Cloning and characterization of a close homologue of human UDP-N-acetyl-alpha-D-galactosamine : polypeptide N-acetylgalactosaminyltransferase-T3, designated GalNAc-T6 - Evidence for genetic but not functional redundancy

The UDP-GalAc:polypeptide N-acetylgalactosaminyltransferase, designated Gal NAc-T3, exhibits unique functions, Specific acceptor substrates are used by GalNAc-T3 and not by other GalNAc-transferases. The expression pattern of GalNAc-T3 is restricted, and loss of expression is a characteristic feature of poorly differentiated pancreatic tumors. In the present study, a sixth human UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase, designated GalNAc-T6, with high similarity to GalNAc-T3, was characterized. GalNAc-T6 exhibited high sequence similarity to GalNAc-T3 throughout the coding regio n, in contrast to the limited similarity that exists between homologous gly cosyltransferase genes, which is usually restricted to the putative catalyt ic domain. The genomic organizations of GALNT3 and GALNT6 are identical wit h the coding regions placed in 10 exons, but the genes are localized differ ently at 2q31 and 12q13, respectively. Acceptor substrate specificities of GalNAc-T3 and -T6 were similar and different from other GalNAc-transferases . Northern analysis revealed distinct expression patterns, which were confi rmed by immunocytology using monoclonal antibodies. In contrast to GalNAc-T 3, GalNAc-T6 was expressed in WI38 fibroblast cells, indicating that GalNAc -T6 represents a candidate for synthesis of oncofetal fibronectin, The resu lts demonstrate the existence of genetic redundancy of a polypeptide GalNAc -transferase that does not provide full functional redundancy.