Many plants contain carbohydrate-binding proteins known as lectins. Recent advances in the characterization, cloning and structural analysis allowed to classify plant lectins in seven families of structurally and evolutionary related proteins. Within each lectin family the overall fold and structure of the carbohydrate-binding site (s) are conserved. This structural conservation is reflected in the very similar specificity of lectins belonging to the families of the amaranthins, the chitin-binding lectins composed of hevein domains, the Cucurbitaceae phloem lectins, the monocot mannose-binding lectins and the type 2 ribosome-inactivating proteins. Within the family of jacalin-related lectins the same fold gives raise to two structurally similar binding sites but with a different specificity, and in the legume lectin family a single structure allows the formation of binding sites with a wide range of specificities. An analysis of the structure/specificity relationships of plant lectins leads to important conclusions. First, most lectin families exhibit a highly conserved specificity whereas others cover a broad range of specificities. Second, some carbohydrates are recognized by multiple structurally different carbohydrate-binding motifs. Third, the development of multiple binding motifs for mannose, chitin, and Gal/GalNAc highlights the importance for the plant of a system to sense the presence of these glycans. A closer examination of the specificity further indicates that most plant lectins are not targeted against plant carbohydrates but preferentially bind foreign glycans.
It has been revealed that high-mannose type free N-glycans with one N-acetylglucosamine and plant complex type N-glycans with N-acetylchitobiose segment occur ubiquitously in developing or growing plant cells (hypocotyls of seedlings or developing seeds). Taking account of the reducing-end structure of such free N-glycans, the high-mannose type and plant complex type free glycans would be produced by the action of endo-β-N-acetylglucosaminidase (ENGase) and peptide: N-glycanase (PNGase), respectively. Functional analysis of free N-glycans becomes an interesting subject of plant physiology, since such oligosaccharides have been postulated to have a function as an auxin-like signaling molecule for tomato fruit ripening. In this review, the author provides an overview of the current knowledge of the structural features and putative function of these free N-glycans occurring in plant cells, linking to the functional features of the plant endoglycosidase.
N-Acetylchitooligosaccharides (oligochitin, chitin oligosaccharides) of a specific size can act as potent elicitor signals for suspension-cultured rice cells as well as various plant cells which include many monocots and some dicots. We recently isolated and characterized a highly elicitor-active glucopentaose from the cell wall β-Glucan from rice blast disease fungus. The results indicated that rice and soybean cells recognize different structural units of fugal glucans as elicitor signals. Because this elicitor treatment can induce many defense reactions, it has been serving as an excellent model system for the study of the signal transduction cascade leading to the activation of defense-related genes. It is critically important to identify and characterize the receptor molecules which perceive the elicitor signal to clarify the whole signal transduction cascade. A 75kDa chitin oligosaccharide binding protein in the plasma membrane of suspension-cultured rice cells was identified as a putative receptor for the elicitor and purified. Recent studies on the structure and function of the binding proteins for these oligosaccharide elicitors will provide a clue to understanding how these elicitors are perceived and transduced in rice and other plant cells and also how such recognition systems have evolved.
Heparin and heparan sulphate (HS) are characterised as a group of linear and highly sulphated polysaccharides of glycosaminoglycan (GAG) type. Heparin is produced mainly by connective tissue mast cells, whereas heparan sulfate is synthesized virtually by most cell types. Recently, interest is increasing in this group of polysaccharides due largely to its multiple biological functions and potential association with disease (1). In fact, heparin and HS have been implicated in modulating various biological processes, such as blood clotting, cell adhesion, growth factor signalling, and viral infection. The biological functions of heparin and HS largely depend on interactions of the negatively charged polysaccharide chain with a variety of proteins, such as proteases, protease inhibitors, growth factors, extracellular matrix components and viral proteins (2). In some cases, a single domain, generally consisting of less than 10 monosaccharide units but with a defined saccharide structure, is required for specific interaction with a selected protein. Moreover, the polysaccharide chain may form complexes with two or more proteins, identical or different. To induce biological response, these saccharide domains must be organized in a proper way. The enormous structural diversity of heparin/HS is created by a complex biosynthetic pathway (3). The biosynthesis of the polysaccharide chain is initiated by the formation of a precursor polymer composed of repeating disaccharide units of alternating D-glucuronic acid and N-acetyl-D-glucosamine, [-GlcA-GlcNAc-]n. While the chain is elongating, the polymer is modified by a series of sequential reactions including N-deacetylation/N-sulphation of GlcNAc, C5-epimerization of GlcA to L-iduronic acid (IdoA) and O-sulphation of IdoA at position C2 and of GlcN residues at position C6. In addition, to a lesser extent, O-sulphation of GlcA at position C2 and of GlcN at position C3 may also occur and appear to be biologically important. Most of the enzymes involved in HS biosynthesis have been characterised in molecular detail. Interestingly, some of these enzymes exist in several genetic isoforms with distinct substrate specificities (2, 4, 5). This thesis has been conducted to address two specific questions regarding the biosynthetic pathway and the structureactivity relationship of heparin/HS. Firstly, how 2-O-sulfation of GlcA and IdoA residues is accomplished in the biosynthetic process of heparin/HS. Secondly, how different saccharide domains must be organized in order to induce biological response.