Higher plants developed structurally different motifs to recognize foreign glycans

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.