The acceptor substrate specificity of human β4-galactosyltransferase V indicates its potential function in O-glycosylation

In order to assess the function of the different human UDP-Gal:GlcNAc beta 4-galactosyltransferases, the cDNAs of two of them, beta 4-GalT I and beta 4-GalT V, were expressed in the baculovirus/insect cell expression system, The soluble recombinant enzymes produced mere purified from the medium and used to determine their in vitro substrate specificities. The specific activity of the recombinant beta 4-GalT V was more than 15 times lower than that of beta 4-GalT I, using GlcNAc beta-S-pNP as an acceptor. Whereas beta 4-GalT I efficiently acts on all substrates hating a terminal beta-linked GlcNAc, beta 4-GalT V appeared to be far more restricted in acceptor usage. beta 4-GalT V acts with high preference on accepters that contain the GlcNAc beta 1-->6GalNAc structural element, as found in O-linked core 2-, 4- and 6-based glycans, but not on substrates related to N-linked or blood group I-active oligosaccharides. These results suggest that beta 4-GalT V may function in the synthesis of lacNAc units on O-linked chains, particularly in tissues which do not express beta 4-GalT I, such as brain. (C) 1999 Federation of European Biochemical Societies.