Purification and cDNA cloning of UPDP-GlcNAc:GclNAcβ1-3Galβ1-4Glc(NAc)-R [GlcNAc to Gall]β1,6N-acetyglucosaminyltransferase from rat small intestine: A major carrier of dIGnt activity in rat small intestine

A rat intestinal β1,6N-acetylglucosaminyltransferase (β1-6GnT) responsible for the formation of the β1,6-branched poly-N-acetyllactosamine structure has been purified to apparent homogeneity by successive column chromatographic procedures using an assay wherein pyridylaminated lacto-N-triose II (GlcNAcβ1-3Galβ1-4Glc-PA) was used as an acceptor substrate and the reaction product was GlcNAcβ1-3(GlcNAcβ1-6 Galβ1-4Glc-PA. The purified enzyme catalyzed the conversion of the polylactosamine acceptor GlcNAcβ1-3′LacNAc into GlcNAcβ1-3′(GlcNAcβ1-6′) LacNAc (dIGnT activity), but it could not transfer GlcNAc to LacNAcβ1-3′LacNAc (cIGnT activity). This enzyme could also convert mucin core 1 and core 3 analogs, Galβ1-3GalNAcα1-O-paranitrophenyl (pNP) and GlcNAcβ1-3GalNAcα1-O-pNP, into Galβ1-3(GlcNAcβ1-6) GalNAcα1-O-pNP (C2GnT activity) and GlcNAcβ1-3(GlcNAcβ1-6)GalNAcα1-O-pNP (C4GnT activity), respectively. Based on the partial amino acid sequences of the purified protein, the cDNA encoding this enzyme was cloned. The COS-1 cells transiently transfected with this cDNA had high dI/C2/C4GnT activities in a ratio of 0.34:1.00:0.90, compared with non- or mock-transfected cells. The primary structure shows a significant homology with human and viral mucin-type core 2 β1-6GnTs (C2GnT-Ms), indicating that this enzyme is the rat ortholog of human and viral C2GnT-Ms. This is the first identification and purification of this enzyme as a major carrier of dIGnT activity in the small intestine. This rat ortholog should mostly be responsible for making distal I-branch structures on poly-N-acetyllactosamine sequences in this tissue, as well as making mucin core 2 and core 4 structures, given that it also has high C2/C4GnT activities.