KDN-gp, which is the unique glycoprotein of the rainbow trout egg envelope, was shown to have a small amount of N-linked oligosaccharide units in addition to a large number of O-linked glycan units. Structural analysis based on chemical analysis in combination with 400 MHz 1H NMR spectroscopy revealed the presence of fully KDNosylated bi- and triantennary complex-type oligosaccharide chains, mostly fucosylated at the innermost GlcNAc residue and bisected by the GlcNAc residue linked β1→4 to the β-Man residue. The structures thus determined represent the first demonstration of N-linked glycan unit containing the KDN residues in the KDN-containing glycoproteins (see Chart 1). The KDN-gp of the rainbow trout egg envelope is a molecule that is present in the second layer of the vitelline envelope but is exposed to the outer surface around the micropyle through which sperm can get in at fertilization. Like human hematopoietic cell surface glycoproteins such as glycophorin A and leukosialin, KDN-gp, which is now characterized to contain N-linked complex-type glycan chains as minor components, is heavily O-glycosylated with α2→8-linked oligo/polyKDN-containing glycan units attached O-glycosidically to Ser/Thr residues. Although little is known about the functional roles of these glycan chains, KDN-gp appears to form a model for further study on the function of cell surface receptor for sperm in fertilization.
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