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

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 in detail. The description of the individual lectin families includes (1) a brief historical note, (2) an overview of the occurrence, molecular structure, and primary structure of the lectins, (3) a detailed discussion of the structure of the gene(s) and the biosynthesis and posttranslational processing of the primary translation products, (4) a summary of carbohydrate-binding specificity, (5) if relevant a note on the occurrence of lectin-related proteins, (6) a description of the three-dimensional structure of the lectins and the protomers, (7) a detailed discussion of the molecular evolution, and (8) a critical assessment of the physiological role of each group of lectins. Lectins that cannot be classified into one of the seven groups are discussed separately. General conclusions about the structure, evolution, and function of plant lectins are summarized in the concluding remarks.