A novel variant form of murine beta-1, 6-N-acetylglucosaminyltransferase forming branches in poly-N-acetyllactosamines.

Abstract

A novel form of murine beta-1,6-N:-acetylglucosaminyltransferase that forms branches in poly-N:-acetyllactosamines (designated as IGnT B) was cloned based on sequence homology to the known IGnT (designated as IGnT A). When expressed as proteins, IGnT B showed higher specific activity than IGnT A. The C-terminal 1/4 of IGnT B was identical to that of IGnT A, while the rest of the predicted sequences showed 63% identity. Genomic analysis indicated that IGnT A and IGnT B were derived by alternative splicing; the unique portion was encoded by exon 1, and the common portion was encoded by exons 2 and 3. IGnT B showed an expression profile closely related to that of IGnT A and was strongly expressed in the liver, kidney and intestine, and moderately in the mammary gland, submaxially gland, embryonic stem cells, and embryonal carcinoma cells. The specificity of IGnT B examined using various substrates was indistinguishable from that of IGnT A, which is classified as the central acting IGnT (cIGnT). Thus, IGnT B acted on Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc, but not on GlcNAcbeta1-3Galbeta1-4Glc. It formed branches in both of the internal galactosyl residues of Galbeta1-4Glc-NAcbeta1-3Galbeta1-4GlcNAcbeta1-++ +3Galbeta1-4Glc, and prolonged incubation resulted in production of the di-branched oligosaccharide. Although addition of sialic acid to the terminal galactosyl residue did not abolish the acceptor activity, alpha2-6 sialylation was a preferred one as compared to alpha2-3 sialylation.