ABSTRACT
The lectin isolated from the phytopathogenic basidiomycete Rhizoctonia solani (RSA) is a homodimer of two noncovalently associated monomers of 15.5 kDa. RSA is a basic protein (pI > 9) which consists mainly of beta-sheets. A presumed relationship with ricin-B is supported by the sequence similarity between the N-terminus of RSA and the N-terminal subdomain of ricin-B. Hydrophobic cluster analysis confirms that the N-terminus of both proteins has a comparable folding. RSA exhibits specificity towards Gal/GalNAc whereby the hydroxyls at the C3', C4', and C6' positions of the pyranose ring play a key role in the interaction with simple sugars. The carbohydrate-binding site of RSA apparently accommodates only a single sugar unit. Our results demonstrate an obvious evolutionary relationship between some fungal and plant lectins, but also provide evidence for the occurrence of a lectin consisting of subunits corresponding to a single subdomain of ricin-B.
Subject(s)
Lectins/chemistry , Rhizoctonia/chemistry , Ricin/chemistry , Acetylgalactosamine , Amino Acid Sequence , Binding Sites , Dimerization , Galactose , Isoelectric Point , Lectins/classification , Lectins/genetics , Lectins/isolation & purification , Molecular Sequence Data , Molecular Weight , Plant Lectins , Plants/microbiology , Protein Structure, Secondary , Protein Structure, Tertiary , Rhizoctonia/genetics , Rhizoctonia/pathogenicity , Ricin/classification , Ricin/genetics , Sequence Homology, Amino AcidABSTRACT
The molecular structure and carbohydrate-binding activity of the lectin from bulbs of spring crocus (Crocus vernus) has been determined unambiguously using a combination of protein analysis and cDNA cloning. Molecular cloning revealed that the lectin called C. vernus agglutinin (CVA) is encoded by a precursor consisting of two tandemly arrayed lectin domains with a reasonable sequence similarity to the monocot mannose-binding lectins. Post-translational cleavage of the precursor yields two equally sized polypeptides. Mature CVA consists of two pairs of polypeptides and hence is a heterotetrameric protein. Surface plasmon resonance studies of the interaction of the crocus lectin with high mannose-type glycans showed that the lectin interacts specifically with exposed alpha-1,3-dimannosyl motifs. Molecular modelling studies confirmed further the close relationships in overall fold and three-dimensional structure of the mannose-binding sites of the crocus lectin and other monocot mannose-binding lectins. However, docking experiments indicate that only one of the six putative mannose-binding sites of the CVA protomer is active. These results can explain the weak carbohydrate-binding activity and low specific agglutination activity of the lectin. As the cloning and characterization of the spring crocus lectin demonstrate that the monocot mannose-binding lectins occur also within the family Iridaceae a refined model of the molecular evolution of this lectin family is proposed.
Subject(s)
Agglutinins , Lectins/chemistry , Lectins/genetics , Magnoliopsida/genetics , Mannose-Binding Lectins , Mannose , Amino Acid Sequence , Carbohydrate Conformation , Carbohydrate Sequence , Cloning, Molecular , Evolution, Molecular , Lectins/isolation & purification , Macromolecular Substances , Magnoliopsida/chemistry , Models, Molecular , Molecular Sequence Data , Phylogeny , Plant Lectins , Protein Structure, Secondary , Recombinant Proteins/chemistry , Recombinant Proteins/isolation & purification , Sequence Alignment , Sequence Homology, Amino AcidABSTRACT
The direct interaction of mannose-specific plant lectins with gp120 of HIV-1 was studied by surface plasmon resonance. Inhibition experiments indicated that exposed high mannose type glycans play a key role in the interaction. Most of the lectins specifically accommodate outer alpha1,2-, alpha1,3-, or alpha1,6-linked di- or trimannosides, and especially legume lectins, also interact with the trimannoside core of the complex type glycans. The unexpected affinity of some lectins towards gp120 presumably results from conformational differences in their binding sites. These results demonstrate that mannose-specific plant lectins are powerful tools to study the accessibility and elucidate the function of the gp120 glycans in the recognition and infection of the host cells by HIV-1.
Subject(s)
HIV Envelope Protein gp120/chemistry , HIV-1/chemistry , Lectins/chemistry , Mannose/chemistry , Polysaccharides/chemistry , Binding Sites , Biosensing Techniques , Dose-Response Relationship, Drug , Lectins/pharmacology , Models, Molecular , Protein Binding/drug effects , Protein Binding/physiology , Substrate Specificity , Surface Plasmon ResonanceABSTRACT
A novel plant lectin was isolated from salt-stressed rice (Oryza sativa L.) plants and partially characterized. The lectin occurs as a natural mixture of two closely related isoforms consisting of two identical non-covalently linked subunits of 15 kDa. Both isoforms are best inhibited by mannose and exhibit potent mitogenic activity towards T-lymphocytes. Biochemical analyses and sequence comparisons further revealed that the rice lectins belong to the subgroup of mannose-binding jacalin-related lectins. In addition, it could be demonstrated that the lectins described here correspond to the protein products of previously described salt-stress-induced genes. Our results not only identify the rice lectin as a stress protein but also highlight the possible importance of protein-carbohydrate interactions in stress responses in plants.
