God must love galectins; He made so many of them

Abstract

Galectins are a family of proteins first identified as galactoside-binding lectins in extracts of vertebrate tissue (Barondes et al., 1994a; Various authors, 1997). Sequencing of such proteins isolated from amphibians, birds, fish, and mammals revealed extensive sequence similarity, and in 1994, the galectin family was formally defined (Barondes et al., 1994b) on the basis of both shared sequence and galactoside binding. Four human galectins, which had been discovered in various contexts, and which bore multiple names, were renamed galectin-1 through -4. Since that time, advances in molecular genetics have led to the identification of many new members of the galectin gene family, which have been discovered on the basis of sequence similarity. Here we call attention to these newly established galectins and to additional genes, whose sequence suggests that they too are galectin family members. All galectins share a core sequence consisting of about 130 amino acids, many of which are highly conserved. Crystallography has been used to determine the structure of several galectins, most recently for galectin-7 (Leonidas et al., 1998). The portion of the core sequence which represents the carbohydrate recognition domain (CRD) (Figure 1) is contained between about residues 30 and 90, a segment generally encoded by a single exon. Of the original mammalian members, galectins 1–3 include just one CRD, whereas galectin-4 is composed of two nonidentical tandem core structures with two separate CRDs. Another shared feature was surprising: although galectins are found both in the cytoplasm and extracellularly, none has a secretion signal peptide. Instead, several galectins have been shown to be secreted by an unorthodox mechanism (Hughes, 1997).