Structure
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).
Most Siglecs, such as CD22 and the CD33-related family, contain ITIMs (Immunoreceptor tyrosine-based inhibitory motifs) in their cytosolic region. These act to down-regulate signalling pathways involving phosphorylation, such as those induced by ITAMs (Immunoreceptor tyrosine-based activation motifs). Some, however, like Siglec-14, contain positive amino acid residues that help dock ITAM-containing adaptor proteins such as DAP12.
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