ABSTRACT
The siglecs (sialic acid-binding Ig-type lectins) belong to the Ig superfamily and form the main subpopulation of the Ig-type (I-type) lectins. They are type 1 membrane proteins, characterized by an amino-terminal V-set Ig domain, containing the sialic acid-binding site, followed by various numbers of C2-set Ig domains. Siglecs can be further subdivided into two subsets, the evolutionary conserved, but distantly related (~25%–30% sequence identity) group containing sialoadhesin (Sn, CD169, Siglec-1), CD22 (Siglec-2), and myelin-associated glycoprotein (MAG, Siglec-4) (Figure 17.1A), and the rapidly evolving group of CD33-related siglecs that share high sequence similarity (50%–99% sequence identity) (Figure 17.1B). Sn, CD22, and MAG have been found in all mammals so far examined and MAG additionally in birds [1] and fi sh [2]. In contrast, the repertoire of CD33-related siglecs varies considerably between species, with nine known in humans and only fi ve in mice (Figure 17.1).
