Immunohistochemical demonstration od α,4-N-acetylglucosaminyltransferase that forms GlcNAcα1,4Galβ residues in human gastrointestinal mucosa

alpha ,4-N-acetylglucosaminyltransferase (alpha 4GnT) is a glycosyltransferase that mediates transfer of GlcNAc to beta Gal residues with alpha1,4-linkage, forming GlcNAc alpha1 --> 4Gal beta -->R structures. In normal human tissues, glycoproteins having GlcNAc alpha1 --> 4Gal beta -->R structures at non-reducing terminals are exclusively limited to the mucins secreted from glandular mucous cells of gastric mucosa, Brunner's gland of duodenum, and accessory gland of pancreaticobiliary tract. Recently, we have isolated a cDNA encoding human alpha 4GnT by expression cloning. Although alpha 4GnT plays a key role in producing this unique glycan in vitro, the actual localization of alpha 4GnT was not determined. In this study we examined the localization of alpha 4GnT in various human tissues, including gastrointestinal mucosa, using a newly developed antibody against human alpha 4GnT. The specificity of the antibody was confirmed by analyses of human gastric adenocarcinoma AGS cells transfected by alpha 4GnT cDNA. Expression of alpha 4GnT was largely associated with the Golgi region of mucous cells that produce the mucous glycoproteins having GlcNAc alpha1 --> 4Gal beta -->R, such as the glandular mucous cells of stomach and Brunner's gland. An immunoprecipitation experiment disclosed that two distinct mucin proteins, MUC5AC and MUC6 present in gastric mucin, carried the GlcNA alpha1 --> 4Gal beta -->R structures. These results indicate that alpha 4GnT is critical to form the mucous glycoproteins having GlcNAc alpha1 --> 4Gal beta -->R on MUC6 and MUC5AC in vivo.