Cloning, Characterization, Expression Analysis, and Agglutination Studies of Novel Gene Encoding β-D-Galactose, N-Acetyl-D-Glucosamine and Lactose-Binding Lectin from Rice Bean (Vigna umbellata)

Lectins are glycoproteins and known for their peculiar carbohydrate-binding activity and their insect-pest-resistant properties. Earlier we have published our research finding on novel gene encoding Bowman-Birk type protease inhibitor with insecticidal properties from rice bean. This paper presents first report on cloning, sequencing, and expression of RbL ORF of 843 bp encoding 280 amino acids long lectin precursor from rice bean (Vigna umbellata) seeds. Blast analysis revealed more than 90% similarity of RbL protein with Vigna aconitifolia and Vigna angularis lectins. Phylogenetic analysis also revealed a close relationship between RbL and other legume lectins. Sequence analysis of genomic DNA revealed intronless nature of RbL gene (GenBank accession No. MT043160). The isolated RbL ORF was expressed in E. coli BL-21(DE3) cells and maximum expression was recorded with 0.5 mM IPTG after 4 h incubation at 37 degrees C. Western blotting confirmed RbL protein expression in E. coli. Recombinant protein (His(6)-RbL) of similar to 35 kDa m.wt was purified using Ni-NTA affinity chromatography to the extent of 0.26 mg/ml. In silico analysis characterized RbL protein as acidic, stable, hydrophobic, and secretary protein with one signal peptide cleavage site (A(26)-A(27)) and four N-glycosylation sites. Template-based 3D model of RbL was structured using MODELLER tool and validated as good quality model. Structural analysis revealed dominance of beta-pleated sheets and beta-turns in RbL protein structure. beta-D-galactose, N-acetyl-D-glucosamine, and lactose were predicted as putative ligands for RbL protein. Hydrogen bonding and hydrophobic forces were the major interactions between the predicted ligands and RbL protein. Agglutination and agglutination inhibition assays confirmed the binding specificity of RbL protein with the trypsinized rabbit erythrocytes and with the predicted ligands, respectively. Gene ontology analysis functionally annotated RbL protein as a plant defense protein. The novel information generated in the study is not mere pre-experimental findings but could also lay foundation for future research on exploring RbL gene and encoding protein for different biomedical and biotechnological applications.