Control of O-Glycan Branch Formation

A novel human UDP-GlcNAc:Gal/GlcNAcβ1–3GalNAcα β1,6GlcNAc-transferase, designated C2/4GnT, was identified by BLAST analysis of expressed sequence tags. The sequence of C2/4GnT encoded a putative type II transmembrane protein with significant sequence similarity to human C2GnT and IGnT. Expression of the secreted form of C2/4GnT in insect cells showed that the gene product had UDP- N -acetyl-α- d -glucosamine:acceptor β1,6- N -acetylglucosaminyltransferase (β1,6GlcNAc-transferase) activity. Analysis of substrate specificity revealed that the enzyme catalyzed O -glycan branch formation of the core 2 and core 4 type. NMR analyses of the product formed with core 3- para- nitrophenyl confirmed the product core 4- para- nitrophenyl. The coding region of C2/4GnT was contained in a single exon and located to chromosome 15q21.3. Northern analysis revealed a restricted expression pattern of C2/4GnT mainly in colon, kidney, pancreas, and small intestine. No expression of C2/4GnT was detected in brain, heart, liver, ovary, placenta, spleen, thymus, and peripheral blood leukocytes. The expression of core 2 O -glycans has been correlated with cell differentiation processes and cancer. The results confirm the predicted existence of a β1,6GlcNAc-transferase that functions in both core 2 and core 4 O -glycan branch formation. The redundancy in β1,6GlcNAc-transferases capable of forming core 2 O -glycans is important for understanding the mechanisms leading to specific changes in core 2 branching during cell development and malignant transformation.