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 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 and -12 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.Also found in abundance in muscle, neurons and kidneyActivate apoptosis in T cellsFound upregulated in tumour cellsRegulation of some genes including JNK1 Implicated in tuberculosis defenseMay have a role in apoptosis and cellular repair mediated by p53 Implicated in tuberculosis defenseThymusInduces apoptosis of thymocytes and Th1 cellsImplicated in tuberculosis defenseInvolved in adipocyte differentiation 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 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 and -12 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, -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. The galectin carbohydrate recognition domain (CRD) is constructed from beta-sheet of about 135 amino acids. The two sheets are slightly bent with 6 strands forming the concave side and 5 strands forming the convex side. The concave side forms a groove in which the carbohydrate ligand can bind, and which is long enough to hold about a linear tetrasaccharide. Galectins essentially bind to glycans featuring galactose and its derivatives. However, physiologically, they are likely to require lactose or N-aceyllactosamine for significantly strong binding. Generally, the longer the sugar the stronger the interactions. For example, galectin-9 binds to polylactosamine chains with stronger affinity than to an N-acetyllactosamine monomer. This is because more Van der Waals interactions can occur between sugar and binding pocket. Carbohydrate binding is calcium independent, unlike C-type lectins. The strength of ligand binding is determined by a number of factors: The multivalency of both of ligand and the galectin, the length of the carbohydrate and the mode of presentation of ligand to carbohydrate recognition domain. Different galectins have distinct binding specificities for binding oligosaccharides depending on the tissue in which they are expressed and the function that they possess. However, in each case, galactose is essential for binding. Crystallisation experiments of galectins in complex with N-acetyllactosamine show that binding arises due to hydrogen bonding interactions from the carbon-4 and carbon-6 hydroxyl groups of galactose and carbon-3 of N-acetylglucosamine (GlcNAc) to the side chains of amino acids in the protein. They cannot bind to other sugars such as mannose because this sugar will not fit inside the carbohydrate recognition domain without steric hindrance. Due to the nature of the binding pocket, galectins can bind terminal sugars or internal sugars within a glycan. This allows bridging between two ligands on the same cell or between two ligands on different cells. Galectins are a large family with relatively broad specificity. Thus, they have a broad variety of functions including mediation of cell–cell interactions, cell–matrix adhesion and transmembrane signalling. Their expression and secretion is well regulated, suggesting they may be expressed at different times during development. There are no serious defects when individual galectin genes are deleted in knock-out mouse models. This is because there is substantial overlap for the essential functions. The list of functions for galectins is extensive and it is unlikely they have all been discovered. A handful of the main functions are described below. Galectins are distinct in that they can regulate cell death both intracellularly and extracellularly. Extracellularly, they cross link glycans on the outside of cells and transduce signals across the membrane to directly cause cell death or activate downstream signaling that triggers apoptosis. Intracellularly, they can directly regulate proteins that control cell fate. Many galectins have roles in apoptosis: Galectin-3 has an essential role in negatively regulating T cell receptor (TCR) activation. Crosslinking of T cell receptors and other glycoproteins by galectin-3 on the membrane of T cells prevents clustering of TCRs and ultimately suppresses activation. This prevents auto-activation. Experiments in transgenic mice with deficient N-acetylglucosamine transferase V (GnTV) have increased susceptibility to autoimmune diseases. GnTV is the enzyme required to synthesise polylactosamine chains, which are the ligand for galectin-3 on T cell receptors. This knock-out means galectin-3 cannot prevent auto-activation of TCR so T cells are hypersensitive. Also within the immune system, galectins have been proven to act as chemoattractants to immune cells and activate secretion of inflammatory cytokines. Galectins can both promote and inhibit integrin-mediated adhesion. To enhance integrin-mediated adhesion, they cross link between two glycans on different cells. This brings the cells closer together so integrin binding occurs. They can also hinder adhesion by binding to two glycans on the same cell, which blocks the integrin binding site. Galectin-8 is specific for the glycans bound to integrin and has a direct role in adhesion as well as activating integrin-specific signaling cascades. Galectin-1 and galectin-3 have been found, surprisingly, to associate with nuclear ribonucleoprotein complexes including the spliceosome. Studies revealed that galectin-1 and -3 are required splicing factors, since removal of the galectins by affinity chromatography with lactose resulted in loss of splicing activity. It appears that the splicing capability of galectins is independent of their sugar-binding specificities. Site-directed mutagenesis studies to the carbohydrate recognition domain removes glycan binding but does not prevent association with the spliceosome.