Galectin refers to a family of proteins found in the body. These proteins are defined thanks to the way they bind specifically with beta-galactoside sugars. Sugars like this are bound to proteins via N-linked or O-linked glycosylation. Often, galectins are called S-type lectins because they’re highly dependent on disulphide bonds. They need this bond to help with stability, and it also helps with carbohydrate binding. Currently, there have been a total of 15 galectins discovered in humans and other mammals. All of which are encoded by the LGALS genes. Galectins 1 down to -13 (excluding -5 and -6 which are found in rodents) are the only ones present in humans. The remaining three galectins (-11, -14, and -15) are only found in goats and sheep.
Due to the fact, galectins are a big family, it’s hard to specify one blanket function. As a result, they have many different functions in many different cells. Some galectins are found to help with the mediation between intercellular interactions, cell-matrix adhesion, and transmembrane signalling.
Galectins are unique because they’re known to play a regulatory role in cell death both extra and intracellularly. On an extracellular level, they send signals through membranes to elicit cell death. Their intracellular function involves the regulation of proteins that ultimately control the fate of a cell. Consequently, it is deduced that a primary function of galectins is to play a vital role in apoptosis.
Galectin -3 is also known to suppress T-cell receptor activation. Along with this, galectins are able to prevent integrin-mediated cell adhesion, as well as promoting it.
Mechanism & Interactions
Galectins will bind to glycans that include galactose and other derivatives of galactose. For the binding to be as strong as possible, it’s suggested that lactose or N-acetyllactosamine are present. The longer the sugar is that galectins bind to, the stronger their interactions will be. This is due to the fact that Van Der Waals interactions can occur more frequently between sugars and a binding pocket. Apoptosis is regulated by galectins thanks to the control of T cells in the thymus. Galectin -1 and -9 are both secreted by epithelial cells in the thymus.
Research suggests there are a total of three different types of structure with regards to the galectin family. These are dimeric, chimera, or tandem. A dimeric galectin is known as a prototypical galectin, and is also a homodimer. They have two galectin subunits that are identical to one another. Galectins -1, -2, -5, -7, -10, -11, -13, -14, and -15 are considered dimeric in their structure.
Galectins falling under the tandem structure will have a minimum of two carbohydrate recognition domains - CRD - inside one polypeptide. These CRDs are connected by a small peptide domain. Galectins in this category are; -4, -6, -8, -9, and -12.
This leaves one final galectin; -3. This is the only chimera galectin, with one CRD and a long non-lectin domain. This galectin can bridge between many ligands and help create adhesive networks. This is down to the fact is can exist in a monomeric form or associate through its non-lectin domain.