The tumor suppressor EXT-like gene EXTL2 encodes an alpha 1, 4-N-acetylhexosaminyltransferase that transfers N-acetylgalactosamine and N-acetylglucosamine to the common glycosaminoglycan-protein linkage region - The key enzyme for the chain initiation of heparan sulfate

We previously demonstrated a unique alpha-N-acetylgalactosaminyltransferase that transferred N-acetylgalactosamine (GalNAc) to the tetrasaccharide-ser ine, GlcA beta 1-3Gal beta 1-3Gal beta 1-4Xyl beta 1-O-Ser (GlcA represents glucuronic acid), derived from the common glycosaminoglycan-protein linkag e region, through an alpha 1,4-linkage. In this study, we purified the enzy me from the serum-free culture medium of a human sarcoma cell line. Peptide sequence analysis of the purified enzyme revealed 100% identity to the mul tiple exostoses-like gene EXTL2/EXTR1, a member of the hereditary multiple exostoses (EXT) gene family of tumor suppressors. The expression of a solub le recombinant form of the protein produced an active enzyme, which transfe rred alpha-GalNAc from UDP-[H-3]GalNAc to various acceptor substrates inclu ding GlcA beta 1-3Gal beta 1-3Gal beta 1-4Xyl beta 1-O-Ser. Interestingly, the enzyme also catalyzed the transfer of N-acetylglucosamine (GlcNAc) from UDP-[H-3]GlcNAc to GlcA beta 1-3Gal beta 1-O-naphthalenemethanol, which wa s the acceptor substrate for the previously described GlcNAc transferase I involved in the biosynthetic initiation of heparan sulfate. The GlcNAc tran sferase reaction product was sensitive to the action of heparitinase I, est ablishing the identity of the enzyme to be alpha 1,4-GlcNAc transferase. Th ese results altogether indicate that EXTL2/EXTR2 encodes the alpha 1,4-N-ac etylhexosaminyl-transferase that transfers GalNAc/GlcNAc to the tetrasaccha ride representing the common glycosaminoglycan-protein linkage region and t hat is most likely the critical enzyme that determines and initiates the he parin/heparan sulfate synthesis, separating it from the chondroitin sulfate /dermatan sulfate synthesis.