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

Abstract

In order to assess the function of the different human UDP-Gal:GlcNAc β4-galactosyltransferases, the cDNAs of two of them, β4-GalT I and β4-GalT V, were expressed in the baculovirus/insect cell expression system. The soluble recombinant enzymes produced were purified from the medium and used to determine their in vitro substrate specificities. The specific activity of the recombinant β4-GalT V was more than 15 times lower than that of β4-GalT I, using GlcNAcβ-S- pNP as an acceptor. Whereas β4-GalT I efficiently acts on all substrates having a terminal β-linked GlcNAc, β4-GalT V appeared to be far more restricted in acceptor usage. β4-GalT V acts with high preference on acceptors that contain the GlcNAcβ1→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 β4-GalT V may function in the synthesis of lacNAc units on O-linked chains, particularly in tissues which do not express β4-GalT I, such as brain.