Application of a lectin from the mushroom Polysporus squamosus for the histochemical detection of the NeuAcalpha2,6Galbeta1,4Glc/GlcNAc sequence of N-linked oligosaccharides: a comparison with the Sambucus nigra lectin.

The lectin from the mushroom Polysporus squamosus (PSL) has an extended carbohydrate combining site, which exhibits a high specificity and affinity toward the NeuAc5alpha2,6Galbeta1,4Glc/GlcNAc trisaccharide sequence of asparagine-linked oligosaccharides. Therefore, PSL should be a superior reagent to the lectin from Sambucus nigra (SNA), which does not discriminate between alpha2,6-linked NeuAc5 present either in asparagine- or serine/threonine-linked oligosaccharides. We have prepared a digoxigenin-conjugated PSL and applied it for histochemistry and blotting. We observed a more restricted staining pattern by PSL as compared to SNA in paraffin sections from different rat organs. Pretreatment of sections with N-glycanase F abolished PSL staining indicating that it interacts only with asparagine-linked oligosaccharides. Furthermore, PSL staining was neuraminidase sensitive. In contrast, SNA staining was only partially sensitive to N-glycanase F pretreatment demonstrating that it was in part due to alpha2,6-linked NeuAc5 present in serine/threonine-linked oligosaccharides. The most striking observation in this regard was that PSL, in contrast to SNA, did not stain the mucus of sheep submandibular gland, which is extremely rich in serine/threonine-linked Neu5Acalpha2,6N-acetylgalactosamine. Furthermore, in some tissues neuraminidase pretreatment resulted in increased intensity of SNA staining probably due to binding to exposed terminal N-acetylgalactosamine residues. Collectively, these results indicate that PSL is a useful tool for the histochemical detection of alpha2,6-linked NeuAc5 in asparagine-linked oligosaccharides.