Domain structure of gpNu1, a phage lambda DNA packaging protein

The terminase enzyme from bacteriophage lambda is responsible for the insertion of a dsDNA genome into the confines of the viral capsid. The holoenzyme is composed of gpA and gpNu1 subunits in a gpA(1) x gpNu1(2) stoichiometry. While genetic studies have described regions within the two proteins responsible for DNA binding, capsid binding, and subunit interactions in the holoenzyme complex, biochemical characterization of these domains is limited. We have previously described the cloning, expression, and biochemical characterization of a soluble DNA binding domain of the terminase gpNu1 subunit (Met1 to Lys100) and suggested that the hydrophobic region spanning Lys100 to Pro141 defines a domain responsible for self-association interactions, and that is important for cooperative DNA binding [Yang et al. (1999) Biochemistry 38, 465-477]. We further suggested that the genetically defined gpA-interactive domain in the C-terminal half of the protein is limited to the C-terminal approximately 40 amino acids of gpNu1. Here we describe the cloning, expression, and biochemical characterization of gpNu1DeltaP141, a deletion mutant of gpNu1 that comprises the DNA binding domain and the putative hydrophobic self-assembly domain of the full-length protein. Purified gpNu1DeltaP141 shows a strong tendency to aggregate in solution; However, the protein remains soluble in 0.4 M guanidine hydrochloride, and circular dichroism (CD) and fluorescence spectroscopic studies demonstrate that the protein is folded under these conditions. Moreover, CD spectroscopy and thermally induced unfolding studies suggest that the DNA binding domain and the self-association domain represent independent folding domains of gpNu1DeltaP141. The mutant protein interacts weakly with the gpA subunit, but does not form a catalytically competent holoenzyme complex, suggesting that the C-terminal 40 residues are important for appropriate subunit interactions. Importantly, gpNu1DeltaP141 binds DNA tightly, but with less specificity than does full-length protein, and the data suggest that the C-terminal residues are further required for specific DNA binding activity. The implications of these results in the assembly of a functional holoenzyme complex are discussed.     

Pseudocatalase from Lactobacillus plantarum: evidence for a homopentameric structure containing two atoms of manganese per subunit

An improved procedure for the isolation of the pseudocatalase of Lactobacillus plantarum has been devised, and the quaternary structure and manganese content of this enzyme have been reexamined. Sedimentation equilibrium of the native enzyme at several salt concentrations gave a molecular weight of 172 000. The subunit weight, obtained by sedimentation equilibrium in 6.4 M guanidinium chloride, with or without prior reduction and carboxymethylation, was 34 kilodaltons. The amino acid composition indicated 150 Arg + Lys, and after exhaustive tryptic digestion, 32 peptides were resolved. These data suggest that the pseudocatalase is a homopentamer. Cross-linking with dimethyl suberimidate, followed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, yielded five major bands, another indication of pentameric structure. The manganese content was found to be 1.8-2.4 per subunit. S20,W was found to be 9.6 S and f/f0 = 1.2, suggesting a globular structure of Stokes radius 44 A.     

Sequential unfolding of the two-domain protein Pseudomonas stutzeri cytochrome c(4)

P. stutzeri cytochrome c(4) is a di-haem protein, composed of two globular domains each with His-Met coordinated haem, and a hydrogen bond network between the domains. The domain foldings are highly symmetric but with specific differences including structural differences of ligand coordination, and different spin states of the oxidised haem groups. We have studied unfolding of oxidised P. stutzeri cyt c(4) induced thermally and by chemical denaturants. Horse heart cyt c was a reference molecule. Isothermal unfolding induced by guanidinium chloride and acid was followed by Soret, alpha/beta, and 701-nm band absorption, and by far-UV circular dichroism spectroscopy. Multifarious patterns emerge, but the two domains clearly unfold sequentially. One phase, assigned to unfolding of the N-terminal domain, proceeds at guanidinium concentrations up to approximately 1.0 M. This is followed by two overlapping phases at higher concentrations. The intermediate state maintains Fe-Met coordination, assigned to the C-terminal domain. Interdomain interaction is reflected in decreasing values of the cooperativity parameters. Differential scanning calorimetry shows a single peak, but two peaks appear when guanidinium chloride up to 0.4 M is present. This reflects different chemical action in chemical and thermal unfolding. Acid-induced unfolding kinetics was addressed by pH jumps using diode array stopped-flow techniques. Three kinetic phases in the 701 nm Fe-Met marker band, and four phases in the Soret and alpha/beta bands, spanning 4-1000 ms could be distinguished on pH jumps from 7.5 to the range 2.5-3.5. In this range of time and pH cyt c appears to unfold in no more than two phases. Spectral properties of the kinetic intermediates could be identified. Sequential domain unfolding, formation of high-spin states, and an intermediate state with Fe-Met coordination to a single haem group are features of the unfolding kinetics.     

Nucleotides regulate the conformational state of the small terminase subunit from bacteriophage lambda: implications for the assembly of a viral genome-packaging motor

Terminase enzymes are responsible for "packaging" of viral DNA into a preformed procapsid. Bacteriophage lambda terminase is composed of two subunits, gpA and gpNu1, in a gpA(1).gpNu1(2) holoenzyme complex. The larger gpA subunit is responsible for preparation of viral DNA for packaging, and is central to the packaging motor complex. The smaller gpNu1 subunit is required for site-specific assembly of the packaging motor on viral DNA. Terminase assembly at the packaging initiation site is regulated by ATP binding and hydrolysis at the gpNu1 subunit. Characterization of the catalytic and structural interactions between the DNA and nucleotide binding sites of gpNu1 is thus central to our understanding of the packaging motor at the molecular level. The high-resolution structure of the DNA binding domain of gpNu1 (gpNu1-DBD) was recently determined in our lab [de Beer, T., et al. (2002) Mol. Cell 9, 981-991]. The structure reveals the presence of a winged-helix-turn-helix DNA binding motif, but the location of the ATPase catalytic site in gpNu1 remains unknown. In this work, nucleotide binding to the gpNu1-DBD was probed using acrylamide fluorescence quenching and fluorescence-monitored ligand binding studies. The data indicate that the minimal DBD dimer binds both ATP and ADP at two equivalent but highly cooperative binding sites. The data further suggest that ATP and ADP induce distinct conformations of the dimer but do not affect DNA binding affinity. The implications of these results with respect to the assembly and function of a terminase DNA-packaging motor are discussed.     

Variation in quantity and extractability of the 148-kilodalton cartilage protein with age.

The occurrence of non-collagenous matrix proteins was studied in samples of tracheal cartilage from steers of different ages. The amounts of the 148 kDa cartilage protein in 4M-guanidinium chloride extracts and in subsequent trypsin digests of the extraction residues were determined by radioimmunoassay. Surprisingly, the 148 kDa-protein antigenicity was not changed by tryptic digestion, even though the protein was extensively degraded. The amount of the 148 kDa protein increased dramatically with age, both in the guanidinium chloride extract and in the subsequent tryptic digest, and reached maximal values at about 3 and 8 years respectively. The increase of the guanidinium chloride-soluble pool preceded that of the trypsin-digestible pool, possibly indicating a metabolic relationship. The ratio between the trypsin-digestible and guanidinium chloride-soluble pools increased continuously, and at 12 years of age close to 90% of the total 148 kDa protein detected was insoluble in guanidinium chloride. At all ages, the bulk of the cartilage collagen was insoluble both to extraction with guanidinium chloride and to tryptic digestion. The decreasing extractability of the 148 kDa protein was therefore not secondary to changes in the solubility of the collagen network. Other cartilage proteins, such as the link proteins and the 36 kDa protein, showed much smaller quantitative variations of a different character.     

Unfolding transitions of fibronectin and its domains. Stabilization and structural alteration of the N-terminal domain by heparin.

Changes in the conformational state of human plasma fibronectin and several of its fragments were studied by fluorescence emission, intrinsic fluorescence polarization and c.d. spectroscopy under conditions of guanidinium chloride-and temperature-induced unfolding. Fragments were chosen to represent all three types of internal structural homology in the protein. Low concentration (less than 2 M) of guanidinium chloride induced a gradual transition in the intact protein that was not characteristic of any of the isolated domains, suggesting the presence of interdomain interactions within the protein. Intermediate concentrations of guanidinium chloride (2-3 M) and moderately elevated temperatures (55-60 degrees C) induced a highly co-operative structural transition in intact fibronectin that was attributable to the central 110 kDa cell-binding domain. High temperatures (greater than 60 degrees C) produced a gradual unfolding in the intact protein attributable to the 29 kDa N-terminal heparin-binding and 40 kDa collagen-binding domains. Binding of heparin to intact fibronectin and to its N-terminal fragment stabilized the proteins against thermal unfolding. This was reflected in increased delta H for the unfolding transitions of the heparin-bound N-terminal fragment, as well as decreased accessibility to solvent perturbants of internal chromophores in this fragment when bound to heparin. These results help to account for the biological efficacy of the interaction between the fibronectin N-terminal domain and heparin, despite its relatively low affinity.     

Phosphorylation of the amino-terminal head domain of the middle molecular mass 145-kDa subunit of neurofilaments. Evidence for regulation by second messenger-dependent protein kinases

To begin to understand the regulation and roles of neurofilament phosphorylation, we localized the phosphorylated domains on the 140-145-kDa neurofilament subunit (NF-M) and identified the protein kinases that may specifically phosphorylate the sites within these domains in vivo. Mouse retinal ganglion cells were labeled in vivo by injecting mice intravitreally with [32P]orthophosphate, and neurofilament-enriched fractions were obtained from the optic axons. Two-dimensional phosphopeptide map analysis of NF-M after digestion with alpha-chymotrypsin and trypsin revealed seven major (M8-M14) and at least eight minor (M1-M7 and M15) phosphopeptides. Two-dimensional phosphopeptide map analyses of NF-M phosphorylated in vitro by individual purified or endogenous axonal cytoskeleton-associated protein kinases showed that five peptides (M9-M13) were substrates for the heparin-sensitive second messenger-independent protein kinase(s). Protein kinase A and/or protein kinase C phosphorylated eight other peptides (M1-M8). Two alpha-chymotryptic peptides (C1 and C2) that were phosphorylated by protein kinase A but not by the endogenous independent kinase(s) were isolated by high performance liquid chromatography on a reverse-phase C8 column. Partial sequence analysis of peptides C1 (S R V S G P S ...) and C2 (S R G S P S T V S ...) showed that the peptides were localized on the head domain of NF-M at 25 and 41 residues from the amino terminus, respectively. Tryptic digest of peptide C1 (less than 12 kDa) generated the phosphopeptides M1-M6. Peptide C2 was a breakdown product of peptide C1. Since the polypeptide sites targeted by second messenger-independent kinase(s) associated with neurofilaments are localized on the carboxyl-terminal domain, separate aspects of NF-M function appear to be regulated by separate kinase systems that selectively phosphorylate head or tail domains of the polypeptide.     

Bacteriophage lambda gpNu1 and Escherichia coli IHF proteins cooperatively bind and bend viral DNA: implications for the assembly of a genome-packaging motor

Terminase enzymes are common to both prokaryotic and eukaryotic double-stranded DNA viruses and are responsible for packaging viral DNA into the confines of an empty procapsid shell. In all known cases, the holoenzymes are heteroligomers composed of a large subunit that possesses the catalytic activities required for genome packaging and a small subunit that is responsible for specific recognition of viral DNA. In bacteriophage lambda, the DNA recognition protein is gpNu1. The gpNu1 subunit interacts with multiple recognition elements within cos, the packaging initiation site in viral DNA, to site-specifically assemble the packaging machinery. Motor assembly is modulated by the Escherichia coli integration host factor protein (IHF), which binds to a consensus sequence also located within cos. On the basis of a variety of biochemical data and the recently solved NMR structure of the DNA binding domain of gpNu1, we proposed a novel DNA binding mode that predicts significant bending of duplex DNA by gpNu1 (de Beer et al. (2002) Mol. Cell 9, 981-991). We further proposed that gpNu1 and IHF cooperatively bind and bend viral DNA to regulate the assembly of the packaging motor. Here, we characterize cooperative gpNu1 and IHF binding to the cos site in lambda DNA using a quantitative electrophoretic mobility shift (EMS) assay. These studies provide direct experimental support for the long presumed cooperative assembly of gpNu1 and IHF at the cos sequence of lambda DNA. Further, circular permutation experiments demonstrate that the viral and host proteins each introduce a strong bend in cos-containing DNA, but not nonspecific DNA substrates. Thus, specific recognition of viral DNA by the packaging apparatus is mediated by both DNA sequence information and by structural alteration of the duplex. The relevance of these results with respect to the assembly of a viral DNA-packaging motor is discussed.     

Isolation of proteins by gel filtration in 6M guanidinium chloride: application to RNA tumor viruses

Separation of RNA tumor virus proteins by gel filtration in 6m guanidinium chloride indicates that this method will effectively separate polypeptides in milligram amounts whose molecular weights differ by as little as 15 percent. Such separations are achieved because proteins in 6m guanidinium chloride have a random coil conformation with diffusion coefficients considerably smaller than the corresponding native proteins. Consequently, protein bands that emerge from a gel filtration column in 6m guanidinium chloride are remarkably sharp. A 60–70% yield of renatured RNA tumor virus proteins could be obtained following dialysis against a dilute solution of mercaptoethanol. The major proteins of avian RNA tumor viruses were obtained as pure components by gel filtration in 6m guanidinium chloride. Mammalian RNA tumor viruses appear to have a more complex protein structure, since, in some cases, additional purification was required to obtain the pure proteins. This method of protein separation might be used as the initial stop in the isolation of components from other biological macrostructures.     

1H nuclear magnetic relaxation dispersion of Cu,Zn superoxide dismutase in the native and guanidinium-induced unfolded forms

Potentialities and limitations of the use of (1)H NMRD technique for the characterization of the hydration properties of unfolded or partially folded states of proteins are discussed. The copper(I) form of monomeric Cu,Zn superoxide dismutase in its folded state and in the presence of 4M guanidinium chloride is taken as case system. The dispersion profile, analyzed with an extended relaxation matrix analysis, indicates the presence of long-lived water molecules in the folded state. The observed increase in relaxation at high field upon addition of guanidinium chloride indicates an increase in the number of solvation protons interacting with the protein and exchanging with a time shorter than the protein reorientational time. The observed effect is consistent with an exposed protein surface of SOD in the presence of 4M guanidinium chloride smaller than what could be expected for a random coil.     

Design and folding of a multidomain protein

To test whether the folding process of a large protein can be understood on the basis of the folding behavior of the domains that constitute it, we coupled two well-studied small -helical proteins, the B-domain of protein A (60 amino acids) and Rd-apocytochrome b562 (Rd-apocyt b562, 106 amino acids), by fusing the C-terminal helix of the B-domain of protein A with the N-terminal helix of Rd-apocyt b562 without changing their hydrophobic core residues. The success of the design was confirmed by determining the structure of the engineered protein with multidimensional NMR methods. Kinetic studies showed that the logarithms of the folding/unfolding rate constants of the engineered protein are linearly dependent on concentrations of guanidinium chloride in the measurable range from 1.7 to 4 M. Their slopes (m-values) are close to those of Rd-apocyt b562. In addition, the 1H-15N HSQC spectrum taken at 1.5 M guanidinium chloride reveals that only the Rd-apocyt b562 domain in the designed protein remained folded. These results suggest that the two domains have weak energetic coupling. Interestingly, the redesigned protein folds faster than Rd-apocyt b562, suggesting that the fused helix stabilizes the rate-limiting transition state.     

Biophysical characterization of the DNA binding domain of gpNu1, a viral DNA packaging protein

Terminase enzymes are common to double-stranded DNA viruses. These enzymes "package" the viral genome into a pre-formed capsid. Terminase from bacteriophage lambda is composed of gpA (72.4 kDa) and gpNu1 (20.4 kDa) subunits. We have described the expression and biochemical characterization of gpNu1DeltaK100, a construct comprising the N-terminal 100 amino acids of gpNu1 (Yang, Q., de Beer, T., Woods, L., Meyer, J., Manning, M., Overduin, M., and Catalano, C. E. (1999) Biochemistry 38, 465-477). Here we present a biophysical characterization of this construct. Thermally induced loss of secondary and tertiary structures is fully reversible. Surprisingly, although loss of tertiary structure is cooperative, loss of secondary structure is non-cooperative. NMR and limited proteolysis data suggest that approximately 30 amino acids of gpNu1DeltaK100 are solvent-exposed and highly flexible. We therefore constructed gpNu1DeltaE68, a protein consisting of the N-terminal 68 residues of gpNu1. gpNu1DeltaE68 is a dimer with no evidence of dissociation or further aggregation. Thermally induced unfolding of gpNu1DeltaE68 is reversible, with concomitant loss of both secondary and tertiary structure. The melting temperature increases with increasing protein concentration, suggesting that dimerization and folding are, at least in part, coupled. The data suggest that gpNu1DeltaE68 represents the minimal DNA binding domain of gpNu1. We further suggest that the C-terminal approximately 30 residues in gpNu1DeltaK100 adopt a pseudo-stable alpha-helix that extends from the folded core of the protein. A model describing the role of this helix in the assembly of the packaging apparatus is discussed.     

Plant microbody proteins. II. Purification and characterization of the major protein component (SP-63) of peroxisome membranes.

The major component of membranes of microbodies from green leaves of Lens culinaris is a protein of a subunit molecular weight of 63 000. This protein, referred to as SP-63, seems to be unique to microbodies and could not be detected when plastids or mitochondria were analyzed. It is probably a structural protein and is thus not solubilized by cholate, Triton X-100, chloroform/methanol, or 0.2M KCl. Solubilization from purified membranes was achieved with guanidinium chloride or sodium dodecylsulphate. The protein was separated from minor contaminating components by chromatography on Sepharose 4B or Sephadex G-150 employing 0.1% sodium dodecylsulphate or 4M urea as eluent. It was shown to be homogeneous upon sodium dodecylsulphate gel electrophoresis and did not give a positive glycoprotein stain.     

The stability of cell surface protein to surfactants and denaturants

The effects of several denaturants and detergents on the structure and stability of cell surface protein have been evaluated by circular dichroism and fluorescence measurements. Cell surface protein undergoes a single broad transition in both urea and guanidinium chloride. Although guanidinium chloride is twice as effective as urea on a molar basis, both appear to eliminate all of the organized structure present in the native molecule. Nonionic surfactants and lysolecithin have little effect on cell surface protein. However, sodium dodecyl sulfate increases the alpha helical content and cetyltrimethylammonium bromide increases the beta structure of cell surface protein. The reorganization of the polypeptide backbone requires the loss of certain restraints imposed by tertiary interactions as evidenced by a decrease in ellipticity in the far ultraviolet and in the polarization of tryptophanyl fluorescence. These results along with the data of a previous paper (Alexander, S. S., Jr., Colonna, G., Yamada, K. M., Pastan, I., and Edelhoch, H. (1978) J. Biol. Chem. 253, 5820--5824) suggest the presence of structural domains distributed along the flexible polypeptide chain of cell surface protein.     

The structure of a high-Mr subunit of durum-wheat (Triticum durum) gluten.

A high-Mr subunit was prepared from durum wheat (Triticum durum). Viscometric analysis showed that the molecule is rod-shaped, with molecular dimensions of about 50 nm x 1.75 nm (500 A x 17.5 A) in 0.05 M-acetic acid/0.01 M-glycine and 49 nm x 1.79 nm (490 A x 17.9 A) in aq. 50% (v/v) propan-1-ol (+/- 0.01 M-glycine) at 30 degrees C. C.d. spectroscopy in the same solvents indicated the presence of beta-turns, but little alpha-helix [7% in 50% (v/v) propan-1-ol] and no beta-sheet. However, when dissolved in trifluoroethanol the protein contains about 30% alpha-helix, and viscometric analysis gives dimensions of about 62 nm x 1.53 nm (620 A x 15.3 A). It is proposed, on the basis of these studies and previously published structural prediction, that the repetitive central domain of the high-Mr subunit forms a loose spiral based on repetitive beta-turns, whereas the shorter non-repetitive N- and C-terminal domains are alpha-helical in trifluoroethanol, but random coil in other solvents. The Mr of the high-Mr subunit determined from the intrinsic viscosity in 6.0 M-guanidinium chloride was 65,000, compared with 84,000 determined in 5.0 M-guanidinium thiocyanate. The latter value is consistent with the Mr values for related proteins whose complete amino acid sequences are known, and it was concluded that the protein is incompletely denatured in the former solvent. This was confirmed by c.d. spectroscopy in increasing concentrations (1-6 M) of guanidinium chloride.     

Probing local conformational changes during equilibrium unfolding of firefly luciferase: fluorescence and circular dichroism studies of single tryptophan mutants

Firefly luciferase is a monomeric protein composed of two globular domains. There is a wide cleft between the two domains. The N-terminal domain can be further divided into A-, B-, and C-subdomains. Previous studies showed that in vitro unfolding of firefly luciferase induced by guanidinium chloride can be described as a four-state equilibrium with two inactive intermediates (Herbst, R., et al. (1997) J. Biol. Chem. 272, 7099-7105). In order to monitor spectroscopically the conformational changes that occur in the different domains and subdomains during the multi-state unfolding process, we constructed a series of single-tryptophan mutants. These mutants were purified and characterized and shown to retain essentially all of the structural properties of the wild-type luciferase. Under equilibrium conditions, the unfolding of each mutant protein were studied by means of fluorescence and circular dichroism. The results show that different conformational changes occur in specific regions, suggesting a sequential unfolding process for firefly luciferase. Under 2.5 M GdmCl, whereas the N-domain unfolds partially holding half of the secondary structure content, the C-domain unfolds almost completely. In the equilibrium intermediate I(2), the secondary structure might stem mostly from the A- and B- subdomains.     

Cloning, expression, and biochemical characterization of hexahistidine-tagged terminase proteins.

The terminase enzyme from bacteriophage lambda is composed of two viral proteins (gpA, 73.2 kDa; gpNu1, 20.4 kDa) and is responsible for packaging viral DNA into the confines of an empty procapsid. We are interested in the genetic, biochemical, and biophysical properties of DNA packaging in phage lambda and, in particular, the nucleoprotein complexes involved in these processes. These studies require the routine purification of large quantities of wild-type and mutant proteins in order to probe the molecular mechanism of DNA packaging. Toward this end, we have constructed a hexahistidine (hexa-His)-tagged terminase holoenzyme as well as hexa-His-tagged gpNu1 and gpA subunits. We present a simple, one-step purification scheme for the purification of large quantities of the holoenzyme and the individual subunits directly from the crude cell lysate. Importantly, we have developed a method to purify the highly insoluble gpNu1 subunit from inclusion bodies in a single step. Hexa-His terminase holoenzyme is functional in vivo and possesses steady-state and single-turnover ATPase activity that is indistinguishable from wild-type enzyme. The nuclease activity of the modified holoenzyme is near wild type, but the reaction exhibits a greater dependence on Escherichia coli integration host factor, a result that is mirrored in vivo. These results suggest that the hexa-His-tagged holoenzyme possesses a mild DNA-binding defect that is masked, at least in part, by integration host factor. The mild defect in hexa-His terminase holoenzyme is more significant in the isolated gpA-hexa-His subunit that does not appear to bind DNA. Moreover, whereas the hexa-His-tagged gpNu1 subunit may be reconstituted into a holoenzyme complex with wild-type catalytic activities, gpA-hexa-His is impaired in its interactions with the gpNu1 subunit of the enzyme. The results reported here underscore that a complete biochemical characterization of the effects of purification tags on enzyme function must be performed prior to their use in mechanistic studies.     

Studies of the cyanogen bromide fragments of the apoprotein of human serum low density lipoproteins.

The apoprotein of human serum low density lipoproteins was reduced and carboxymethylated and then cleaved by cyanogen bromide (CNBr). The peptides which were produced from this cleavage (90% yield, based upon loss of methionine) were resolved by SDS polyacrylamide gel electrophoresis into 10 major bands, each having an amino acid composition very similar to that of intact reduced and carboxymethylated LDL apoprotein. The fractionation of the CNBr fragments by preparative gel filtration was dependent upon the nature of the eluting solvent. NH4OH and SDS solvents eluted all of the material in the void volume. In 6 M guanidinium chloride solvents several peaks were, however, resolved, each having an amino acid composition similar to that of the unfractionated products. Whereas no NH2-terminal was detected in reduced and carboxylmethylated LDL apoprotein, automated Edman degradation of the protein following treatment with CNBr revealed the presence of several NH2-termini. The results suggest that LDL apoprotein may be made of segments of, at least, very similar amino acid composition and that both the protein itself and derivative fragments have a great tendency to aggregate even in denaturing solvents.     

Molecular weight determination of bovine carbonic anhydrase

The molecular weight of bovine carbonic anhydrase was determined by osmometric and sedimentation equilibrium methods. The solvents used were 0.15 M KCl and 6.0 M guanidinium chloride. The value found was 28300 ± 300 which is lower than the values found by other investigators.As a part of the studies the intrinsic viscosities of the enzyme in 4.5 M guanidinium thiocyanate and 6.0 M guanidinium chloride were also ascertained. The values found, 25.4 ml/g and 24.7 ml/g, respectively, are smaller than expected on the basis of the molecular weight. This finding, however, is in agreement with the low value. 0.72 × 10^?3 cm³ mol/g² of the second virial coefficient in 6.0 M guanidinium chloride.     

Stability of the major allergen Brazil nut 2S albumin (Ber e 1) to physiologically relevant in vitro gastrointestinal digestion

The major 2S albumin allergen from Brazil nuts, Ber e 1, was subjected to gastrointestinal digestion using a physiologically relevant in vitro model system either before or after heating (100 degrees C for 20 min). Whilst the albumin was cleaved into peptides, these were held together in a much larger structure even when digested by using a simulated phase 1 (gastric) followed by a phase 2 (duodenal) digestion system. Neither prior heating of Ber e 1 nor the presence of the physiological surfactant phosphatidylcholine affected the pattern of proteolysis. After 2 h of gastric digestion, approximately 25% of the allergen remained intact, approximately 50% corresponded to a large fragment of M(r) 6400, and the remainder comprised smaller peptides. During duodenal digestion, residual intact 2S albumin disappeared quickly, but a modified form of the 'large fragment' remained, even after 2 h of digestion, with a mass of approximately 5000 Da. The 'large fragment' comprised several smaller peptides that were identified, by using different MS techniques, as deriving from the large subunit. In particular, sequences corresponding to the hypervariable region (Q37-M47) and to another peptide (P42-P69), spanning the main immunoglobulin E epitope region of 2S albumin allergens, were found to be largely intact following phase 1 (gastric) digestion. They also contained previously identified putative T-cell epitopes. These findings indicate that the characteristic conserved skeleton of cysteine residues of 2S albumin family and, particularly, the intrachain disulphide bond pattern of the large subunit, play a critical role in holding the core protein structure together even after extensive proteolysis, and the resulting structures still contain potentially active B- and T-cell epitopes.