Contribution of disulfide bonds to stability of the folding intermediate of alpha-lactalbumin

Effects of Metal Ions, Temperature, and a Denaturant on the Oxidative Folding Pathways of Bovine α-Lactalbumin

Bovine α-lactalbumin (αLA) has four disulfide (SS) bonds in the native form (N). On the oxidative folding pathways of this protein, two specific SS folding intermediates, i.e., (61–77, 73–91) and des[6–120], which have two and three native SS bonds, respectively, accumulate predominantly in the presence of Ca²⁺. In this study, we reinvestigated the pathways using a water-soluble cyclic selenoxide reagent, trans-3,4-dihydroxyselenolane oxide (DHS^ox), as a strong and quantitative oxidant to oxidize the fully reduced form (R). In the presence of ethylenediaminetetraacetic acid (EDTA) (under a metal-free condition), SS formation randomly proceeded, and N did not regenerate. On the other hand, two specific SS intermediates transiently generated in the presence of Ca²⁺. These intermediates could be assigned to (61–77, 73–91) and des[6–120] having two common SS bonds, i.e., Cys61-Cys77 and Cys73-Cys91, near the calcium binding pocket of the β-sheet domain. Much faster folding to N was observed in the presence of Mn²⁺, whereas Na⁺, K⁺, Mg²⁺, and Zn²⁺ did not affect the pathways. The two key intermediates were susceptible to temperature and a denaturant. The oxidative folding pathways revealed were significantly different from those of hen egg white lysozyme, which has the same SS-bonding pattern as αLA, suggesting that the folding pathways of SS-containing proteins can alter depending on the amino acid sequence and other factors, even when the SS-bond topologies are similar to each other.

Interactions responsible for secondary structure formation during folding of equine beta-lactoglobulin

Equine beta-lactoglobulin forms a compact intermediate at an acidic pH (A state). It also forms an expanded and helical conformation at low temperatures (C state). The structure of a single disulfide mutant C66A/C160A is similar to the A state in the presence of salts, while it is similar to the C state at low anion concentrations. We have investigated the temperature-dependent change in the secondary structure using circular dichroism and proline scanning mutagenesis. At low anion concentrations, the helical content increased linearly as temperature decreased. In the presence of salts, the A state was cooperatively transformed into the C state at low temperatures. This suggests the importance of hydrophobic interactions for stabilizing the A state. Peptides encompassing native-like and non-native alpha-helices were synthesized to investigate the interactions responsible for helix formation in the A and C states. These did not form stable helices, indicating that not only the helices in the A state but also the helices in the C state are stabilized by long-range interactions. A longer fragment, CHIBL, which encompasses the structured region in the A and C states, showed a helical structure. Proline-substituted mutants of CHIBL showed CD spectral changes similar to the corresponding mutants of the full-length protein in the C state. Therefore, CHIBL has a structure similar to the corresponding region of the full-length protein in the C state. This result indicates that interactions responsible for helix formation in the C state reside in the sequence of CHIBL, and that the sequences outside CHIBL are essential for secondary structure formation in the A state.

Structural and thermodynamic consequences of removal of a conserved disulfide bond from equine beta-lactoglobulin

A disulfide bond between cysteine 66 and cysteine 160 of equine beta-lactoglobulin was removed by substituting cysteine residues with alanine. This disulfide bond is conserved across the lipocalin family. The conformation and stability of the disulfide-deleted mutant protein was investigated by circular dichroism. The mutant protein assumes a native-like structure under physiological conditions and assumes a helix-rich molten globule structure at acid pH or at moderate concentrations of urea as the wild-type protein does. The urea-induced unfolding experiment shows that the stability of the native conformation was reduced but that of the molten globule intermediate is not significantly changed at pH 4 by removal of the disulfide bond. On the other hand, the molten globule at acid pH was destabilized by removal of the disulfide bond. This difference in the stabilizing effect of the disulfide bond was interpreted by the effect of the disulfide in keeping the molecule compact against the electrostatic repulsion at acid pH. In contrast to the wild-type protein, the circular dichroism spectrum in the molten globule state at acid pH depends on anion concentration, suggesting that the expansion of the molecule through electrostatic repulsion induces alpha-helices as observed in the cold denatured state of the wild-type protein.

Chaperone-like activity and surface hydrophobicity of 70S ribosome

Ribosomes have been shown to mediate refolding of proteins in vitro. In order to understand the mechanism of action, we have explored the 70S ribosome surface for hydrophobicity, one of the important aspects in chaperone-target protein interaction. We find that the 70S ribosome displays significant hydrophobicity on its surface when probed with the hydrophobic fluorophore 8-anilino-1-naphthalene sulfonate. To understand the functional significance of this hydrophobicity we investigated the ability of the ribosome to prevent aggregation of insulin B chain and alpha-lactalbumin induced by reducing the interchain and intrachain disulfide bond respectively with dithiothreitol (DTT) and photo aggregation of gamma-crystallin at 37 degrees C. The 70S ribosome offers complete protection towards light-induced aggregation of gamma-crystallin (at 1:2 (w/w) ratio of crystallin:ribosome) and DTT-induced aggregation of alpha-lactalbumin (at 1:3) and there is appreciable protection (at 1:3) against the aggregation of insulin B chain. We also investigated the role of 70S ribosome in refolding of bovine carbonic anhydrase. Ribosomes improved the folding yield in a concentration-dependent manner. These results clearly demonstrate a general chaperone-like activity of 70S ribosome and implicate its surface hydrophobicity.

Aggregate formation and the structure of the aggregates of disulfide-reduced proteins

Aggregate formation and the structure of the aggregates of disulfide-reduced proteins were investigated using alpha-lactalbumin and lysozyme as model proteins. First, reducing conditions were adjusted so that only one of the four disulfide bonds present in each native protein was cleaved. These three-disulfide (3SS) proteins are known to adopt almost native conformations, yet formed precipitates with a basic peptide, lactoferricin, and heparin and heparin fragment, respectively, at concentrations at which native proteins mixed with these compounds remained clear. The 3SS-lysozyme also formed precipitates in the absence of these ligands. Thus, subtle structural changes could lead to aggregation. Electron microscopy revealed fibrillar structures in the aggregates of extensively reduced proteins in the absence of ligands but not in their presence, which shows that the reduction of disulfide bonds suffices for fibril formation and that ligands inhibit fibril formation.

Active-site mutagenesis of tetanus neurotoxin implicates TYR-375 and GLU-271 in metalloproteolytic activity

Tetanus neurotoxin (TeNT) blocks neurotransmitter release by cleaving VAMP/synaptobrevin, a membrane associated protein involved in synaptic vesicle fusion. Such activity is exerted by the N-terminal 50kDa domain of TeNT which is a zinc-dependent endopeptidase (TeNT-L-chain). Based on the three-dimensional structure of botulinum neurotoxin serotype A (BoNT/A) and serotype B (BoNT/B), two proteins closely related to TeNT, and on X-ray scattering studies of TeNT, we have designed mutations at two active site residues to probe their involvement in activity. The active site of metalloproteases is composed of a primary sphere of residues co-ordinating the zinc atom, and a secondary sphere of residues that determines proteolytic specificity and activity. Glu-261 and Glu-267 directly co-ordinates the zinc atom in BoNT/A and BoNT/B respectively and the corresponding residue of TeNT was replaced by Asp or by the non conservative residue Ala. Tyr-365 is 4.3A away from zinc in BoNT/A, and the corresponding residue of TeNT was replaced by Phe or by Ala. The purified mutants had CD, fluorescence and UV spectra closely similar to those of the wild-type molecule. The proteolytic activity of TeNT-Asp-271 (E271D) is similar to that of the native molecule, whereas that of TeNT-Phe-375 (Y375F) is lower than the control. Interestingly, the two Ala mutants are completely devoid of enzymatic activity. These results demonstrate that both Glu-271 and Tyr-375 are essential for the proteolytic activity of TeNT.

Recombinant and truncated tetanus neurotoxin light chain: cloning, expression, purification, and proteolytic activity

Tetanus neurotoxin (TeNT) consists of two disulfide-linked polypeptide chains, heavy (H) and light (L). The L chain is a zinc endopeptidase protein highly specific for vesicle-associated membrane protein (VAMP), which is an essential component of the exocytosis apparatus. Here we describe the cloning of the L chain of TeNT from Clostridium tetani strain Y-IV-3 (WS 15) and its expression in Escherichia coli as a glutathione S-transferase fusion protein. The full-length recombinant L chain, corresponding to residues 1-457, was obtained as a mixture of proteins of slightly different mass with identical N-terminal ends. To obtain a product useful for structural analysis and crystallization, a COOH-terminally truncated L chain (residues 1-427) was cloned, expressed, and purified with high yield. This truncated L chain is more active than the full-length and wild-type proteins in the hydrolysis of VAMP. Preliminary experiments of crystallization of the truncated recombinant L chain gave encouraging results.

The chaperone-like alpha-crystallin forms a complex only with the aggregation-prone molten globule state of alpha-lactalbumin

The chaperone-like alpha-crystallin prevents aggregation of several proteins by interacting with their non-native states. Alpha-Lactalbumin adopts different non-native states under different experimental conditions. We have investigated the interaction of alpha-crystallin with three non-identical non-native states, using fluorescence, circular dichroism, and gel filtration chromatography. The compact molten globule state of apo-alpha-lactalbumin in tris buffer does not interact with alpha-crystallin. The expanded, flexible molten globule-like state of reduced apo-alpha-lactalbumin (formed at pH 7.2) also does not interact with alpha-crystallin. Only the aggregation-prone non-native state of reduced apo-alpha-lactalbumin formed at pH 6.0 interacts with alpha-crystallin to form a stable complex. The alpha-crystallin bound reduced apo-alpha-lactalbumin exhibits properties similar to those of a molten globule. Our results show that alpha-crystallin interacts only with the aggregation prone molten globule state of reduced apo-alpha-lactalbumin but not with the other non-aggregating molten globule states of the protein.

Conformational changes of alpha-lactalbumin induced by the stepwise reduction of its disulfide bridges: the effect of the disulfide bridges on the structural stability of the protein in sodium dodecyl sulfate solution

Four disulfide bridges of bovine alpha-lactalbumin (alpha-lact) were selectively reduced to obtain its derivatives with three, two, and zero disulfide bridges (designated as 3SS, 2SS, and 0SS alpha-lact, respectively). The original helicity was almost maintained in 3SS alpha-lact missing only the Cys6-Cys120 bridge. Upon the reduction of both Cys28-Cys111 and Cys6-Cys120 bridges, various changes occurred in the protein. In particular, the maximum fluorescence of 1-anilinonaphthalene-8-sulfonic acid was observed in this stage. Upon the reduction of all disulfide bridges, the hydrophobic box of the protein, formed by Trp60, Ile95, Tyr103, and Trp104, was disrupted and an internal helical structure was destroyed. The conformation of each derivative was examined mainly in a solution of sodium dodecyl sulfate. In the surfactant solution, the helicity increased from 33% to 37% in 3SS alpha-lact, from 26% to 31% in 2SS alpha-lact, and from 18% to 37% in 0SS alpha-lact, as against from 34% to 44% in intact alpha-lact. On the other hand, the tryptophan fluorescence of each derivative was affected in very low surfactant concentrations, suggesting that the tertiary structure considerably changed prior to the secondary structural change in the surfactant solution.

Disulphide-coupled protein folding pathways

Protein folding pathways that involve disulphide bond formation can be determined in great detail. Those of bovine pancreatic trypsin inhibitor, alpha-lactalbumin and ribonucleases A and T1 are compared and contrasted. In each species, whatever conformation favours one disulphide bond over another is stabilized to the same extent by the presence of that disulphide bond in the disulphide intermediates. The pathways differ markedly in the nature of that conformation: in bovine pancreatic trypsin inhibitor a crucial intermediate is partly folded, in alpha-lactalbumin the intermediates tend to adopt to varying extents the molten globule conformation, while in the ribonucleases the early disulphide intermediates are largely unfolded, and none predominate. In each case, however, the slowest step is formation of a disulphide bond that will be buried in a stable folded conformation; the most rapid step is formation of an accessible disulphide bond on the surface of a folded conformation. Quasi-native species with the native conformation, but incomplete disulphide bonds, can either increase or decrease the rate of further disulphide formation.

Structural studies on the zinc-endopeptidase light chain of tetanus neurotoxin

Tetanus neurotoxin (TeNT) blocks neuroexocytosis via a zinc-endopeptidase activity highly specific for vescicle-associated membrane protein(VAMP)/synaptobrevin. TeNT is the prototype of clostridial neurotoxins, a new family of metalloproteinases. They consist of three domains and the proteolytic activity is displayed by the 50-kDa light chain (L chain). The L chain was isolated here in the native state from bacterial filtrates of Clostridium tetani and its structure was studied via circular dichroism (CD) and fluorescence spectroscopy. The secondary structure content (27% alpha-helix and 43% beta-sheet), estimated by far-ultraviolet CD measurements, was in reasonable agreement with that obtained by standard predictive methods (25% alpha-helix and 49% beta-sheet). Moreover, the hypothetical zinc-binding motif, encompassing residues His-Glu-Leu-Ile-His, was correctly predicted to be in alpha-helical conformation, as also expected on the basis of the geometrical requirements for a correct coordination of the zinc ion. Both near-ultraviolet CD and fluorescence data strongly suggest that the single Trp43 residue is buried and constrained in a hydrophobic environment, likely distant from the zinc ion located in the active-site cleft. The contribution of the bound zinc ion to the overall conformation of TeNT L chain was investigated by different and complementary techniques, including spectroscopic (far- and near-ultraviolet CD, fluorescence, second derivative absorption spectroscopy) as well as proteolytic probes. The results indicate that the zinc ion plays little, if any, role in determining the structural properties of the L chain molecule. Similarly, the metal-free apo-enzyme and the holo-protein share common stability features evaluated in respect to different physico-chemical parameters (pH, temperature and urea concentration). These results parallel those obtained on thermolysin, a zinc-dependent neutral endoprotease from Bacillus thermoproteolyticus, where both conformational and stability properties are unchanged upon zinc removal.

Mitochondrial presequences can induce aggregation of unfolded proteins

We have studied the interactions between various synthetic peptides and two model unfolded proteins, reduced alpha-lactalbumin and reduced and carboxymethylated alpha-lactalbumin. We found that mitochondrial presequences could induce aggregation of the unfolded alpha-lactalbumins but not of the native alpha-lactalbumin. The presequence-induced aggregation of unfolded alpha-lactalbumin was dependent on electrostatic interactions and on the amphiphilicity of the presequences. Since positive charge and amphiphilicity are necessary for the targeting function of mitochondrial presequences, presequence-induced aggregation may be responsible for the instability of mitochondrial precursor proteins and may need to be inhibited by binding factors in the cytosol.

Native disulfide bonds greatly accelerate secondary structure formation in the folding of lysozyme

To assess the respective roles of local and long-range interactions during protein folding, the influence of the native disulfide bonds on the early formation of secondary structure was investigated using continuous-flow circular dichroism. Within the first 4 ms of folding, lysozyme with intact disulfide bonds already had a far-UV CD spectrum reflecting large amounts of secondary structure. Conversely, reduced lysozyme remained essentially unfolded at this early folding time. Thus, native disulfide bonds not only stabilize the cfinal conformation of lysozyme but also provide, in early folding intermediates, the necessary stabilization that favors the formation of secondary structure.

Hydrogen exchange in native and denatured states of hen egg-white lysozyme

The hydrogen exchange kinetics of 68 individual amide protons in the native state of hen lysozyme have been measured at pH 7.5 and 30 degrees C by 2D NMR methods. These constitute the most protected subset of amides, with exchange half lives some 10(5)-10(7) times longer than anticipated from studies of small model peptides. The observed distribution of rates under these conditions can be rationalized to a large extent in terms of the hydrogen bonding of individual amides and their burial from bulk solvent. Exchange rates have also been measured in a reversibly denatured state of lysozyme; this was made possible under very mild conditions, pH 2.0 35 degrees C, by lowering the stability of the native state through selective cleavage of the Cys-6-Cys-127 disulfide cross-link (CM6-127 lysozyme). In this state the exchange rates for the majority of amides approach, within a factor of 5, the values anticipated from small model peptides. For a few amides, however, there is evidence for significant retardation (up to nearly 20-fold) relative to the predicted rates. The pattern of protection observed under these conditions does not reflect the behavior of the protein under strongly native conditions, suggesting that regions of native-like structure do not persist significantly in the denatured state of CM6-127 lysozyme. The pattern of exchange rates from the native protein at high temperature, pH 3.8 69 degrees C, resembles that of the acid-denatured state, suggesting that under these conditions the exchange kinetics are dominated by transient global unfolding. The rates of folding and unfolding under these conditions were determined independently by magnetization transfer NMR methods, enabling the intrinsic exchange rates from the denatured state to be deduced on the basis of this model, under conditions where the predominant equilibrium species is the native state. Again, in the case of most amides these rates showed only limited deviation from those predicted by a simple random coil model. This reinforces the view that these denatured states of lysozyme have little persistent residual order and contrasts with the behavior found for compact partially folded states of proteins, including an intermediate detected transiently during the refolding of hen lysozyme.

Conformational aspects of the Cu2+ binding to alpha-lactalbumin. Characterization and stability of the Cu-bound state

By circular dichroism experiments the existence of a typical Cu2(+)-bound state is demonstrated for bovine- and for goat alpha-lactalbumin. As in the near-UV region an important ligand to metal charge-transfer band overlaps with the aromatic band of the protein, a subtraction method is developed in order to determine the net effect of Cu2+ ions on the protein conformation. The Cu2(+)-bound state, characterized by a vanishing tertiary structure and a substantial loss of secondary structure, clearly differs from the well-known Ca2(+)-, apo-, and acid conformers. At room temperature, the Cu2+ binding has already decreased the alpha-helix content of bovine alpha-lactalbumin to the extent that further unfolding by thermal or guanidine hydrochloride denaturation behaves in a non-cooperative way. Since for goat alpha-lactalbumin the Cu2+ binding to His-68 is much less important than for bovine alpha-lactalbumin, we observe a somewhat different conformational behaviour for goat alpha-lactalbumin. The results of this conformational circular dichroism study are confirmed by isothermal calorimetric data.

Mechanism of acid-induced folding of proteins

We have previously shown [Goto, Y., Calciano, L. J., & Fink, A. L. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 573-577] that beta-lactamase, cytochrome c, and apomyoglobin are maximally unfolded at pH 2 under conditions of low ionic strength, but a further decrease in pH, by increasing the concentration of HCl, refolds the proteins to the A state with properties similar to those of a molten globule state. To understand the mechanism of acid-induced refolding of protein structure, we studied the effects of various strong acids and their neutral salts on the acid-unfolded states of ferricytochrome c and apomyoglobin. The conformational transition of cytochrome c was monitored at 20 degrees C by using changes in the far-UV CD and in the Soret absorption at 394 nm, and that of apomyoglobin was monitored by changes in the far-UV CD. Various strong acids (i.e., sulfuric acid, perchloric acid, nitric acid, trichloroacetic acid, and trifluoroacetic acid) refolded the acid-unfolded cytochrome c and apomyoglobin to the A states as was the case with HCl. For both proteins neutral salts of these acids caused similar conformational transitions, confirming that the anions are responsible for bringing about the transition. The order of effectiveness of anions was shown to be ferricyanide greater than ferrocyanide greater than sulfate greater than thiocyanate greater than perchlorate greater than iodide greater than nitrate greater than trifluoroacetate greater than bromide greater than chloride.(ABSTRACT TRUNCATED AT 250 WORDS)