Abstract
There are two major families of carbohydrate-binding proteins (lectins) in the animal kingdom: the family of calcium-dependent (C-type) lectins and the family of metal-independent β-galactoside-binding lectins (so-called S-type). As one of the approaches to an understanding of reasons for their being, a general comparison of these protein families is attempted. C-type lectins form a vast family consisting of various extracellular proteins including both lectins and lectinrelated proteins. Although they have diverse molecular architectures and functions, insofar as they are known, each molecule shows rather limited biological functions and histological distributions. Such C-type lectins can be regarded as “bricolage products”, developed in the course of molecular evolution having made the best use of the carbohydraterecognition domain (CRD). On the other hand, S-type lectins form a relatively small family. At the moment, only three types of soluble metal-independent β-galactoside-binding lectins are members of this family. Though their biological roles remain elusive, recent findings of apparent homologues in some of the most primitive multicellular animals, such as sponge and nematode, strongly suggest their fundamental importance. Marked evidence that they are initially synthesized as cytoplasmic proteins implies so far unknown cytoplasmic function(s). They are possibly involved in the “essential minimum” functions for all multicellular animals in cooperation with their partner glycoconjugates, such as lactosaminoglycans. All of. these contrastive properties represent the S-type lectins as “antithesis” to the C-type lectins.
