SIGLEC
Siglecs (Sialic acid-binding immunoglobulin-type lectins) are cell surface proteins that bind sialic acid. They are found primarily on the surface of immune cells and are a subset of the I-type lectins. There are 14 different mammalian Siglecs, providing an array of different functions based on cell surface receptor-ligand interactions.
The first described candidate Siglec was Sialoadhesin (Siglec-1/CD169) a lectin-like adhesion protein on macrophages. Parallel studies by Ajit Varki and colleagues on the previously cloned CD22 (a B cell surface protein involved in adhesion and activation) showed direct evidence for sialic acid recognition. The subsequent cloning of Sialoadhesin by Crocker revealed homology to CD22 (Siglec-2), CD33 (Siglec-3) and myelin-associated glycoprotein (MAG/Siglec-4), leading to the proposal for a family of "Sialoadhesins". Varki then suggested the term Siglec as a better alternative and as a subset of I-type (Ig-type) lectins. This nomenclature was agreed upon and has been adopted by almost all investigators working on these molecules (by convention, Siglecs are always capitalised.) Several additional Siglecs (Siglecs 5–12) have been identified in humans that are highly similar in structure to CD33 and so are collectively referred to as "CD33-related Siglecs". Further Siglecs have been identified including Siglec-14 and Siglec-15. Some Siglecs are absolutely conserved between mice and humans including Sialoadhesin, CD22, MAG and Siglec-15. Others such as Siglec-8 and Siglec-9 have homologues in mice and rats (Siglec-F and Siglec-E respectively in both). Humans have a higher number of Siglecs than mice and so the numbering system was based on the human proteins.
Siglecs are Type I transmembrane proteins where the NH3+-terminus is in the extracellular space and the COO−-terminus is cytosolic. Each Siglec contains an N-terminal V-type immunoglobulin domain (Ig domain) which acts as the binding receptor for sialic acid. These lectins are placed into the group of I-type lectins because the lectin domain is an immunoglobulin fold. All Siglecs are extended from the cell surface by C2-type Ig domains which have no binding activity. Siglecs differ in the number of these C2-type domains. As these proteins contain Ig domains, they are members of the Immunoglobulin superfamily (IgSF).
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