ABSTRACT
A number of Ca2+-dependent lectins have been found in various invertebrates. Many of them belong to C-type lectin family, which contain C-type carbohydrate recognition domains (CRDs) [1,2], and assumed to be involved in defense against foreign pathogens [3]. Four Ca2+-dependent, Gal/ GalNAc-speci c lectins (CEL-I, II, III, and IV) have been isolated from the body uid of the marine invertebrate (sea cucumber), Cucumaria echinata [4]. Among these, CEL-I and CEL-IV belong to the C-type lectin family [5,6], while CEL-III is a novel lectin that exhibits strong hemolytic and cytotoxic activities [7,8]. Based on several structural and biochemical analyses, it was subsequently found that CEL-III belongs to ricin-type (R-type) lectin family in spite of the Ca2+ dependency and its hemolytic activity is mediated by formation of ion-permeable pores composed of its oligomers in the target cell membranes [9]. Self-oligomerization of CEL-III in the membranes is mediated by conformational changes of the protein, triggered by binding to the cell surface carbohydrate chains. The three-dimensional structure of this lectin, along with its complex with speci c carbohydrate, has recently been solved by x-ray crystallographic analysis and has provided important clues to speculate the mechanism of the hemolytic action. This chapter describes characteristic features of CEL-III as a hemolytic lectin and its putative mechanism of hemolytic activity on the basis of the structural information.
