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 designa ted as lectins, agglutinins, or hemagglutinins. Due to the obvious differen ces in molecular structure, biochemical properties, and carbohydrate-bindin g specificity, plant lectins are usually considered a complex and heterogen eous group of proteins. Recent advances in the structural analysis of lecti ns and molecular cloning of lectin genes enable subdividision of plant lect ins in a limited number of subgroups of structurally and evolutionary relat ed proteins. Four major lectin families, namely, the legume lectins, the ch itin-binding lectins composed of hevein domains, the type 2 ribosome-inacti vating proteins, and the monocot mannose-binding lectins comprise the major ity of all currently known plant lectins. In addition to these four large f amilies the jacalin-related lectins, the amaranthin family, and the Cucurbi taceae 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) a n 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 translati on products, (4) a summary of carbohydrate-binding specificity, (5) if rele vant 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 assessm ent of the physiological role of each group of lectins. Lectins that cannot be classified into one of the seven groups are discussed separately. Gener al conclusions about the structure, evolution, and function of plant lectin s are summarized in the concluding remarks.