Human low-molecular-weight salivary mucin expresses the sialyl Lewis(x) determinant and has L-selectin ligand activity

摘要

Previously we showed that the low-molecular-weight mucin (MG2, encoded by MUC7), a major component of human submandibular/sublingual saliva, is a bacterial receptor that coats the tooth surface. Here we tested the hypothesis that the structure of its carbohydrate residues contains important information about its function. Purified MG2 (M-r 120 000) was digested with trypsin, and the resulting M-r 90 000 fragment, which carried primarily O-linked oligosaccharides, was subjected to reductive beta-elimination. The released oligosaccharides were characterized by using nuclear magnetic resonance spectroscopy and mass spectrometry. Of the 41 different structures we detected, the most prominent included NeuAc alpha 2-->3Gal beta 1-->3GalNAc-ol (sialyl-T antigen), Gal beta 1-->4(Fuc alpha 1-->3)GlcNAc beta 1-->6(Gal beta 1-->3)GalNAc-ol [type 2 core with Lewis(x) (Le(x)) determinant], and NeuAc alpha 2-->3Gal beta 1-->4(Fuc alpha 1-->3)GlcNAc beta 1-->6(Gal beta 1-->3) GalNAc-ol [type 2 core with sialyl Le(x) (sLe(x)) determinant]. We also detected di-, tri-, and pentasaccharides with one sulfate group. Le(x), sLe(x), and related sulfated structures are ligands for selectins, adhesion molecules that mediate leukocyte trafficking. Therefore, we investigated whether MG2 was a selectin ligand. In an enzyme-linked immunosorbent assay, L-selectin chimeras interacted with immobilized MG2 in a Ca2+-dependent manner. L-Selectin chimeras also bound to MG2 immobilized on nitrocellulose. Together, these results suggest that the saccharides that MG2 carries could specify some of its important functions, which may include mediating leukocyte interactions in the oral cavity. [References: 73]