Galectin

Galectins are a class of proteins that bind specifically to β-galactoside sugars, such as N-acetyllactosamine (Galβ1-3GlcNAc or Galβ1-4GlcNAc), which can be bound to proteins by either N-linked or O-linked glycosylation. They are also termed S-type lectins due to their dependency on disulphide bonds for stability and carbohydrate binding. There have been about 15 galectins discovered in mammals, encoded by the LGALS genes, which are numbered in a consecutive manner. Only galectin-1, -2, -3, -4, -7, -7B, -8, -9, -9B, 9C, -10, -12, -13, -14, and -16 have been identified in humans. Galectin-5 and -6 are found in rodents, whereas galectin-11 and -15 are uniquely found in sheep and goats. Members of the galectin family have also been discovered in other mammals, birds, amphibians, fish, nematodes, sponges, and some fungi. Unlike the majority of lectins they are not membrane bound, but soluble proteins with both intra- and extracellular functions. They have distinct but overlapping distributions but found primarily in the cytosol, nucleus, extracellular matrix or in circulation. Although many galectins must be secreted, they do not have a typical signal peptide required for classical secretion. The mechanism and reason for this non-classical secretion pathway is unknown. There are three different forms of galectin structure: dimeric, tandem or chimera. Dimeric galectins, also called prototypical galectins, are homodimers, consisting of two identical galectin subunits that have associated with one another. The galectins that fall under this category are galectin-1, -2, -5, -7, -10, -11, -14 and -15. Tandem galectins contain at least two distinct carbohydrate recognition domains (CRD) within one polypeptide, thus are considered intrinsically divalent. The CRDs are linked with a small peptide domain. Tandem galectins include galectin-4, -6, -8, -9 and -12. The final galectin is galectin-3 which is the only galectin found in the chimera category in vertebrates. Galectin-3 has one CRD and a long non-lectin domain.