The barley lectin, horcolin, binds high-mannose glycans in a multivalent fashion, enabling high-affinity, specific inhibition of cellular HIV infection

Antitumor Activity of a Lectin Purified from Punica granatum Pulps against Ehrlich Ascites Carcinoma (EAC) Cells

BACKGROUND

Lectins are carbohydrate-binding proteins with various pharmacological activities, such as antimicrobial, antidiabetic, antioxidant, and anticancer. Punica granatum fruit extract has traditional uses, however, the anti-cancer activity of purified lectin isolated from P. granatum pulp is yet to be reported.

OBJECTIVE

The goals of this study are purification, characterization of the lectin from P. granatum, and examination of the purified lectin's anticancer potential.

METHODS

Diethylaminoethyl (DEAE) ion-exchange chromatography was used to purify the lectin, and SDSPAGE was used to check the purity and homogeneity of the lectin. Spectrometric and chemical analysis were used to characterize the lectin. The anticancer activity of the lectin was examined using in vivo and in vitro functional assays.

RESULTS

A lectin, designated as PgL of 28.0 ± 1.0 kDa molecular mass, was isolated and purified from the pulps of P. granatum and the lectin contains 40% sugar. Also, it is a bivalent ion-dependent lectin and lost its 75% activity in the presence of urea (8M). The lectin agglutinated blood cells of humans and rats, and sugar molecules such as 4-nitrophenyl-α-D-manopyranoside and 2- nitrophenyl -β- D-glucopyranoside inhibited PgL's hemagglutination activity. At pH ranges of 6.0-8.0 and temperature ranges of 30°C -80°C, PgL exhibited the highest agglutination activity. In vitro MTT assay showed that PgL inhibited Ehrlich ascites carcinoma (EAC) cell growth in a dose-dependent manner. PgL exhibited 39 % and 58.52 % growth inhibition of EAC cells in the mice model at 1.5 and 3.0 mg/kg/day (i.p.), respectively. In addition, PgL significantly increased the survival time (32.0 % and 49.3 %) of EAC-bearing mice at 1.5 and 3.0 mg/kg/day doses (i.p.), respectively, in comparison to untreated EAC-bearing animals (p < 0.01). Also, PgL reduced the tumor weight of EAC-bearing mice (66.6 versus 39.13%; p < 0.01) at the dose of 3.0 mg/kg/day treatment. Furthermore, supplementation of PgL restored the haematological parameters toward normal levels deteriorated in EAC-bearing animals by the toxicity of EAC cells.

CONCLUSION

The results indicated that the purified lectin has anticancer activity and has the potential to be developed as an effective chemotherapy agent.

Genome-Wide and Transcriptome Analysis of Jacalin-Related Lectin Genes in Barley and the Functional Characterization of HvHorcH in Low-Nitrogen Tolerance in Arabidopsis

The jacalin-related lectins (JRLs) are widely distributed in plants and are involved in plant development and multiple stress responses. However, the characteristics of the HvJRL gene family at the genome-wide level and the roles of JRLs in barley's response to low-nitrogen (LN) stress have been rarely reported. In this study, 32 HvJRL genes were identified and unevenly distributed at both ends of the seven chromosomes in barley. HvJRL proteins generally exhibited low sequence similarity but shared conserved jacalin domains by multiple sequence analysis. These proteins were classified into seven subfamilies based on phylogenetic analysis, with a similar gene structure and conserved motifs in the same subfamily. The HvJRL promoters contained a large number of diverse cis-elements associated with hormonal response and stress regulation. Based on the phylogenetic relationships and functionally known JRL homologs, it was predicted that some HvJRLs have the potential to serve functions in multiple stress responses but not nutrition deficiency stress. Subsequently, nine differentially expressed genes (DEGs) encoding eight HvJRL proteins were identified in two barley genotypes with different LN tolerance by transcriptome analysis. Furthermore, 35S:HvHorcH transgenic Arabidopsis seedlings did enhance LN tolerance, which indicated that HvHorcH may be an important regulator of LN stress response (LNSR). The HvJRL DEGs identified herein could provide new candidate genes for LN tolerance studies.

Atomic visualization of flipped-back conformations of high mannose glycans interacting with cargo lectins: An MD simulation perspective

Protein-carbohydrate interactions play a crucial role in mediating several biomolecular recognition events. We attempt to unravel its intricacies by understanding how carbohydrate-binding proteins interpret the glycan code. We aim to decipher lectin-mediated recognition in the endoplasmic reticulum (ER), which plays a crucial role in ER-mediated quality control (ER-QC). The ER-QC functions in three phases-protein folding, transport, and degradation. Altered protein QC leads to ER-related storage disorders. Cargo transport proteins-Ergic53 and Vip36-necessary for maintaining cellular homeostasis-are our primary focus. They recognize monoglucosylated/high mannose N-glycans on the folded glycoproteins. This article reports on the first dynamic investigation of the ER cargo lectins in complex with the high mannose glycans using an advanced sampling technique-replica exchange molecular dynamics to decipher the inherent conformational heterogeneity and the binding mechanism. The study involves simulations for the proteins complexed with three high mannose glycans-Man8B, Man9, and mono-glucosylated glycan. The recognition process is captured using MD simulations to achieve mechanistic insights and characterize the dynamics of glycans in their native and bound states via dihedral angle analysis. Results indicate that the flipped conformation of the glycans was crucial in differentiating their interaction with the proteins. Similar conformers of the glycans are preferred for Ergic53 and Vip36 in their glycan recognition events. Ergic53 preferred Man8B while it was Man9 for Vip36, in coherence with the previous experimental reports. These simulations provide a computational microscopic purview of the mechanism at both spatial and temporal scales. The results correlate with the published experimental data on the specificities of these lectins.

Lectins from plant and algae act as anti-viral against HIV, influenza and coronaviruses

BACKGROUND

Carbohydrate-lectin interactions are extremely specific as the lectin is capable of recognising monomeric and oligomeric sugars in a reversible manner. It has been known for a long time that lectins have antibacterial, antifungal, and insecticidal activities. Recently, it has been reported that many lectins can prevent the virus growth by interacting with the viral envelop surface glycoprotein. Spike protein, which is found on the surface of some enveloped viruses, is heavily mannosylated and will have strong affinity for mannose specific lectins. According to the findings, lectins have a high binding affinity for the glycans of the SARS-CoV-2 spike glycoprotein, which contains N-glycosylation sites. As a result, various lectins are being researched and developed as anti-viral agents.

RESULTS

According to our in silico studies, the amino acid residues Asn487, Tyr489, Gln493, Lys417, and Tyr505 of the receptor binding domain (RBD) of SARS-CoV-2 formed an interaction with the model lectin Lablab purpureus lectin. Similar interaction for SARS-CoV-2 spike protein was observed with Griffithsin lectin (algal source) as well. These observations demonstrate that lectins could be one of the potential molecules for neutralising coronavirus infection.

CONCLUSION

This review focuses on anti-viral lectins isolated and characterized from plants and algae (last 5 years) and showed anti-viral properties against HIV, Influenza, and coronaviruses.

Synthesis and Immunological Evaluation of Pentamannose-Based HIV-1 Vaccine Candidates

Dense glycosylation and the trimeric conformation of the human immunodeficiency virus-1 (HIV-1) envelope protein limit the accessibility of some cellular glycan processing enzymes and end up with high-mannose-type N-linked glycans on the envelope spike, among which the Man₅GlcNAc₂ structure occupies a certain proportion. The Man₅GlcNAc₂ glycan composes the binding sites of some potent broadly neutralizing antibodies, and some lectins that can bind Man₅GlcNAc₂ show HIV-neutralizing activity. Therefore, Man₅GlcNAc₂ is a potential target for HIV-1 vaccine development. Herein, a highly convergent and effective strategy was developed for the synthesis of Man₅ and its monofluoro-modified, trifluoro-modified, and S-linked analogues. We coupled these haptens to carrier protein CRM197 and evaluated the immunogenicity of the glycoconjugates in mice. The serological assays showed that the native Man₅ conjugates failed to induce Man₅-specific antibodies in vivo, while the modified analogue conjugates induced stronger antibody responses. However, these antibodies could not bind the native gp120 antigen. These results demonstrated that the immune tolerance mechanism suppressed the immune responses to Man₅-related structures and the conformation of glycan epitopes on the synthesized glycoconjugates was distinct from that of native glycan epitopes on gp120.

Structure and Carbohydrate Recognition by the Nonmitogenic Lectin Horcolin

Lectins are sugar-binding proteins that have shown considerable promise as antiviral agents because of their ability to interact with envelope glycoproteins present on the surface of viruses such as HIV-1. However, their therapeutic potential has been compromised by their mitogenicity that stimulates uncontrolled division of T-lymphocytes. Horcolin, a member of the jacalin family of lectins, tightly binds the HIV-1 envelope glycoprotein gp120 and neutralizes HIV-1 particles but is nonmitogenic. In this report, we combine X-ray crystallography and NMR spectroscopy to obtain atomic-resolution insights into the structure of horcolin and the molecular basis for its carbohydrate recognition. Each protomer of the horcolin dimer adopts a canonical β-prism I fold with three Greek key motifs and carries two carbohydrate-binding sites. The carbohydrate molecule binds in a negatively charged pocket and is stabilized by backbone and side chain hydrogen bonds to conserved residues in the ligand-binding loop. NMR titrations reveal a two-site binding mode and equilibrium dissociation constants for the two binding sites determined from two-dimensional (2D) lineshape modeling are 4-fold different. Single-binding-site variants of horcolin confirm the dichotomy in binding sites and suggest that there is allosteric communication between the two sites. An analysis of the horcolin structure shows a network of hydrogen bonds linking the two carbohydrate-binding sites directly and through a secondary binding site, and this coupling between the two sites is expected to assume importance in the interaction of horcolin with high-mannose glycans found on viral envelope glycoproteins.

Legume Lectins with Different Specificities as Potential Glycan Probes for Pathogenic Enveloped Viruses

Pathogenic enveloped viruses are covered with a glycan shield that provides a dual function: the glycan structures contribute to virus protection as well as host cell recognition. The three classical types of N-glycans, in particular complex glycans, high-mannose glycans, and hybrid glycans, together with some O-glycans, participate in the glycan shield of the Ebola virus, influenza virus, human cytomegalovirus, herpes virus, human immunodeficiency virus, Lassa virus, and MERS-CoV, SARS-CoV, and SARS-CoV-2, which are responsible for respiratory syndromes. The glycans are linked to glycoproteins that occur as metastable prefusion glycoproteins on the surface of infectious virions such as gp120 of HIV, hemagglutinin of influenza, or spike proteins of beta-coronaviruses. Plant lectins with different carbohydrate-binding specificities and, especially, mannose-specific lectins from the Vicieae tribe, such as pea lectin and lentil lectin, can be used as glycan probes for targeting the glycan shield because of their specific interaction with the α1,6-fucosylated core Man3GlcNAc2, which predominantly occurs in complex and hybrid glycans. Other plant lectins with Neu5Ac specificity or GalNAc/T/Tn specificity can also serve as potential glycan probes for the often sialylated complex glycans and truncated O-glycans, respectively, which are abundantly distributed in the glycan shield of enveloped viruses. The biomedical and therapeutical potential of plant lectins as antiviral drugs is discussed.

The Jacalin-Related Lectin HvHorcH Is Involved in the Physiological Response of Barley Roots to Salt Stress

Salt stress tolerance of crop plants is a trait with increasing value for future food production. In an attempt to identify proteins that participate in the salt stress response of barley, we have used a cDNA library from salt-stressed seedling roots of the relatively salt-stress-tolerant cv. Morex for the transfection of a salt-stress-sensitive yeast strain (Saccharomyces cerevisiae YSH818 Δhog1 mutant). From the retrieved cDNA sequences conferring salt tolerance to the yeast mutant, eleven contained the coding sequence of a jacalin-related lectin (JRL) that shows homology to the previously identified JRL horcolin from barley coleoptiles that we therefore named the gene HvHorcH. The detection of HvHorcH protein in root extracellular fluid suggests a secretion under stress conditions. Furthermore, HvHorcH exhibited specificity towards mannose. Protein abundance of HvHorcH in roots of salt-sensitive or salt-tolerant barley cultivars were not trait-specific to salinity treatment, but protein levels increased in response to the treatment, particularly in the root tip. Expression of HvHorcH in Arabidopsis thaliana root tips increased salt tolerance. Hence, we conclude that this protein is involved in the adaptation of plants to salinity.

Mevo lectin specificity towards high-mannose structures with terminal αMan(1,2)αMan residues and its implication to inhibition of the entry of Mycobacterium tuberculosis into macrophages

Mannose-binding lectins can specifically recognize and bind complex glycan structures on pathogens and have potential as anti-viral and anti-bacterial agents. We previously reported the structure of a lectin from an archaeal species, Mevo lectin, which has specificity towards terminal α1,2 linked manno-oligosaccharides. Mycobacterium tuberculosis (M. tuberculosis) expresses mannosylated structures including, lipoarabinomannan (ManLAM) on its surface and exploits C-type lectins to gain entry into the host cells. ManLAM structure has mannose capping with terminal αMan(1,2)αMan residues and is important for recognition by innate immune cells. Here, we aim to address the specificity of Mevo lectin towards high-mannose type glycans with terminal αMan(1,2)αMan residues and its effect on M. tuberculosis internalization by macrophages. ITC studies demonstrated that Mevo lectin shows preferential binding towards manno-oligosaccharides with terminal αMan(1,2)αMan structures, and showed a strong affinity for ManLAM, whereas it binds weakly to Mycobacterium smegmatis (M. smegmatis) lipoarabinomannan (MsmLAM), which displays relatively fewer and shorter mannosyl caps. Crystal structure of Mevo lectin complexed with a Man7D1 revealed the multivalent cross-linking interaction, which explains avidity-based high affinity for these ligands when compared to previously studied manno-oligosaccharides lacking the specific termini. Functional studies suggest that M. tuberculosis internalization by the macrophage was impaired by binding of Mevo lectin to ManLAM present on the surface of M. tuberculosis. Selectivity shown by Mevo lectin towards glycans with terminal αMan(1,2)αMan structures, and its ability to compromise the internalization of M. tuberculosis in vitro, underscore the potential utility of Mevo lectin as a research tool to study host-pathogen interactions.