Galectin

Galectins are a family of proteins defined by their binding specificity for 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 15 galectins discovered in mammals, encoded by the LGALS genes, which are numbered in a consecutive manner. Only galectin-1, -2, -3, -4, -7, -8, -9, -10, -12 and -13 have been identified in humans. Galectin-5 and -6 are found in rodents, whereas galectin-11, -14 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, -13, -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. Galectin-3 can exist in monomeric form or can associate via the non-lectin domain into multivalent complexes up to a pentameric form. This allows galectin-3 to bridge effectively between different ligands and form adhesive networks. The formation of multimers is concentration dependent. When Galectin-3 is at a low concentration it is monomeric and likely to inhibit adhesion. It binds to adhesion proteins such as integrins and blocks further binding to other cells or the extracellular matrix. When concentrations of galectin-3 are high it forms large complexes that assist in adhesion by bridging between cells or cells and the extracellular matrix. Many isoforms of galectins have been found due to different splicing variants. For example, Galectin-8 has seven different mRNAs encoding for both tandem and dimeric forms. The type of galectin-8 that is expressed is dependent on the tissue. Galectin-9 has three different isoforms which differ in the length of the linker region.

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