Structural characterization of the oligosaccharide chains of native and crystallized boar seminal plasma spermadhesin PSP-I and PSP-II glycoforms

The PSP-I/PSP-II heterodimer is the major protein of boar seminal plasma. Both subunits are glycoproteins of the spermadhesin family and each contains a single N-glycosylation site. After enzymatic release of the oligosaccharides from isolated PSP-I and PSP-TI, mainly neutral and monosialylated oligosaccharides, and small amounts of disialylated oligosaccharides, were recovered from both proteins. Twenty-two neutral oligosaccharides, 11 monosialylated glycans and three disialylated carbohydrate chains were characterized using mass spectrometric and NMR techniques. PSP-I and PSP-II share the same glycans but differ in their relative molar ratios. Most glycan structures are proximally alpha 1-6-fucosylated, diantennary complex-type bearing nonsialylated or alpha 2-6-sialylated N-acetyllactosamine or di-N-acetyllactosamine antennae. The majority of nonsialylated N-acetyllactosamine antennae bear terminal alpha 1-3-linked Gal residues. In addition, the N-acetylglucosamine residue of nonsialylated N-acetyl and di-N-acetyllactosamine antennae can be modified by an alpha 1-3-linked fucose residue. Structures of higher antennarity, as well as structures 3,6-branched at galactose residues, were found in smaller amounts. In one oligosaccharide, N-acetylneuraminic acid is substituted by N-glycolylneuraminic acid. Mass spectrometric analysis of PSP-I and PSP-II glycoforms isolated from crystallized PSP-I/PSP-II heterodimer showed the coexistence of major PSP-I and PSP-II glycoforms in the hexagonal crystals. Oligosaccharides with the NeuNAc alpha 2-6GalNAc beta 1-4GlcNAc-R motif block adhesive and activation-related events mediated by CD22, suggesting a possible immunoregulatory activity for PSP-I/PSP-II.