protect its structural integrity in vivo. Studies are now needed to assess the activity of the different AACT variants, and to identify the molecular determinants for the nuclear localization of AACT expressed in the cytosol.

Cleaved serpin refolds into the relaxed state via a stressed conformer

Serine proteinase inhibitors (serpins) are believed to fold in vivo into a metastable "stressed" state with cleavage of their P1-P1' bond resulting in reactive center loop insertion and a thermostable "relaxed" state. To understand this unique folding mechanism, we investigated the refolding processes of the P1-P1'-cleaved forms of wild type ovalbumin (cl-OVA) and the R339T mutant (cl-R339T). In the native conditions, cl-OVA is trapped as the stressed conformer, whereas cl-R339T attains the relaxed structure. Under urea denaturing conditions, these cleaved proteins completely dissociated into the heavy (Gly(1)-Ala(352)) and light (Ser(353)-Pro(385)) chains. Upon refolding, the heavy chains of both proteins formed essentially the same initial burst refolding intermediates and then reassociated with the light chain counterparts. The reassociated intermediates both refolded into the native states with indistinguishable kinetics. The two refolded proteins, however, had a notable difference in thermostability. cl-OVA refolded into the stressed form with T(m) = 68.4 degrees C, whereas cl-R339T refolded into the relaxed form with T(m) = 85.5 degrees C. To determine whether cl-R339T refolds directly to the relaxed state or through the stressed state, conformational analyses by anion-exchange chromatography and fluorescence measurements were executed. The results showed that cl-R339T refolds first to the stressed conformation and then undergoes the loop insertion. This is the first demonstration that the P1-P1'-cleaved serpin peptide capable of loop insertion refolds to the stressed conformation. This highlights that the stressed conformation of serpins is an inevitable intermediate state on the folding pathway to the relaxed structure.

Cooperative Unfolding of a Metastable Serpin to a Molten Globule Suggests a Link Between Functional and Folding Energy Landscapes

Alpha-1 antitrypsin (alpha(1)-AT) is a member of the serpin class of protease inhibitors, and folds to a metastable state rather than its thermodynamically most stable native state. Upon cleavage by a target protease, alpha(1)-AT undergoes a dramatic conformational change to a stable form, translocating the bound protease more than 70 A to form an inhibitory protease-serpin complex. Numerous mutagenesis studies on serpins have demonstrated the trade-off between the stability of the metastable state on the one hand and the inhibitory efficiency on the other. Studies of the equilibrium unfolding of serpins provide insight into this connection between structural plasticity and metastability. We studied equilibrium unfolding of wild-type alpha(1)-AT using hydrogen-deuterium/exchange mass spectrometry to characterize the structure and the stability of an equilibrium intermediate that was observed in low concentrations of denaturant in earlier studies. Our results show that the intermediate observed at low concentrations of denaturant has no protection from hydrogen-deuterium exchange, indicating a lack of stable structure. Further, differential scanning calorimetry of alpha(1)-AT at low concentrations of denaturant shows no heat capacity peak during thermal denaturation, indicating that the transition from the intermediate to the unfolded state is not a cooperative first-order-like phase transition.. Our results show that the unfolding of alpha(1)-AT involves a cooperative transition to a molten globule form, followed by a non-cooperative transition to a random-coil form as more guanidine is added. Thus, the entire alpha(1)-AT molecule consists of one cooperative structural unit rather than multiple structural domains with different stabilities. Furthermore, our results together with previous mutagenesis studies suggest a possible link between an equilibrium molten globule and a functional intermediate that may be populated during the protease inhibition.

Aeropin from the extremophile Pyrobaculum aerophilum bypasses the serpin misfolding trap

Serpins are metastable proteinase inhibitors. Serpin metastability drives both a large conformational change that is utilized during proteinase inhibition and confers an inherent structural flexibility that renders serpins susceptible to aggregation under certain conditions. These include point mutations (the basis of a number of important human genetic diseases), small changes in pH, and an increase in temperature. Many studies of serpins from mesophilic organisms have highlighted an inverse relationship: mutations that confer a marked increase in serpin stability compromise inhibitory activity. Here we present the first biophysical characterization of a metastable serpin from a hyperthermophilic organism. Aeropin, from the archaeon Pyrobaculum aerophilum, is both highly stable and an efficient proteinase inhibitor. We also demonstrate that because of high kinetic barriers, aeropin does not readily form the partially unfolded precursor to serpin aggregation. We conclude that stability and activity are not mutually exclusive properties in the context of the serpin fold, and propose that the increased stability of aeropin is caused by an unfolding pathway that minimizes the formation of an aggregation-prone intermediate ensemble, thereby enabling aeropin to bypass the misfolding fate observed with other serpins.

Molecular gymnastics: serpin structure, folding and misfolding

The native state of serpins represents a long-lived intermediate or metastable structure on the serpin folding pathway. Upon interaction with a protease, the serpin trap is sprung and the molecule continues to fold into a more stable conformation. However, thermodynamic stability can also be achieved through alternative, unproductive folding pathways that result in the formation of inactive conformations. Our increasing understanding of the mechanism of protease inhibition and the dynamics of native serpin structures has begun to reveal how evolution has harnessed the actual process of protein folding (rather than the final folded outcome) to elegantly achieve function. The cost of using metastability for function, however, is an increased propensity for misfolding.

Role of the carbohydrate chain and two phosphate moieties in the heat-induced aggregation of hen ovalbumin

We investigated the effect of the carbohydrate chain and two phosphate moieties on heat-induced aggregation of hen ovalbumin. The dephosphorylated form of ovalbumin was obtained by treating the original protein with acid phosphatase. The single carbohydrate chain was removed by digestion of heat-denatured ovalbumin with glycopeptidase F, and the resulting polypeptide without this carbohydrate chain was correctly refolded to acquire protease-resistance. Thermal unfolding can be approximated by a mechanism involving a two-state transition between the folded and unfolded states with a midpoint temperature of 76 degrees C for the original form, of 74 degrees C for the dephosphorylated form, and of 71 degrees C for the carbohydrate-free form. The conformational stability of the original form was higher than that of the carbohydrate-free form. When the three forms of ovalbumin were heated to 80 degrees C and then cooled rapidly in an ice bath, the polypeptide chains were compactly collapsed to metastable intermediates with secondary structures whose properties were indistinguishable. Upon incubation at 60 degrees C, renaturation was possible for a large portion of the intermediates of the original form, but for only a small portion of those of the carbohydrate-free form. Light scattering experiments showed that in the presence of sulfate anions, the intermediates of the carbohydrate-free form aggregated to a greater extent than did those of the original form. The intermediates of the carbohydrate-free form bound to the chaperonin GroEL with about 10-fold higher affinity than those of the original form. It follows that the carbohydrate chain and the two phosphate moieties do not affect hydrophobic collapse in the kinetic refolding of hen ovalbumin but play an important role in the slow rearrangement. They block the off-pathway reaction that competes with correct refolding by effectively decreasing surface hydrophobicity.

Identification of residual structure within denatured antichymotrypsin: implications for serpin folding and misfolding

The native serpin fold is metastable and possesses the inherent ability to convert into more stable, but inactive, conformations. In order to understand why serpins attain the native fold instead of other more thermodynamically favourable folds we have investigated the presence of residual structure within denatured antichymotrypsin (ACT). Through mutagenesis we created a single tryptophan variant of ACT in which a Trp residue (276) is situated on the H-helix, located within a region known as the B/C barrel. The presence of residual structure around Trp 276 in 5 M guanidine hydrochloride (GdnHCl) was shown by fluorescence and circular dichroism spectroscopy and fluorescence lifetime experiments. The residual structure was disrupted in the presence of 5 M guanidine thiocyanate (GdnSCN). Protein refolding studies showed that significant refolding could be achieved from the GdnHCl denatured state but not the GdnSCN denatured form. The implications of these data on the folding and misfolding of the serpin superfamily are discussed.

Latent antithrombin and its detection, formation and turnover in the circulation

It is now apparent that the inactivated latent and cleaved conformers of antithrombin (AT) are of pathological significance. Using a single-run electrophoretic technique that allows the quantitative assessment of these conformers in 2 microL plasma, we show that near 3% of the total AT in the circulations of normal individuals is in latent conformation. Only trace amounts of cleaved AT were observed. The slow decline in AT activity on incubation of plasma at 37 degrees C was shown to be almost wholly due to a transition of native AT to its inactive latent form. Also initial studies in the rabbit indicate that the latent form, like the cleaved, has an identical circulatory half-life to that of native AT. We deduce that the steady concentration of latent AT in the circulation is due to the transition of some 10(12) molecules of AT per second balanced by an equivalent clearance of the latent form. Examples of clinical applications of the new technique include its use as a comprehensive single-step screen for genetic variants associated with AT deficiency, and notably the potential it provides to monitor the changes responsible for the loss of AT in the shock syndromes.

Virus inactivation and protein recovery in a novel ultraviolet-C reactor

BACKGROUND AND OBJECTIVES

Ultraviolet-C (UVC) irradiation is a viral-inactivation method that was dismissed by many plasma fractionators as a result of the potential for protein damage and the difficulty in delivering uniform doses. A reactor with novel spiral flow hydraulic mixing was recently designed for uniform and controlled UVC treatment. The objective of this study was to investigate virus inactivation and protein recovery after treatment through the new reactor.

MATERIALS AND METHODS

Virus- and mock-spiked Alpha1-proteinase inhibitor (Alpha1-PI) solutions were treated with UVC. The virus samples were assayed for residual infectivity and amplified by the polymerase chain reaction (PCR). The mock-spiked samples were assayed for protein integrity.

RESULTS

Greater than 4 log10 of all test viruses were inactivated, regardless of the type of nucleic acid or presence of an envelope. Unlike previous studies, viruses with the smallest genomes were found to be those most sensitive to UVC irradiation, and detection of PCR amplicons > or = 2.0 kb was correlated to viral infectivity. Doses that achieved significant virus inactivation yielded recovery of > 90% protein activity, even in the absence of quenchers.

CONCLUSIONS

The results demonstrate the effectiveness of UVC treatment, in the novel reactor, to inactivate viruses without causing significant protein damage, and confirm the utility of large PCR amplicons as markers for infectious virus.

Different Conformational Changes within the F-Helix Occur during Serpin Folding, Polymerization, and Proteinase Inhibition†

The intrinsic metastability of the serpin native state is the thermodynamic driving force for both proteinase inhibition and the formation of inactive polymers. A number of mechanisms has been proposed to explain how both these conformational changes are achieved. However, one aspect that has received little attention is the movement of the F-helix, which physically impedes both these events. We have applied a protein engineering approach to investigate the conformational changes of this helix during proteinase inhibition, serpin folding, and polymerization. We systematically mutated two highly conserved hydrophobic residues on the F-helix, V161 and I157, and in addition, removed a hydrogen bond between D149 and the first turn of the helix. Our data demonstrate that while all three interactions are important for the stability and folding of the molecule, their contribution during inhibition and polymerization differ. The presence of I157 is crucial to all conformational changes as its loss results in inactivation of the serpin and rapid polymerization. The replacement of D149 does not affect activity but significantly increases the polymerization rate. The interactions formed by V161 play an important role only in maintaining the native conformation. Taken together, these data suggest that the F-helix undergoes a reversible conformational change in both its N- and C-termini during proteinase inhibition only the C-terminus undergoes changes during polymerization, but there is a global change required for folding.

How do proteins avoid becoming too stable? Biophysical studies into metastable proteins

The vast majority of theoretical and experimental folding studies have shown that as a protein folds, it attempts to adopt a conformation that occurs at its lowest free energy minimum. However, studies on a small number of proteins have now shown that this is a generality. In this review we discuss recent data on how two proteins, alpha-lytic protease and alpha1-antitrypsin, successfully fold to their metastable native states, whilst avoiding more stable but inactive conformations.

A novel mode of polymerization of alpha1-proteinase inhibitor

Patients homozygous for the Z mutant form of alpha1-proteinase inhibitor (alpha1-PI) have an increased risk for the development of liver disease because of the accumulation in hepatocytes of inclusion bodies containing linear polymers of mutant alpha1-PI. The most widely accepted model of polymerization proposes that a linear, head-to-tail polymer forms by sequential insertion of the reactive center loop (RCL) of one alpha1-PI monomer between the central strands of the A beta-sheet of an adjacent monomer. This model derives primarily from two observations: peptides that are homologous with the RCL insert into the A beta-sheet of alpha1-PI monomer and this insertion prevents alpha1-PI polymerization. Normal alpha1-PI monomer does not spontaneously polymerize; however, here we show that the disulfide-linked dimer of normal alpha1-PI spontaneously forms linear polymers in buffer. The monomers within this dimer are joined head-to-head. Thus, the arrangement of monomers in these polymers must be different from that predicted by the loop-A sheet model. Therefore, we propose a new model for alpha1-PI polymer. In addition, polymerization of disulfide-linked dimer is not inhibited by the presence of the peptide even though dimer appears to interact with the peptide. Thus, RCL insertion into A beta-sheets may not occur during polymerization of this dimer.

Acid Denaturation of alpha1-antitrypsin: characterization of a novel mechanism of serpin polymerization

The native serpin architecture is extremely sensitive to mutation and environmental factors. These factors induce the formation of a partially folded species that results in the production of inactive loop-sheet polymers. The deposition of these aggregates in tissue, results in diseases such as liver cirrhosis, thrombosis, angioedema and dementia. In this study, we characterize the kinetics and conformational changes of alpha(1)-antitrypsin polymerization at pH 4 using tryptophan fluorescence, circular dichroism, turbidity changes and thioflavin T binding. These biophysical techniques have demonstrated that polymerization begins with a reversible conformational change that results in partial loss of secondary structure and distortion at the top of beta-sheet A. This is followed by two bimolecular processes. First, protodimers are formed, which can be dissociated by changing the pH back to 8. Then, an irreversible conformational change occurs, resulting in the stabilization of the dimers with a concomitant increase in beta-sheet structure, allowing for subsequent polymer extension. Electron microscopy analysis of the polymers, coupled with the far-UV CD and thioflavin T properties of the pH 4 polymers suggest they do not form via the classical loop-beta-sheet A linkage. However, they more closely resemble those formed by the pathological variant M(malton). Taken together, these data describe a novel kinetic mechanism of serine proteinase inhibitor polymerization.

Probing the role of the F-helix in serpin stability through a single tryptophan substitution

Serpins form loop-sheet polymers through the formation of a partially folded intermediate. Through mutagenesis and biophysical analysis, we have probed the conformational stability of the F-helix, demonstrating that it is almost completely unfolded in the intermediate state. The replacement of Tyr160 on the F-helix of alpha1-antitrypsin to alanine results in the loss of a conserved hydrogen bond that dramatically reduces the stability of the protein to both heat and solvent denaturation, indicating the importance of Tyr160 in the stability of the molecule. The mutation of Tyr160 to a tryptophan residue, within a fluorescently silent variant of alpha1-antitrypsin, results in a fully active, stable serpin. Fluorescence analysis of the equilibrium unfolding behavior of this variant indicates that the F-helix is highly disrupted in the intermediate conformation. Iodide quenching experiments demonstrate that the tryptophan residue is exposed to a similar extent in both the intermediate and unfolded states. Cumulatively, these data indicate that the F-helix plays an important role in controlling the early conformational changes involved in alpha1-antitrypsin unfolding. The implications of these data on both alpha1-antitrypsin function and misfolding are discussed.

Probing reactive center loop insertion in serpins: a simple method for ovalbumin

Insertion of the reactive center loop in beta-sheet A in serpins has been typically inferred from the increased stability of the cleaved form to thermal- and urea-induced denaturation. We describe a convenient and rapid fluorescence-based method that differentiates the loop-inserted form from the loop-exposed form in ovalbumin, a prototypic noninhibitory serpin. Recombinant wild-type and R345A ovalbumins in the intact form bind ANS with equilibrium dissociation constants of 116 and 125 microM and a maximal fluorescence increase of 200 and 264%, respectively, in pH 6.8 buffer. Cleavage of the two proteins with porcine pancreatic elastase results in a 1.6- and 2.6-fold increase in the ANS-binding affinity. While cleavage of the reactive center loop in rR345A ovalbumin results in a approximately 200% increase in the ANS fluorescence, the rWT protein exhibits a approximately 50% decrease. Similar experiments with alpha(1)-proteinase inhibitor and antithrombin, two inhibitory serpins that exhibit reactive center loop insertion, show a decrease in ANS fluorescence on cleavage with porcine pancreatic elastase and thrombin, respectively. Denaturation studies in guanidinium hydrochloride indicate that the reactive center loop is inserted in the main body of the serpin in the cleaved form of rR345A mutant, while it is exposed in the cleaved form of rWT ovalbumin. These results demonstrate that ANS fluorescence change is an indicator of the loop-inserted or loop-exposed form in these recombinant ovalbumins, and thus could be advantageously used for probing reactive center loop insertion in ovalbumins. The major increase in fluorescence for the rR345A mutant on cleavage primarily arises from a change in ANS binding rather than from the generation of an additional ANS-binding site.

Role of the connectivity of secondary structure segments in the folding of alpha(1)-antitrypsin

The native form of serpins (serine protease inhibitors) is metastable, which is critical to their biological functions. Spontaneous conversion from the native form of serpins into a more stable conformation, called the "latent" form, is restricted. To examine whether the connectivity of strand 1 of beta-sheet C to the hydrophobic core is critical to the serpin's preferential folding to the metastable native conformation, we designed a circularly-permuted mutant of alpha(1)-antitrypsin, the prototype serpin, in which strand 1C is disconnected from the hydrophobic core. Conformation of the circular permutant was similar to that of the latent form, as revealed by equilibrium unfolding, limited proteolysis, and spectroscopic properties. Our results support the notion that rapid folding of the hydrophobic core with concomitant incorporation of strand 1C into beta-sheet C traps the serpin molecule into its native metastable conformation.

Kinetic stability of designed glycosylation mutants of Coprinus cinereus peroxidase

The effect of glycans and surface mutations on protein unfolding induced by heat or urea has been studied. Removal of the only native high mannose type glycan in the N142P, N142T, and N142D CIP mutants reduced the lifetime to half of that of wtCIP at irreversible conditions of unfolding. The effect was moderate at reversible conditions. Five glycomutants designed to have 0, 1, 2, 4 and 6N glycans showed a correlation between increased carbohydrate mass and increased stability toward irreversible unfolding. The results are in agreement with a dampening effect of glycans on backbone fluctuation in both the native and the unfolded states. However, experiments in reversible conditions were less clear because of additional effects of an increasing number of amino acid substitutions and aggregation. Examples of strong effects from minor surface changes were also observed.

Conformational change and intermediates in the unfolding of alpha 1-antichymotrypsin

Serpins are the prototypical members of the conformational disease family, a group of proteins that undergoes a change in shape that subsequently leads to tissue deposition. One specific example is alpha(1)-antichymotrypsin (ACT), which undergoes misfolding and aggregation that has been implicated in emphysema and Alzheimer's disease. In this study we have used guanidine hydrochloride (GdnHCl)-induced denaturation to investigate the conformational changes involved in the folding and unfolding of ACT. When the reaction was followed by circular dichroism spectroscopy, one stable intermediate was observed in 1.5 m GdnHCl. The same experiment monitored by fluorescence revealed a second intermediate formed in 2.5 m GdnHCl. Both these intermediates bound the hydrophobic dye ANS. These data suggest a four-state model for ACT folding N <--> I(1) <--> I(2) <--> U. I(1) and I(2) both have a similar loss of secondary structure (20%) compared with the native state. In I(2), however, there is a significant loss of tertiary interactions as revealed by changes in fluorescence emission maximum and intensity. Kinetic analysis of the unfolding reaction indicated that the native state is unstable with a fast rate of unfolding in water of 0.4 s(-1). The implications of these data for both ACT function and associated diseases are discussed.

Characterizing the functionality of recombinant T-cell receptors in vitro: a pMHC tetramer based approach

The very low affinity of the T-cell receptor (TCR) for the peptide-major histocompatibility complex (pMHC) has made it very challenging to design assays for testing the functionality of these molecules on small scales, which in turn has severely hampered the progress in developing expression and refolding methodologies for the TCR. We have now developed an ELISA assay for detecting pMHC binding to functional recombinant TCRs. It uses tetramers of biotinylated pMHCs bound to a neutravidin-horseradish peroxidase conjugate and detects the presence of functional TCR, bound in a productive orientation to an immobilized anti-Cbeta antibody. Specificity can be stringently demonstrated by inhibition with monomeric pMHCs. The assay is very sensitive and specific, and requires only very small amounts of protein. It has allowed us to study the unstable recombinant TCR P14, which we expressed and refolded from Escherichia coli. The TCR P14 is directed against the most abundant epitope of LCMV. We have confirmed the specificity of the interaction by BIAcore, and were able to determine the dissociation constant of the interaction of the P14 TCR and of the gp33-pMHC as 6 microM. This affinity ranks it among the tighter ones of TCR-pMHC interactions, and unusually low affinity thus does not seem to be the cause of the modest protective power of these T-cells, compared to others elicited in the anti-LCMV response. This strategy of multimerizing one partner and immobilizing the other in both a native form and productive orientation should be generally useful for characterizing the weak interactions of cell-surface molecules.

Characterization of the thyroxine-binding site of thyroxine-binding globulin by site-directed mutagenesis

The principal transport protein for T4 in human blood, thyroxine-binding globulin (TBG), binds T4 with an exceptionally high affinity (Ka = 10(10) M(-1)). Its homology to the superfamily of the serpins has recently been used in the design of chimeric proteins, providing experimental evidence that an eight-stranded beta-barrel domain encompasses the ligand-binding site. We have now characterized the T4 binding site by site-directed mutagenesis. Sequence alignment of TBG from several species revealed a phylogenetically highly conserved stretch of amino acids comprising strands 2B and 3B of the beta-barrel motif. Mutations within this region (Val228Glu, Cys234Trp, Thr235Trp, Thr235Gln, Lys253Ala, and Lys253Asp), designed to impose steric hindrance or restriction of its mobility, had no significant influence on T4 binding. However, binding affinity was 20-fold reduced by introduction of an N-linked glycosylation site at the turn between strands 2B and 3B (Leu246Thr) without compromising the proper folding of this mutant as assessed by immunological methods. In most other serpins, this glycosylation site is highly conserved and has been shown to be crucial for cortisol binding of corticosteroid-binding globulin, the only other member of the serpins with a transport function. The ligand-binding site could thus be located to a highly aromatic environment deep within the beta-barrel. The importance of the binding site's aromatic character was investigated by exchanging phenylalanines with alanines. Indeed, these experiments revealed that substitution of Phe249 in the middle of strand 3B completely abolished T4 binding, while the substitution of several other phenylalanines had no effect.

Three-state unfolding and self-association of maspin, a tumor-suppressing serpin

Maspin is a tumor suppressor protein expressed by normal human mammary epithelium but not by many breast tumor cell lines. Recombinant human maspin (rMaspin) inhibits tumor cell motility, invasion, and metastasis and thus has potential value as an anti-cancer therapeutic. Maspin is a member of the serpin family and, although the molecular mechanism by which maspin acts is unknown, recent work suggests that tissue plasminogen activator is a potential target. A puzzling observation in previous cell culture studies was loss of rMaspin activity at higher protein concentrations. One hypothesis to explain these results is self-association of rMaspin at the higher concentrations, which would be consistent with the tendency of serpins to form noncovalent polymers. This hypothesis is addressed by examining the relationship between rMaspin stability and self-association. Urea denaturation of rMaspin at pH 7 and 25 degrees C and at protein concentrations ranging from 0.01 to 0.2 mg/ml has been monitored by circular dichroism and intrinsic tryptophan fluorescence. Denaturation profiles show a protein concentration dependence and indicate the presence of at least one unfolding intermediate. The results suggest that destabilization of native monomeric rMaspin leads to partial unfolding and formation of an intermediate which can self-associate.

Modularity of serpins. A bifunctional chimera possessing alpha1-proteinase inhibitor and thyroxine-binding globulin properties

An exciting application of protein engineering is the creation of proteins with novel functions by the retrofitting of native proteins. Such attempts might be facilitated by the idea of a mosaic architecture of proteins out of structural units. Even though numerous theoretical concepts deal with the delineation of structural "modules," their potential in the design of proteins has not yet been sufficiently exploited. To address this question we used a gain of function approach by designing modular chimeric molecules out of two structurally homologous but functionally diverse members of the superfamily of serine-proteinase inhibitors, alpha1-proteinase inhibitor and thyroxine-binding globulin. Substitution of two of four alpha1-proteinase inhibitor modules (Lys222 to Leu288 and Pro362 to Lys394, respectively), identified by alpha-backbone distance analysis, with their thyroxine-binding globulin homologues resulted in a bifunctional chimera with inhibition of human leukocyte elastase and high affinity thyroxine binding. To our knowledge, this is the first report on a bifunctional chimera engineered from modules of homologous globular proteins. Our results demonstrate how a modular concept can facilitate the design of new functional proteins by swapping structural units chosen from members of a protein superfamily.

Probing the unfolding pathway of alpha1-antitrypsin

Protein misfolding plays a role in the pathogenesis of many diseases. alpha1-Antitrypsin misfolding leads to the accumulation of long chain polymers within the hepatocyte, reducing its plasma concentration and predisposing the patient to emphysema and liver disease. In order to understand the misfolding process, it is necessary to examine the folding of alpha1-antitrypsin through the different structures involved in this process. In this study we have used a novel technique in which unique cysteine residues were introduced at various positions into alpha1-antitrypsin and fluorescently labeled with N, N'-dimethyl-N-(iodoacetyl)-N'-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)ethylenediamine. The fluorescence properties of each protein were studied in the native state and as a function of guanidine hydrochloride-mediated unfolding. The studies found that alpha1-antitrypsin unfolded through a series of intermediate structures. From the position of the fluorescence probes, the fluorescence quenching data, and the molecular modeling, we show that unfolding of alpha1-antitrypsin occurs via disruption of the A and C beta-sheets followed by the B beta-sheet. The implications of these data on both alpha1-antitrypsin function and polymerization are discussed.

The mechanism of alpha 1-antitrypsin polymerization probed by fluorescence spectroscopy

The polymerization of alpha1-antitrypsin within the hepatic cell leads to alpha1-antitrypsin deficiency. Both the conformational changes and the kinetics of the polymerization process are poorly understood. Here we describe fluorescence experiments investigating the polymerization reaction using the fluorescent probe4, 4'-dianilino-1,1'-binaphthyl-5,5'-disulfonate (bis-ANS) which bound to both native and polymerized alpha1-antitrypsin. Biphasic changes in bis-ANS fluorescence were observed during formation of alpha1-antitrypsin polymers. Initially a rapid increase in fluorescence signal was observed; it was followed by a gradual reduction in fluorescence signal. The first phase is a conformational change in which the A beta-sheet of alpha1-antitrypsin opens, whereas the second phase represents the insertion of the reactive center loop into the A beta-sheet of another molecule and therefore determines the rate of the polymerization process.