Towards the treatment of polyglutamine diseases: the modulatory role of protein context

Protein aggregation is a key mechanism involved in neurodegeneration associated with Alzheimer's, Parkinson's and Huntington's diseases. Nine diseases (including Huntington's) arise from polyglutamine (polyQ) expansion above a repeat threshold of approximately 37 glutamines, and neuronal toxicity correlates with the process of protein aggregation. The similar toxic gain-of-function mechanism of the nine diseases supports the hypothesis that disease onset and progression is dependent upon polyQ expansion. However, there is an increasing body of literature demonstrating that the protein context of the polyQ tract has an essential role in the disease process. The composition of regions flanking repeats can alter the biochemical and biophysical properties of the polyQ region. A number of the disease proteins are proteolytically cleaved, with release of the polyQ-containing fragment initiating aggregation. Interactions of flanking domains with other molecules can also influence aggregation and cellular localization, which are critical factors for toxicity. More recently, there is evidence that domains flanking the polyQ tract can also aggregate independent of the polyQ tract, and that this significantly alters the rate at which the polyQ regions form fibrillar aggregates and the properties of these aggregates. In this review we consider the role of protein context in modulating the polyQ diseases and the therapeutic potential of targeting non-polyQ protein properties.

Dissection of protease-activated receptor-1-dependent and -independent responses to thrombin in skeletal myoblasts

Thrombin exerts a number of effects on skeletal myoblasts in vitro. It stimulates proliferation and intracellular calcium mobilization and inhibits differentiation and apoptosis induced by serum deprivation in these cells. Many cellular responses to thrombin are mediated by protease-activated receptor-1 (PAR-1). Expression of PAR-1 is present in mononuclear myoblasts in vitro, but repressed when fusion occurs to form myotubes. In the current study, we used PAR-1-null mice to determine which of thrombin's effects on myoblasts are mediated by PAR-1. Thrombin inhibited fusion almost as effectively in cultures prepared from the muscle of PAR-1-null myoblasts as in cultures prepared from wild-type mice. Apoptosis was inhibited as effectively in PAR-1-null myoblasts as in wild-type myoblasts. These effects in PAR-1-null myoblasts were mediated by a secreted inhibitor of apoptosis and fusion, as demonstrated previously for normal rat myoblasts. Thrombin failed to induce an intracellular calcium response in PAR-1-null myoblast cultures, although these cells were able to mobilize intracellular calcium in response to activation of other receptors. PAR-1-null myoblasts also failed to proliferate in response to thrombin. These results demonstrate that thrombin's effects on myoblast apoptosis and fusion are not mediated by PAR-1 and that PAR-1 is the only thrombin receptor capable of inducing proliferation and calcium mobilization in neonatal mouse myoblasts.

The role of strand 1 of the C beta-sheet in the structure and function of alpha(1)-antitrypsin

Serpins inhibit cognate serine proteases involved in a number of important processes including blood coagulation and inflammation. Consequently, loss of serpin function or stability results in a number of disease states. Many of the naturally occurring mutations leading to disease are located within strand 1 of the C beta-sheet of the serpin. To ascertain the structural and functional importance of each residue in this strand, which constitutes the so-called distal hinge of the reactive center loop of the serpin, an alanine scanning study was carried out on recombinant alpha(1)-antitrypsin Pittsburgh mutant (P1 = Arg). Mutation of the P10' position had no effect on its inhibitory properties towards thrombin. Mutations to residues P7' and P9' caused these serpins to have an increased tendency to act as substrates rather than inhibitors, while mutations at P6' and P8' positions caused the serpin to behave almost entirely as a substrate. Mutations at the P6' and P8' residues of the C beta-sheet, which are buried in the hydrophobic core in the native structure, caused the serpin to become highly unstable and polymerize much more readily. Thus, P6' and P8' mutants of alpha(1)-antitrypsin had melting temperatures 14 degrees lower than wild-type alpha(1)-antitrypsin. These results indicate the importance of maintaining the anchoring of the distal hinge to both the inhibitory mechanism and stability of serpins, the inhibitory mechanism being particularly sensitive to any perturbations in this region. The results of this study allow more informed analysis of the effects of mutations found at these positions in disease-associated serpin variants.

Probing the equilibrium denaturation of the serpin alpha(1)-antitrypsin with single tryptophan mutants; evidence for structure in the urea unfolded state

The native conformation of proteins in the serpin superfamily is metastable. In order to understand why serpins attain the native state instead of more stable conformations we have begun investigations into the equilibrium-unfolding of alpha(1)-antitrypsin. alpha(1)-Antitrypsin contains two tryptophan residues, Trp194 and Trp238, situated on the A and B beta-sheets, respectively. Site-directed mutagenesis was used to construct two single-tryptophan variants. Both variants were fully active and had similar secondary structure and stabilities to alpha(1)-antitrypsin. The denaturation of alpha(1)-antitrypsin and its variants was extremely similar when followed by far-UV CD, indicating the presence of a single intermediate. Fluorescence analysis of the unfolding behavior of each single tryptophan variant indicated that the sole tryptophan residue reported the structural changes within its immediate environment. These data suggest that the A beta-sheet is expanded in the intermediate state whilst no structural change around the B beta-sheet has occurred. In the urea-induced unfolded state, Trp238 does not become fully solvated, suggesting the persistence of structure around this residue. The implications of these data on the folding, misfolding and function of the serpin superfamily are discussed.

Osmolytes as modulators of conformational changes in serpins

Protein misfolding and aggregation play an integral role in many diseases. The misfolding of the serpin (SERine Proteinase INhibitor) alpha1-antitrypsin results in the accumulation of insoluble polymers within hepatocytes and alpha1-antitrypsin deficiency in plasma, predisposing patients to liver cirrhosis and emphysema. We have examined the effect of three naturally occurring osmolytes, sarcosine, glycine betaine and trimethylamine N-oxide, on conformational changes in alpha1-antitrypsin. All three solutes protected native alpha1-antitrypsin against thermally induced polymerisation and inactivation in a concentration-dependent manner. Further spectroscopic analysis showed that sarcosine stabilises the native conformation of alpha1-antitrypsin, thus hindering its conversion to an intermediate state and subsequent polymerisation. On refolding in the presence of sarcosine, alpha1-antitrypsin formed a heterogeneous population, with increasing proportions of molecules adopting an inactive conformation in higher concentrations of the osmolyte. These data show that sarcosine can be used to prevent abnormal structural changes in native alpha1-antitrypsin, but is ineffective in facilitating the correct folding of the protein. The implications of these results in the context of conformational changes and states adopted by alpha1-antitrypsin are discussed.

Complex between Peptostreptococcus magnus protein L and a human antibody reveals structural convergence in the interaction modes of Fab binding proteins

BACKGROUND

Peptostreptococcus magnus protein L (PpL) is a multidomain, bacterial surface protein whose presence correlates with virulence. It consists of up to five homologous immunoglobulin binding domains that interact with the variable (VL) regions of kappa light chains found on two thirds of mammalian antibodies.

RESULTS

We refined the crystal structure of the complex between a human antibody Fab fragment (2A2) and a single PpL domain (61 residues) to 2.7 A. The asymmetric unit contains two Fab molecules sandwiching a single PpL domain, which contacts similar VL framework regions of two light chains via independent interfaces. The residues contacted on VL are remote from the hypervariable loops. One PpL-Vkappa interface agrees with previous biochemical data, while the second is novel. Site-directed mutagenesis and analytical-centrifugation studies suggest that the two PpL binding sites have markedly different affinities for VL. The PpL residues in both interactions are well conserved among different Peptostreptococcus magnus strains. The Fab contact positions identified in the complex explain the high specificity of PpL for antibodies with kappa rather than lambda chains.

CONCLUSIONS

The PpL-Fab complex shows the first interaction of a bacterial virulence factor with a Fab light chain outside the conventional combining site. Structural comparison with two other bacterial proteins interacting with the Fab heavy chain shows that PpL, structurally homologous to streptococcal SpG domains, shares with the latter a similar binding mode. These two bacterial surface proteins interact with their respective immunoglobulin regions through a similar beta zipper interaction.

Prevention of polymerization of M and Z alpha1-Antitrypsin (alpha1-AT) with trimethylamine N-oxide. Implications for the treatment of alpha1-at deficiency

alpha1-Antitrypsin (alpha1-AT) is the most abundant circulating proteinase inhibitor. The Z variant results in profound plasma deficiency as the mutant polymerizes within hepatocytes. The retained polymers are associated with cirrhosis, and the lack of circulating protein predisposes to early onset emphysema. We have investigated the role of the naturally occurring solute trimethylamine N-oxide (TMAO) in modulating the polymerization of normal M and disease-associated Z alpha1-AT. TMAO stabilized both M and Z alpha1-AT in an active conformation against heat-induced polymerization. Spectroscopic analysis demonstrated that this was due to inhibition of the conversion of the native state to a polymerogenic intermediate. However, TMAO did not aid the refolding of denatured alpha1-AT to a native conformation; instead, it enhanced polymerization. These data show that TMAO can be used to control the conformational transitions of folded alpha1-AT but that it is ineffective in promoting folding of the polypeptide chain within the secretory pathway.

Intrinsic fluorescence changes and rapid kinetics of proteinase deformation during serpin inhibition

The X-ray crystal structure of the serpin-proteinase complex suggested that the serpin deformed the proteinase thereby inactivating the molecule. Using a variant of alpha(1)-antitrypsin in which both tryptophan residues have been replaced by phenylalanine, we have shown that the proteinase becomes partially unfolded during serpin inhibition. The tryptophan free variant, alpha(1)-antitrypsin((FF)), is fully active as an inhibitor of thrombin. Thrombin has a fluorescence emission maximum of 340 nm which blue shifts to 346 nm, concomitant with a 40% increase in intensity, upon formation of the serpin-proteinase complex indicative of substantial conformational change within the proteinase. Stopped-flow analysis of the fluorescence changes within the proteinase indicated a two-step mechanism. A fast bimolecular reaction with a rate constant of 2.8x10(6) M(-1) s(-1) is followed by a slow unimolecular process with a rate of 0.26 s(-1) that is independent of concentration. We propose that the first rate is formation of an initial complex which is then followed by a slower process involving the partial unfolding of the proteinase during its translocation to the opposite pole of the serpin.

Studies on a single immunoglobulin-binding domain of protein L from Peptostreptococcus magnus: the role of tyrosine-53 in the reaction with human IgG

Chemical modification experiments with tetranitromethane (TNM) have been used to investigate the role of tyrosine residues in the formation of the complex between PpL (the single Ig-binding domain of protein L, isolated from P. magnus strain 3316) and the kappa light chain (kappa-chain). Reaction of PpL with TNM causes the modification of 1.9 equiv. of tyrosine (Tyr(51) and Tyr(53)) and results in an approx. 140-fold decrease in affinity for human IgG. Similar experiments with mutated PpL proteins suggest that nitration predominantly inactivates the protein by modification of Tyr(53). Reduction of the nitrotyrosine groups to aminotyrosine by incubation with sodium hydrosulphite does not restore high affinity for IgG. Modification of kappa-chain by TNM resulted in the nitration of 3.1+/-0.09 tyrosine residues. When the PpL-kappa-chain complex was incubated with TNM, 4.1+/-0.04 tyrosine residues were nitrated, indicating that one tyrosine residue previously modified by the reagent was protected from TNM when the proteins are in complex with each other. The K(d) for the equilibrium between PpL, human IgG and their complex has been shown by ELISA to be 112+/-20 nM. A similar value (153+/-33 nM) was obtained for the complex formed between IgG and the Tyr(64)-->Trp mutant (Y64W). However, the K(d) values for the equilibria involving the PpL mutants Y53F and Y53F,Y64W were found to be 3.2+/-0.2 and 4.6+/-1 microM respectively. These suggest that the phenol group of Tyr(53) in PpL is important to the stability of the PpL-kappa-chain complex.

Structure of a serpin-enzyme complex probed by cysteine substitutions and fluorescence spectroscopy

The x-ray crystal structure of the serpin-proteinase complex is yet to be determined. In this study we have investigated the conformational changes that take place within antitrypsin during complex formation with catalytically inactive (thrombin(S195A)) and active thrombin. Three variants of antitrypsin Pittsburgh (an effective thrombin inhibitor), each containing a unique cysteine residue (Cys(232), Cys(P3'), and Cys(313)) were covalently modified with the fluorescence probe N,N'-dimethyl-N-(iodoacetyl)-N'-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)ethylenediamine. The presence of the fluorescent label did not affect the structure or inhibitory activity of the serpin. We monitored the changes in the fluorescence emission spectra of each labeled serpin in the native and cleaved state, and in complex with active and inactive thrombin. These data show that the serpin undergoes conformational change upon forming a complex with either active or inactive proteinase. Steady-state fluorescence quenching measurements using potassium iodide were used to further probe the nature and extent of this conformational change. A pronounced conformational change is observed upon locking with an active proteinase; however, our data reveal that docking with the inactive proteinase thrombin(S195A) is also able to induce a conformational change in the serpin.

Protease-activated receptor-2 mediates proliferative responses in skeletal myoblasts

Protease-activated receptor-2 (PAR-2) is a G protein-coupled receptor that is cleaved by proteases within the N terminus, exposing a new tethered ligand that binds and activates the receptor. Activators of PAR-2 include trypsin and mast cell tryptase. Skeletal myoblasts are known to express PAR-1, a thrombin receptor. The current study was undertaken to determine whether myoblasts express PAR-2. Primary neonatal rat and mouse skeletal myoblast cultures were shown to express PAR-2 in polymerase chain reaction and immunocytochemical studies. Expression of PAR-2 was also demonstrated by immunohistochemistry in developing mouse skeletal muscle in vivo. Trypsin or a synthetic peptide corresponding to the rat PAR-2 tethered ligand caused a dose-dependent elevation in intracellular calcium in cultured rat myoblasts, with an EC(50) of 13 nM or 56 microM, respectively. Studies aimed at identifying the function of PAR-2 in myoblasts demonstrated no effect of the receptor-activating peptide on survival or fusion in serum-deprived myoblasts. The PAR-2-activating peptide did, however, stimulate proliferation of serum-deprived myoblasts. These results demonstrate that skeletal muscle cells express PAR-2, activation of which leads to stimulation of myoblast proliferation.

The inositol polyphosphate 5-phosphatases and the apurinic/apyrimidinic base excision repair endonucleases share a common mechanism for catalysis

Inositol polyphosphate 5-phosphatases (5-phosphatase) hydrolyze the 5-position phosphate from the inositol ring of phosphatidylinositol-derived signaling molecules; however, the mechanism of catalysis is only partially characterized. These enzymes play critical roles in regulating cell growth, apoptosis, intracellular calcium oscillations, and post-synaptic vesicular trafficking. The UCLA fold recognition server (threader) predicted that the conserved 300-amino acid catalytic domain, common to all 5-phosphatases, adopts the fold of the apurinic/apyrimidinic (AP) base excision repair endonucleases. PSI-BLAST searches of GENPEPT, using the amino acid sequence of AP endonuclease exonuclease III, identified all members of the 5-phosphatase family with highly significant scores. A sequence alignment between exonuclease III and all known 5-phosphatases revealed six highly conserved motifs containing residues that corresponded to the catalytic residues in the AP endonucleases. Mutation of each of these residues to alanine in the mammalian 43-kDa, or yeast Inp52p 5-phosphatase, resulted in complete loss of enzyme activity. We predict the 5-phosphatase enzymes share a similar mechanism of catalysis to the AP endonucleases, consistent with other common functional similarities such as an absolute requirement for magnesium for activity. Based on this analysis, functional roles have been assigned to conserved residues in all 5-phosphatase enzymes.

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.

The citrate ion increases the conformational stability of alpha(1)-antitrypsin

Sodium citrate has previously been shown to convert native alpha(1)-antitrypsin into the inactive latent state and cause alpha(1)-antitrypsin to polymerize via the C-sheet pathway instead of the more common A-sheet pathway. In order to begin to understand these dramatic effects, we have examined the influence of low concentrations of sodium citrate upon the structure, stability and function of alpha(1)-antitrypsin. In 0.5 M citrate, the midpoint of guanidine hydrochloride-induced unfolding was increased by 1.8 M and the rate of heat inactivation was decreased approximately 30-fold compared with Tris or phosphate buffer. alpha(1)-Antitrypsin was fully active in the presence of a range of citrate concentrations (0. 1-0.5 M), forming a stable 1:1 complex with chymotrypsin. The association rate constant between alpha(1)-antitrypsin and chymotrypsin was decreased with increasing citrate concentration. Fluorescence and circular dichroism spectroscopy demonstrated no significant changes in the tertiary structure due to the presence of citrate. However, the insertion rate of exogenous reactive-center loop peptide increased with increasing citrate concentration, indicating some structural changes in the A beta-sheet region. Taken together, these data suggest that in the presence of 0.5 M citrate alpha(1)-antitrypsin adopts a highly stable but active conformation.

Cleaved antitrypsin polymers at atomic resolution

Alpha1-antitrypsin deficiency, which can lead to both emphysema and liver disease, is a result of the accumulation of alpha1-antitrypsin polymers within the hepatocyte. A wealth of biochemical and biophysical data suggests that alpha1-antitrypsin polymers form via insertion of residues from the reactive center loop of one molecule into the beta-sheet of another. However, this long-standing hypothesis has not been confirmed by direct structural evidence. Here, we describe the first crystallographic evidence of a beta-strand linked polymer form of alpha1-antitrypsin: the crystal structure of a cleaved alpha1-antitrypsin polymer.

Serpins in the Caenorhabditis elegans genome

Data mining in genome sequences can identify distant homologues of known protein families, and is most powerful if solved structures are available to reveal the three-dimensional implications of very dissimilar sequences. Here we describe putative serpin sequences identified with very high statistical significance in the Caenorhabditis elegans genome. When mapped onto vertebrate serpins such as alpha1-antitrypsin, they suggest novel structural features. Some appear complete, some show extensive deletions, and others appear to contain only the C-terminal part of the known serpin fold, probably in partnership with N-terminal regions that have conformations unlike those of known serpins. The observation of such striking sequence similarity, in proteins that must have significantly different overall structures, substantially extends the structural characteristics of the serpin family of proteins.

Interactions between a single immunoglobulin-binding domain of protein L from Peptostreptococcus magnus and a human kappa light chain

The placement of a tryptophan residue into a single Ig-binding-domain of protein L from Peptostreptococcus magnus has been used to examine the binding interactions between the binding domain and kappa light chains (kappa-chains). The fluorescence intensity of the mutant domain increases on the formation of a complex with kappa-chains. This has been used to determine the Kd of the complex under a range of conditions by using both pre-equilibrium and equilibrium methods. The Kd values determined for the complex with kappa-chains at a number of different pH values are very close to those obtained with the wild-type domain, indicating that the mutation has not substantially affected its binding properties. Examination of the reaction between the mutant domain and kappa-chains by stopped-flow fluorescence shows that complex formation takes place by two discrete, sequential processes. A fast bimolecular reaction, with a rate constant of 8.3x10(5) M-1. s-1 (at pH8.0 and 25 degrees C), is followed by a slow unimolecular process with a rate (1.45 s-1) that is independent of the concentration of the reactants. This suggests that a conformational change occurs after the initial encounter complex is formed. The dissociation of the complex at equilibrium occurs in a single process of rate 0.095 s-1 at pH8.0 and 25 degrees C. Stopped-flow CD studies show that a slow decrease in ellipticity at 275 nm occurs with a rate of 1.3 s-1 when wild-type protein binds to kappa-chains, suggesting that the conformational transition might involve a change in environment around one or more tyrosine residues.

Thrombin, a survival factor for cultured myoblasts

Three members of the family of protease-activated receptors (PARs), PARs-1, -3 and -4, have been identified as thrombin receptors. PAR-1 is expressed by primary myoblast cultures, and expression is repressed once myoblasts fuse to form myotubes. The current study was undertaken to investigate the hypothesis that thrombin inhibits myoblast fusion. Primary rodent myoblast cultures were deprived of serum to promote myoblast fusion and then cultured in the presence or absence of thrombin. Thrombin inhibited myoblast fusion, but another notable effect was observed; 50% of control cells were apoptotic within 24 h of serum deprivation, whereas less than 15% of thrombin-treated cells showed signs of apoptosis. Proteolysis was required for the effect of thrombin, but no other serine protease tested mimicked the action of thrombin. Neither a PAR-1- nor a PAR-4-activating peptide inhibited apoptosis or fusion, and myoblast cultures were negative for PAR-3 expression. Myoblasts exposed to thrombin for 1 h and then changed to medium without thrombin accumulated apoptosis inhibitory activity in their medium over the subsequent 20 h. Thus the protective action of thrombin appears to be effected through cleavage of an unidentified thrombin receptor, leading to secretion of a downstream apoptosis inhibitory factor. These results demonstrate that thrombin functions as a survival factor for myoblasts and is likely to play an important role in muscle development and repair.

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 intracellular serpin proteinase inhibitor 6 is expressed in monocytes and granulocytes and is a potent inhibitor of the azurophilic granule protease, cathepsin G

The monocyte and granulocyte azurophilic granule proteinases elastase, proteinase 3, and cathepsin G are implicated in acute and chronic diseases thought to result from an imbalance between the secreted proteinase(s) and circulating serpins such as alpha1-proteinase inhibitor and alpha1-antichymotrypsin. We show here that the intracellular serpin, proteinase inhibitor 6 (PI-6), is present in monocytes, granulocytes, and myelomonocytic cell lines. In extracts from these cells, PI-6 bound an endogenous membrane-associated serine proteinase to form an sodium dodecyl sulfate (SDS)-stable complex. Using antibodies to urokinase, elastase, proteinase 3, or cathepsin G, we demonstrated that the complex contains cathepsin G. Native cathepsin G and recombinant PI-6 formed an SDS-stable complex in vitro similar in size to that observed in the extracts. Further kinetic analysis demonstrated that cathepsin G and PI-6 rapidly form a tight 1:1 complex (ka = 6.8 +/- 0.2 x 10(6) mol/L-1s-1 at 17 degrees C; Ki = 9.2 +/- 0.04 x 10(-10) mol/L). We propose that PI-6 complements alpha1-proteinase inhibitor and alpha1-antichymotrypsin (which control extracellular proteolysis) by neutralizing cathepsin G that leaks into the cytoplasm of monocytes or granulocytes during biosynthesis or phagocytosis. Control of intracellular cathepsin G may be particularly important, because it has recently been shown to activate the proapoptotic proteinase, caspase-7.

The production and characterisation of an immobilised chaperonin system

Here we report a method of immobilising the chaperonins GroEL and GroES to a glass matrix. The immobilised chaperone system has been used to successfully refold target proteins denatured by guanidine hydrochloride and produce substantially higher levels of active protein than occur on dilution into aqueous solution alone. The chaperone system has been shown to refold proteins from each of the three categories of GroEL substrate. The refolding of the enzyme glycerol dehydrogenase from Bacillus stearothermophilus shows a two-fold increase in activity in the presence of immobilised GroEL compared to that in free solution. The lactate dehydrogenase from B. stearothermophilus also shows a two-fold higher yield of activity in the presence of the immobilised GroEL and ATP. The presence of immobilised GroEL in the absence of ATP arrests the refolding of LDH. The enzyme citrate synthetase from porcine heart demonstrates a three-fold increase in activity when refolded in the presence of immobilised GroEL, ATP and free GroES. Similar results are obtained in the presence of free GroEL, immobilised GroES and ATP. The matrix-bound chaperone can be removed from the refolding mixture by centrifugation, producing a reusable system that can be easily isolated and purified from the refolded substrate.

Protein engineering of chimeric Serpins: an investigation into effects of the serpin scaffold and reactive centre loop length

The exposed Serpin reactive centre loop controls the specificity of the serpin proteinase interaction. Mutations within this region have been used to generate novel potentially therapeutic inhibitors. In this study we examine the effect of the serpin scaffold and reactive centre loop length upon the generation of such inhibitors. The reactive centre loop regions, P7-P3', of alpha1-antitrypsin and alpha1-antichymotrypsin were replaced by the corresponding residues of the viral serpin, Serp1, to form AT/Serp1 and ACT/Serp1, respectively. AT/Serp1 formed SDS stable complexes with a range of proteinases with association rate constants for plasmin, tissue plasminogen activator, urokinase, thrombin and factor Xa of approximately 10(4) M(-1)s(-1) and a stoichiometry of inhibition of approximately 1 for all of them. ACT/Serp1, however, formed SDS-stable complexes with only plasmin and thrombin with association rate constant 100-fold slower than AT/Serp1 and an increased stoichiometry of inhibition. The reactive centre loop of ACT/Serp1 is four amino acid residues longer than AT/Serp1. These four additional residues (VETR) were inserted into AT/Serp1 to form AT/Serp1(VETR). AT/Serp1(VETR) formed SDS stable complexes with plasmin, thrombin and tissue plasminogen activator similar to AT/Serp1, however, the association rate constants were 10-fold slower than those observed with AT/Serp1, while the stoichiometry of inhibition remained around 1. These results suggest that the additional reactive centre loop residues effect the rate of initial complex formation by placing the reactive centre loop in a non-ideal conformation. This study demonstrates that both reactive centre loop length and serpin scaffold are important in defining the inhibitory characteristics of a serpin.

Alpha 1-antitrypsin polymerisation can occur by both loop A and C sheet mechanisms

A number of disease states are attributable to alpha1-antitrypsin polymerisation within the endoplasmic reticulum of hepatocytes and subsequent plasma deficiency. Two distinct mechanisms describing the process of alpha1-antitrypsin polymerisation have been proposed, the loop A-sheet and C-sheet mechanisms. We report fluorescence studies using alpha1-antitrypsin covalently modified with pyrene maleimide. These results in conjunction with detailed molecular modelling studies, show that alpha1-antitrypsin is capable of undergoing both loop A-sheet and loop C-sheet mechanisms of polymerisation, depending upon the in vitro buffer conditions.

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.

A study of the interactions between an IgG-binding domain based on the B domain of staphylococcal protein A and rabbit IgG

The nonantigenic interaction between a recombinant immunoglobulin G (IgG)-binding protein based on the B domain of Protein A from Staphylococcus aureus (termed SpA1) and the Fc fragment of rabbit IgG has been investigated. The contribution to binding of four putative hydrogen bond contacts between SpA1 and IgG-Fc were examined by the individual substitution of the residues in SpA1 involved in these interactions by others unable to form hydrogen bonds. It was found that the most important of the hydrogen bonds involved Tyr 18 which, when replaced by Phe, resulted in a twofold decrease in IgG-binding affinity. The residues of SpA1 proposed to make close, mainly hydrophobic, contacts with Fc were replaced by residues with potential electrostatic charge to establish the importance of the hydrophobic interaction in the complex. The IgG-binding affinities of the mutant proteins were compared to the wild-type protein by a competitive enzyme-linked immunosorbent assay. The replacement of individual hydrophobic residues by His generated a number of novel IgG-binding proteins with reduced binding affinity at pH 5.0 but which maintained strong binding affinities at pH 8.0. The elution profile of human IgG1-Fc (Fc fragment of human IgG1) from a column made from an immobilized two-domain mutant protein shows that the complex dissociates at a higher pH relative to that of the non-mutated protein thus offering favorable elution characteristics.

Expression and purification of the human thrombin receptor

The human thrombin receptor has been overexpressed in Sf9 (Spodoptera frugiperda) insect cells using a baculovirus vector. Cell surface expression of the receptor was confirmed by immunocytochemistry with polyclonal antibodies raised against the extracellular domain of the receptor. The expressed receptor was functional; both thrombin and the thrombin receptor agonist peptide produced increases in intracellular calcium in transfected cells. The concentration of thrombin causing the half-maximal increase (EC50) in intracellular calcium was 3.9 nM, whereas the EC50 for the agonist peptide was 2.7 microM. However, the observed maximum increase in intracellular calcium concentration with the agonist peptide (547 nM) was twofold greater than that observed with thrombin (258 nM). The recombinant receptor was purified by immunoaffinity chromatography using a monoclonal antibody raised against the receptor extracellular domain. The purified preparation contained two species with apparent molecular masses of 48 and 90 kDa, both of which were recognized by mono- and polyclonal antibodies against the thrombin receptor. The yield of the purified receptor was 0.78 mg/liter of insect cells suspension culture (10(6) cells/ml). The purified thrombin receptor will be useful in future structural and functional studies.

The effects of reactive centre loop length upon serpin polymerisation

The clinical effects of serpin polymerisation include thromboembolism, emphysema, and liver disease. A through understanding of serpin polymerisation mechanisms and the structures involved will permit the rational design of therapeutic polymerisation inhibitors. Here we show that serpin polymerisation can be delayed by extending the length of the serpin reactive centre loop. The heat stability of three chimeric serpins was examined. One of them, an active alpha 1-antitrypsin variant with a reactive centre loop C-terminal extension of four amino acid residues, was shown to have increased resistance to inactivation by polymerisation. This variant could also form serpin/peptide binary complexes with a reactive centre loop peptide, which indicates that the increase in thermostability was not due to the A-beta-sheet being unable to accept reactive centre loop residues, an essential requirement for polymerisation. Rather, we conclude that the additional residues within the reactive centre loop delay the release of strand 1C from the C-sheet, a process essential for polymer formation.

Recombinant caspase-3 expressed in Pichia pastoris is fully activated and kinetically indistinguishable from the native enzyme

Intracellular cysteine proteinases (caspases) play key roles in inflammation and apoptosis. Recombinant caspases are typically produced in Escherichia coli expression systems with the attendant problems of solubilization, re-folding and activation of the protease. Here we describe the expression of hexahistidine-tagged caspase-3 (CPP32/Yama/Apopain) in the methylotropic yeast Pichia pastoris, and the purification of soluble enzyme from yeast lysates using cobalt affinity chromatography. The recombinant protease is fully activated, stable, and cleaves the synthetic substrate DEVD-AFC (Km 16.8 microM) but not YVAD-AFC. It mediates the cleavage of the apoptotic death substrate poly(ADP-ribose) polymerase in cell extracts, but does not cleave pro-interleukin-1beta. It is inhibited by the peptide DEVD-CHO (Ki 2.2 nM), far less efficiently by YVAD-CMK (Ki 0.3 microM), and not detectably by CrmA. By these criteria, recombinant caspase-3 is indistinguishable from native caspase-3 purified from apoptotic cell extracts. Activation of recombinant caspase-3 occurs in yeast in the absence of any intrinsic caspase activity, suggesting that caspase-3 can auto-activate. However, the purified enzyme was incapable of cleaving pro-caspase-3 indicating that autoactivation of caspase-3 in vivo is not likely to occur unless very high concentrations are achieved.

Crystallization and X-ray analysis of a single fab binding domain from protein L of Peptostreptococcus magnus

Protein L is a multidomain cell wall constituent of certain strains of Peptostreptococcus magnus which binds to the variable domain of immunoglobulin kappa-light chains. A single immunoglobulin-binding domain of M(r) = 9000 from this protein has been isolated and crystallized. The crystals are of space group P4(2)2(1)2, with cell dimensions a = b = 66.9 A, c = 68.3 A, and diffract to at least 2.2 A resolution. The asymmetric unit of the crystal contains two molecules of the protein L domain, related by a noncrystallographic 2-fold axis, as revealed by a self-rotation function calculated with native diffraction data.

Equilibrium and pre-equilibrium fluorescence spectroscopic studies of the binding of a single-immunoglobulin-binding domain derived from protein G to the Fc fragment from human IgG1

A single-immunoglobulin-binding protein based upon the C2 domain of Protein G from Streptococcus has been shown to bind tightly to the Fc fragment of IgG1. The binding interaction results in a decrease in the fluorescence intensity from the sole Trp residue (Trp-48) in this domain. This spectral change has been used to monitor the binding interactions between the two proteins using equilibrium and pre-equilibrium fluorescence spectroscopy. Comparison of the data from the two techniques suggests that a conformational change occurs after the initial formation of the complex. Mutagenesis studies have shown that the Trp residue is important for binding and that replacement by a Phe residue is important for binding and that replacement by a Phe residue leads to a 300-fold decrease in the affinity for Fc gamma 1. Determination of the rate constants kon and koff at different values of pH between 4.0 and 9.0 suggest that variations in Kd are mediated predominantly by changes in kon. Competition experiments between SpG1 and a single-IgG-binding domain from Protein A from Staphylococcus aureus have been used to determine the affinity of the latter for Fc gamma 1.

Elution of human IgG from affinity columns containing immobilised variants of protein A

Immobilised analogues of protein A have been used for affinity chromatographic separation of human IgG. Truncation of the C-terminal region of an engineered IgG-binding domain based upon the B domain from protein A, in combination with site-directed mutagenesis, has led to the generation of a number of proteins with a decreased affinity for IgG. The elution of human IgG from these proteins when immobilised onto a solid support occurs over the pH range 3.2-5.0 with 0.5 M acetate buffer. These proteins may be effective alternatives to standard protein A columns when milder elution conditions are required.

The stability and unfolding of an IgG binding protein based upon the B domain of protein A from Staphylococcus aureus probed by tryptophan substitution and fluorescence spectroscopy

The stability and unfolding of an immunoglobulin (Ig) G binding protein based upon the B domain of protein A (SpAB) from Staphylococcus aureus were studied by substituting tryptophan residues at strategic locations within each of the three alpha-helical regions (alpha 1-alpha 3) of the domain. The role of the C-terminal helix, alpha 3, was investigated by generating two protein constructs, one corresponding to the complete SpAB, the other lacking a part of alpha 3; the Trp substitutions were made in both one- and two-domain versions of each of these constructs. The fluorescence properties of each of the single-tryptophan mutants were studied in the native state and as a function of guanidine-HCl-mediated unfolding, and their IgG binding activities were determined by a competitive enzyme-linked immunosorbent assay. The free energies of folding and of binding to IgG for each mutant were compared with those for the native domains. The effect of each substitution upon the overall structure and upon the IgG binding interface was modelled by molecular graphics and energy minimization. These studies indicate that (i) alpha 3 contributes to the overall stability of the domain and to the formation of the IgG binding site in alpha 1 and alpha 2, and (ii) alpha 1 unfolds first, followed by alpha 2 and alpha 3 together.