Unfolding energetics and stability of banana lectin

Biochemical and structural characterization of a mannose binding jacalin-related lectin with two-sugar binding sites from pineapple (Ananas comosus) stem

A mannose binding jacalin-related lectin from Ananas comosus stem (AcmJRL) was purified and biochemically characterized. This lectin is homogeneous according to native, SDS-PAGE and N-terminal sequencing and the theoretical molecular mass was confirmed by ESI-Q-TOF-MS. AcmJRL was found homodimeric in solution by size-exclusion chromatography. Rat erythrocytes are agglutinated by AcmJRL while no agglutination activity is detected against rabbit and sheep erythrocytes. Hemagglutination activity was found more strongly inhibited by mannooligomannosides than by D-mannose. The carbohydrate-binding specificity of AcmJRL was determined in some detail by isothermal titration calorimetry. All sugars tested were found to bind with low affinity to AcmJRL, with K_a values in the mM range. In agreement with hemagglutination assays, the affinity increased from D-mannose to di-, tri- and penta-mannooligosaccharides. Moreover, the X-ray crystal structure of AcmJRL was obtained in an apo form as well as in complex with D-mannose and methyl-α-D-mannopyranoside, revealing two carbohydrate-binding sites per monomer similar to the banana lectin BanLec. The absence of a wall separating the two binding sites, the conformation of β7β8 loop and the hemagglutinating activity are reminiscent of the BanLec His84Thr mutant, which presents a strong anti-HIV activity in absence of mitogenic activity.

Banana lectin: a brief review

Lectins are a group of proteins of non-immune origin that recognize and bind to carbohydrates without modifying them. Banana is the common name for both herbaceous plants of the genus Musa and for the fruit they produce. They are indeed a promising source for many medicinal applications. Banana lectins have the potential for inhibiting HIV-1 reverse transcriptase activity, suppressing cancer cell proliferation and stimulating macrophage activities. Nevertheless, compared to other plant lectins, there is relatively little information in the literature on banana lectins, particularly with respect to their structure and biological functions. Herein we focus our review on the structure, functions and exploitable properties of banana lectins.

The lectin from Musa paradisiaca binds with the capsid protein of tobacco mosaic virus and prevents viral infection

It has been demonstrated that the lectin from Musa paradisiaca (BanLec-1) could inhibit the cellular entry of human immunodeficiency virus (HIV). In order to evaluate its effects on tobacco mosaic virus (TMV), the banlec-1 gene was cloned and transformed into Escherichia coli and tobacco, respectively. Recombinant BanLec-1 showed metal ions dependence, and higher thermal and pH stability. Overexpression of banlec-1 in tobacco resulted in decreased leaf size, and higher resistance to TMV infection, which includes reduced TMV cellular entry, more stable chlorophyll contents, and enhanced antioxidant enzymes. BanLec-1 was found to bind directly to the TMV capsid protein in vitro, and to inhibit TMV infection in a dose-dependent manner. In contrast to limited prevention in vivo, purified rBanLec-1 exhibited more significant effects on TMV infection in vitro. Taken together, our study indicated that BanLec-1 could prevent TMV infection in tobacco, probably through the interaction between BanLec-1 and TMV capsid protein.

Novel unfolding sequence of banana lectin: Folded, unfolded and natively unfolded-like monomeric states in guanidine hydrochloride

The sequence of unfolding events of dimeric banana lectin (Banlec), as induced by guanidine hydrochloride (GdnHCl), has been investigated by size-exclusion HPLC, fluorescence, far-UV CD, low temperature phosphorescence and selective chemical modification. 8-Anilino-1-naphthalenesulfonate (ANS) binding indicates a structured unfolding intermediate which has been characterized as dissociated monomer by size-exclusion chromatography. Interestingly, the unfolding elution pattern reveals two distinct unfolded states. One is a usual random coil. The other represents a novel species having elution behavior and structural compactness (Stokes radius) similar to dissociated monomer but showing no regular secondary structure as determined by far-UV CD, thus resembling a natively unfolded state. N-Bromosuccinimide (NBS) oxidation shows that single tryptophan residue remains unmodified in dissociated monomer intermediate while the same is oxidized in natively unfolded-like species. Such difference in tryptophan environment in these species is supported by acrylamide quenching studies, and phosphorescence results at 77 K which show a blue-shift of (0,0) band from 414.8 nm to 409.2 nm. The present results reveal subtlety of structural characteristics of unfolded states of Banlec in GdnHCl, which provide important insight in protein unfolding reaction.

Monomeric banana lectin at acidic pH overrules conformational stability of its native dimeric form

Banana lectin (BL) is a homodimeric protein categorized among jacalin-related family of lectins. The effect of acidic pH was examined on conformational stability of BL by using circular dichroism, intrinsic fluorescence, 1-anilino-8-napthalene sulfonate (ANS) binding, size exclusion chromatography (SEC) and dynamic light scattering (DLS). During acid denaturation of BL, the monomerization of native dimeric protein was found at pH 2.0. The elution profile from SEC showed two different peaks (59.65 ml & 87.98 ml) at pH 2.0 while single peak (61.45 ml) at pH 7.4. The hydrodynamic radii (R h) of native BL was 2.9 nm while at pH 2.0 two species were found with R h of 1.7 and 3.7 nm. Furthermore at, pH 2.0 the secondary structures of BL remained unaltered while tertiary structure was significantly disrupted with the exposure of hydrophobic clusters confirming the existence of molten globule like state. The unfolding of BL with different subunit status was further evaluated by urea and temperature mediated denaturation to check their stability. As inferred from high Cm and ΔG values, the monomeric form of BL offers more resistance towards chemical denaturation than the native dimeric form. Besides, dimeric BL exhibited a Tm of 77°C while no loss in secondary structures was observed in monomers even up to 95°C. To the best of our knowledge, this is the first report on monomeric subunit of lectins showing more stability against denaturants than its native dimeric state.

Folding and homodimerization of wheat germ agglutinin

Wheat germ agglutinin (WGA) is emblematic of proteins that specialize in the recognition of carbohydrates. It was the first lectin reported to have a capacity for discriminating between normal and malignant cells. Since then, it has become a preferred model for basic research and is frequently considered in the development of biomedical and biotechnological applications. However, the molecular basis for the structural stability of this homodimeric lectin remains largely unknown, a situation that limits the rational manipulation and modification of its function. In this work we performed a thermodynamic characterization of WGA folding and self-association processes as a function of pH and temperature by using differential scanning and isothermal dilution calorimetry. WGA is monomeric at pH 2, and one of its four hevein-like domains is unfolded at room temperature. Under such conditions, the agglutinin exhibits a fully reversible thermal unfolding that consists of three two-state transitions. At higher pH values, the protein forms weak, nonobligate dimers. This behavior contrasts with that observed for the other plant lectins studied thus far, which form strong, obligate oligomers, indicating a distinctly different molecular basis for WGA function. For dimer formation, the four domains must be properly folded. Nevertheless, depending on the solution conditions, self-association may be coupled with folding of the labile domain. Therefore, dimerization may proceed as a rigid-body-like association or a folding-by-binding event. This hybrid behavior is not seen in other plant lectins. The emerging molecular picture for the WGA assembly highlights the need for a reexamination of existing ligand-binding data in the literature.