2 – Isolation, Physicochemical Characterization, and Carbohydrate-Binding Specificity of Lectins

Many plants contain carbohydrate-binding proteins that are commonly designated as lectins, agglutinins, or hemagglutinins. Due to the obvious differences in molecular structure, biochemical properties, and carbohydrate-binding specificity, plant lectins are usually considered a complex and heterogeneous group of proteins. Recent advances in the structural analysis of lectins and molecular cloning of lectin genes enable subdividision of plant lectins in a limited number of subgroups of structurally and evolutionary related proteins. Four major lectin families, namely, the legume lectins, the chitin-binding lectins composed of hevein domains, the type 2 ribosome-inactivating proteins, and the monocot mannose-binding lectins comprise the majority of all currently known plant lectins. In addition to these four large families the jacalin-related lectins, the amaranthin family, and the Cucurbitaceae phloem lectins are now recognized as separate subgroups. Each of the above-mentioned lectin families is discussed...

Plant Lectins: A Composite of Several Distinct Families of Structurally and Evolutionary Related Proteins with Diverse Biological Roles

The earliest histochemical methods for demonstrating carbohydratecontaining macromolecules served mainly to &rentiate these substances, as a general class, from the other macromolecular constituents in tissue sections: the proteins, nucleic acids, and lipids (93,137). A later stage in the development of this field aimed at distinguishing by histochemistry subclasses of carbohydratecontaining components, e.g., neutral and acidic constituents and subgroups of the acidic substances. The capacity to characterize complex carbohydrates in situ in tissue sections progressed subsequently with the use of carbohydrate-degrading enzymes to locate moieties possessing a specific carbohydrate linkage labile to the enzyme and of immunostaining to identlfy specific epitopes. An added dimension for analysis of glycoconjugate (GC) structure in situ followed the advent of labeled lectins which allow histochemical demonstration of terminal or internal sugars or of sugar sequences. This review attempts to put in perspective the status of carbohydrate histochemistry before and after introduction of the lectin approach. Progress in the development and application of non-lectin methods for light and electron microscopic localization has been reviewed (126,137,153,172,175,205,210,213,234) and will be covered here in summary form. A voluminous literature on lectin histochemistry and its use in a wide range of adult and embryonic tissues and pathological lesions at the light and electron microscopic level has also been reviewed extensively (2,16,17,95,101,131, 142,153,172,175,213,234). The discussion of lectin histochemistry here concerns to a large extent experience from this laboratory. The intent is to emphasize advantages ofthis approach for probing the chemical nature of GCs in precise loci, detecting the presence of GCs not previously recognized in a number of locations, and showing the tremendous variability of GG among histological sites. Histochemical studies have demonstrated GCs possessing dis-

Diversity of cell glycoconjugates shown histochemically : a perspective

A solid phase carbohydrate library was synthesized and screened against Bauhinia purpurea lectin. The library, which contains approximately 1300 di- and trisaccharides, was synthesized with chemical encoding on TentaGel resin so that each bead contained a single carbohydrate. Two ligands that bind more tightly to the lectin than Gal-β-1,3-GalNAc (the known ligand) have been identified. The strategy outlined can be used to identify carbohydrate-based ligands for any receptor; however, because the derivatized beads mimic the polyvalent presentation of cell surface carbohydrates, the screen may prove especially valuable for discovering new compounds that bind to proteins participating in cell adhesion.

Parallel Synthesis and Screening of a Solid Phase Carbohydrate Library

Carbohydrate-derivatized self-assembled monolayers (SAMs) are used as a model system to address issues involving cell-surface carbohydrate–protein interactions. Here we examine the influence of carbohydrate surface density on protein-binding avidity. We show that the binding selectivity of Bauhinia purpurea lectin switches from one carbohydrate ligand to another as the surface density of the carbohydrate ligands increases from values of χsugar ≈ 0.1–1.0. Polyvalent binding is possible at all surface densities investigated; hence, the switch in selectivity is not due simply to the achievement of a critical density that permits polyvalent contacts. Instead, secondary interactions at high surface densities promote a switch in carbohydrate-binding selectivity. These findings may have implications for how changes in the composition and the density of cell-surface carbohydrates influence biological recognition processes and regulatory pathways.

https://www.pnas.org/content/pnas/96/21/11782.full.pdf

Nonstatistical binding of a protein to clustered carbohydrates

Immunochemical studies on the carbohydrate specificity of Maclura pomifera lectin.

Immunochemical studies on the combining site of the d-galactopyranose and 2-acetamido-2-deoxy-d-galactopyranose specific lectin isolated from Bauhinia purpurea alba seeds

Immunochemical studies on blood groups: The internal structure and immunological properties of water-soluble human blood group A substance studied by Smith degradation, liberation, and fractionation of oligosaccharides and reaction with lectins.

Albert M. Wu

Papers

Immunochemical studies on the carbohydrate specificity of Maclura pomifera lectin.

Immunochemical studies on the combining site of the d-galactopyranose and 2-acetamido-2-deoxy-d-galactopyranose specific lectin isolated from Bauhinia purpurea alba seeds

Immunochemical studies on blood groups: The internal structure and immunological properties of water-soluble human blood group A substance studied by Smith degradation, liberation, and fractionation of oligosaccharides and reaction with lectins.