Artocarpus hirsuta lectin. Differential modes of chemical and thermal denaturation

DSC and FCS Studies Reveal the Mechanism of Thermal and Chemical Unfolding of CIA17, a Polydisperse Oligomeric Protein from Coccinia Indica

The mechanism of thermal and chemical unfolding of Coccinia indica agglutinin (CIA17), a chitooligosacharide-specific phloem exudate lectin, was investigated by biophysical approaches. DSC studies revealed that the unfolding thermogram of CIA17 consists of three components (T_m ∼ 98, 106, and 109 °C), which could be attributed to the dissociation of protein oligomers into constituent dimers, dissociation of the dimers into monomers, and unfolding of the monomers. Intrinsic fluorescence studies on the chemical denaturation by guanidinium thiocyanate and guanidinium chloride indicated the presence of two distinct steps in the unfolding pathway, which could be assigned to dissociation of the dimeric protein into monomers and unfolding of the monomers. Results of fluorescence correlation spectroscopic studies could be interpreted in terms of the following model: CIA17 forms oligomeric structures in a concentration dependent manner, with the protein existing as a monomer below 1 nM concentration but associating to form dimers at higher concentrations (K_D ≈ 2.9 nM). The dimers associate to yield tetramers with a K_D of ∼50 μM, which further associate to form higher oligomers with further increase in concentration. These results are consistent with the proposed role of CIA17 as a key player in the defense response of the plant against microbes and insects.

Comparison of chemical and thermal protein denaturation by combination of computational and experimental approaches. II

Chemical and thermal denaturation methods have been widely used to investigate folding processes of proteins in vitro. However, a molecular understanding of the relationship between these two perturbation methods is lacking. Here, we combined computational and experimental approaches to investigate denaturing effects on three structurally different proteins. We derived a linear relationship between thermal denaturation at temperature T(b) and chemical denaturation at another temperature T(u) using the stability change of a protein (ΔG). For this, we related the dependence of ΔG on temperature, in the Gibbs-Helmholtz equation, to that of ΔG on urea concentration in the linear extrapolation method, assuming that there is a temperature pair from the urea (T(u)) and the aqueous (T(b)) ensembles that produces the same protein structures. We tested this relationship on apoazurin, cytochrome c, and apoflavodoxin using coarse-grained molecular simulations. We found a linear correlation between the temperature for a particular structural ensemble in the absence of urea, T(b), and the temperature of the same structural ensemble at a specific urea concentration, T(u). The in silico results agreed with in vitro far-UV circular dichroism data on apoazurin and cytochrome c. We conclude that chemical and thermal unfolding processes correlate in terms of thermodynamics and structural ensembles at most conditions; however, deviations were found at high concentrations of denaturant.

Spectroscopic studies on the irreversible heat-induced structural transition of Pin1

Previously, the mechanism of the thermal unfolding of Pin1 (on-line measurements) was studied, revealing that Pin1 has a relatively high thermal stability. However, it is still questionable whether the unfolding of Pin1 is reversible. In the present work, intrinsic tryptophan fluorescence, ANS fluorescence, RLS, FTIR and CD spectroscopies are used to evaluate the reversibility of the thermal unfolding of Pin1. Intrinsic tryptophan fluorescence studies indicate that structural changes around tryptophan motifs in Pin1 are possibly reversible after heat treatment (even above 98°C), for no significant change in the intensity or λ(max) of the spectra was observed. ANS fluorescence measurements indicate the irreversible exposure of the hydrophobic clusters in Pin1 after heat treatment at 98°C, with increase in the fluorescence intensity and blue shift in λmax. Also, RLS signals of the Pin1-ANS system increased after heat treatment, possibly implying both the unfolding and the aggregation of Pin1. In addition, FTIR and CD results confirmed the irreversible unfolding of the secondary structure in Pin1 after heat treatment above 90°C, showing decreases in both α-helix and β-sheet. In summary, the present work mainly suggests that heat treatment, especially above 90°C, has an important impact on the structural stability of Pin1, and the structural unfolding induced by heat was proved to be irreversible.

Photophysics of ANS. V. Decay modes of ANS in proteins: the IFABP-ANS complex

The fluorescence properties of ANS as bound to proteins are treated. Several points of view concerning the origin of these properties are reviewed and synthesized into one framework. On proteins where the quantum yield (QY) is appreciable, as in organic solvents, the preferred conformation of ANS is often with the phenyl ring nearly (65 degrees -85 degrees ) orthogonal to the naphthalene. The major consequence of this geometry is water exclusion from the critical zone of ANS at which the largest amount of solvent dipolar relaxation originates. This, in turn, leads to a depression of the rate of electron transfer to the surroundings, together with other effects, as noted in the literature and in our lab. Alternative quenching pathways for ANS on the protein vs in water are also elucidated.

Crystallization and preliminary characterization of a highly thermostable lectin from Trichosanthes dioica and comparison with other Trichosanthes lectins

A lectin from Trichosanthes dioica seeds has been purified and crystallized using 25%(w/v) PEG 2K MME, 0.2 M ammonium acetate, 0.1 M Tris-HCl pH 8.5 and 50 microl 0.5%(w/v) n-octyl beta-D-glucopyranoside as thick needles belonging to hexagonal space group P6(4). Unit-cell parameters were a = b = 167.54, c = 77.42 A. The crystals diffracted to a Bragg spacing of 2.8 A. Both the structures of abrin-a and T. kirilowii lectin could be used as a model in structure determination using the molecular-replacement method; however, T. kirilowii lectin coordinates gave better values of reliability and correlation parameters. The thermal, chemical and pH stability of this lectin have also been studied. When heated, its haemagglutination activity remained unaffected up to 363 K. Other stability studies show that 4 M guanidinium hydrochloride (Gdn-HCl) initiates unfolding and that the protein is completely unfolded at 6 M Gdn-HCl. Treatment with urea resulted in a total loss of activity at higher concentrations of denaturant with no major structural changes. The protein remained stable over a wide pH range, from pH 6 to pH 12, except for partial unfolding at extremely alkaline pH. The role of disulfide bonds in the protein stability was found to be insignificant. Rayleigh light-scattering studies showed no molecular aggregation in any of the extreme treated conditions. The unusual stability of this lectin resembles that of type II ribosome-inactivating proteins (type II RIPs), which is also supported by structure determination. The structural features observed in a preliminary electron-density map were compared with the other two available Trichosanthes lectin structures.

Binding of T-Antigen Disaccharides to Artocarpus hirsuta Lectin and Jacalin are Energetically Different

The thermodynamics of binding of Me-alpha(-GalNAc, Gal-beta-1-3GalNAc-alpha-O-Me (T-antigen-alpha), Gal-beta-1-3GalNAc and Gal-alpha-1-6Glc (mellibiose) to Artocarpus hirsuta lectin was studied using fluorescence spectroscopy. The binding affinities of the saccharides are in the order Gal-beta-1-3GalNAc-alpha-O-Me > Me-alpha-GalNAc > Me-alpha-Gal > Gal-beta-1-3GalNAc > Gal-alpha-1-6Glc. The binding affinities were comparable to those for jacalin. However, binding of the saccharides to the A. hirsuta lectin was not affected as strongly by temperature as observed in jacalin and the trend was found to be reversed. Values for AH and AS were found to be positive in A. hirsuta lectin-disaccharide binding despite similar binding affinities. Thus, with 99% structural and 96% sequence homology, with similar sugar specificity and affinity, the energetics of the disaccharide binding of the two lectins seem to be different. Me-alpha-GalNAc binding to A. hirsuta lectin is enthalpically driven, because the association constant decreases with increasing temperature. However, the binding of the T-antigen disaccharides and mellibiose disaccharides to the lectin is entropically driven. The difference in the molecular associations in the packing and variation of the C-terminal length of the beta chain of the A. hirsuta lectin could be reflected in the different disaccharide binding energetics.

A chemotactic response facilitates mosquito salivary gland infection by malaria sporozoites

Sporozoite invasion of mosquito salivary glands is critical for malaria transmission to vertebrate hosts. After release into the mosquito hemocoel,the means by which malaria sporozoites locate the salivary glands is unknown. We developed a Matrigel-based in vitro system to observe and analyze the motility of GFP-expressing Plasmodium berghei sporozoites in the presence of salivary gland products of Anopheles stephensi mosquitoes using temperature-controlled, low-light-level video microscopy. Sporozoites moved toward unheated salivary gland homogenate (SGH) but not to SGH that had been heated at 56°C for 30 min. We also investigated the origin of the attracted population. Attraction to SGH was restricted to hemolymph- and oocyst-derived sporozoites; salivary gland-derived sporozoites were not attracted to SGH. These data imply that sporozoites employ a chemotactic response to high molecular mass proteins or carbohydrate-binding proteins to locate salivary glands. This raises the possibility of utilizing anti-chemotactic factors for the development of mosquito transmission blocking agents.

Studies on recombinant single chain Jacalin lectin reveal reduced affinity for saccharides despite normal folding like native Jacalin

Sugar binding studies, inactivation, unfolding, and refolding of native Jacalin (nJacalin) from Artocarpus integrifolia and recombinant single-chain Jacalin (rJacalin) expressed in Escherichia coli were studied by intrinsic fluorescence and thermal and chemical denaturation approaches. Interestingly, rJacalin does not undergo any proteolytic processing in an E. coli environment. It has 100fold less affinity for methyl-alpha-galactose (Ka: 2.48 x 10(2)) in comparison to nJacalin (Ka: 1.58 x 10(4)), and it also binds Thomsen-Friedenreich (TF) disaccharide (Galbeta1-3GalNAc) with less affinity. Overall sugar binding characteristics of rJacalin are qualitatively similar to that of nJacalin (Gal<MealphaGal<MealphaTFdisaccharide). Circular dichroism studies at near- and far-UV, thermal, and chemical denaturation studies reveal that the rJacalin behaves like nJacalin. Guanidine hydrochloride-induced denaturation, followed by renaturation, yielded total recovery of sugar binding activity of rJacalin in comparison to partial recovery for nJacalin. This signifies the minor changes in the refolding pathways between native and recombinant lectins. The stability of rJacalin is dramatically reduced in the extreme pH range unlike nJacalin. Both lectins do not bind 1-anilino-8-naphthalene sulfonic acid (ANS) in the pH range of 5 to 12 but they do in the pH range of 1-3. Solute quenching studies of the lectin using acrylamide, KI, and CsCl indicated that the tryptophan residues have full accessibility to the neutral quencher and poor accessibility to ionic quenchers. In summary, biophysical and biochemical studies on the native versus recombinant Jacalin suggest that post-translational modification, i.e., the processing of Jacalin into two chains is probably not a prerequisite for sugar binding but may be required for higher affinity.

Immunomodulatory role of native and heat denatured agglutinin from Abrus precatorius

Lectins are known as polyclonal activators of lymphocytes and work through the induction of battery of cytokines, which vary from lectin to lectin. Most widely used biological response modifier Mistletoe lectin (ML-1) in therapy stimulates lymphocytes, macrophages, and natural killer cells and induces both TH1 and TH2 type cytokines. Abrus agglutinin, similar to ML-1 with respect to carbohydrate specificity [gal (beta1-->3) gal/Nac], was studied both in native (NA) and heat denatured (HDA) condition for murine splenocyte proliferation, cytokine secretion, NK-cell activation, and thymocyte proliferation in vitro with a view to assess its potential as an immunomodulator. Both NA and HDA activate splenocytes and induce production of cytokines like IL-2, IFN-gamma and TNF-alphabeta indicating a TH1 type of immune response. Native agglutinin and HDA induced conditioned media of adherent splenocytes could stimulate non-adherent splenocytes and vice versa. Heat denatured agglutinin was able to induce NK-cell activation at much lower concentration than that of NA, but the extent of NK-cell activation was higher for NA. Proliferation of thymocytes by NA and HDA was also observed. This study indicates that Abrus agglutinin could be a potential immunomodulator both in native as well as in heat denatured form.

Unfolding and refolding of Leucoagglutinin (PHA-L), an oligomeric lectin from kidney beans (Phaseolus vulgaris)

The unfolding and refolding of Phaseolus vulgaris Leucoagglutinin, a homotetrameric legume lectin, was studied at pH 2.5 and 7.2 using fluorescence, far- and near-UV circular dichroism (CD) spectroscopy, 8-anilino-1-naphthalene sulfonate (ANS) binding and FPLC techniques. This protein was found to refold even at pH 2.5 and also exhibited high refolding yield around 60% at pH 2.5 and 85% at pH 7.2. The refolding at pH 2.5 takes place with the formation of a dimeric intermediate. Although the hydrodynamic radius of the completely renatured protein and the dimer at pH 2.5 was found to be same, the ANS binding as well as far-UV CD spectra of the two were different. The denaturation kinetics at pH 2.5 followed single exponential pattern with the rate of denaturation being independent of protein concentration. The renaturation kinetics on the other hand was dependent on the protein concentration providing further evidence of an intermediate state during refolding. From these experiments the folding pathway of the protein at pH 2.5 was proposed.

pH-dependent aggregation of oligomeric Artocarpus hirsuta lectin on thermal denaturation

The pH dependence of the activity, aggregation, and secondary structure of Artocarpus hirsuta lectin was studied using intrinsic and extrinsic fluorescence, light scattering, and circular dichroism. The lectin is more stable in the neutral and acidic than in the alkaline pH range, which is also reflected in the binding constants of the lectin to methyl alpha-galactopyranoside (me alpha-gal). The aggregation of the protein due to heat denaturation is prevented at both extremes of pH. The binding of hydrophobic dye to the lectin takes place at pH 1-2, which increases with increasing temperature. The exposure of hydrophobic patches at pH 1 is reversible. The secondary structure of the lectin is intact in the pH range of 1-8 and is distorted above pH 9. Aggregation of the protein due to heat denaturation is also prevented in the presence of guanidine hydrochloride (GdnHCl).

Identification of an equilibrium intermediate in the unfolding process of galectin-1, which retains its carbohydrate-binding specificity

The unfolding process of galectin-1 (Gal-1) in the presence of a denaturing agent was examined using fluorescence and far-UV circular dichroism (CD) spectroscopy determinations, and was found to be completely reversible. The data showed that the transitions of guanidine hydrochloride (GdnHCl)-induced lectin unfolding, in the absence of ligand, were biphasic in nature, clearly showing the existence of at least one stable intermediate. On the other hand, the unfolding in the presence of disaccharide yielded data that could fit very well to a two-state model, indicating a stabilizing effect of the ligand. The folding intermediate was further characterized by size exclusion chromatography, near-UV CD and anilinonaphtalene sulfonate binding, and shown to belong to the molten globule type. Strikingly, this intermediate retained its carbohydrate-binding specificity, as evidenced by the tryptophan fluorescence changes detected upon its interaction with lactose.