β3-Galactosyltransferase-I, -II, and -III

Abstract

In higher eukaryotes, galactose is commonly found in all classes of glycoconjugates, where it is bound as either α- or β-anomer through 1,3- or 1,4-linkage to various carbohydrate acceptor substrates. Families of galactosyltransferases are defined according to the type of linkage catalyzed. Purification studies have suggested the existence of several enzymes in each galactosyltransferase family, assumptions which have been confirmed by the recent cloning of genes encoding galactosyltransferases. However, the number of galactosyltransferase genes isolated has far surpassed these early predictions. The characterization of the members of each galactosyltransferase family has revealed differences in the patterns of tissue expression and in acceptor substrate specificity, although a certain degree of redundancy prevails between galactosyltransferases from a given family. For example, four β3-galactosyltransferase (β3GalT) genes have been described that direct the expression of enzymes linking Galβ1,3 to GlcNAc (Hennet et al. 1998; Kolbinger et al. 1998; Amado et al. 1998; Isshiki et al. 1999; Zhou et al. 1999a). A comparison between β3GalT proteins unraveled several conserved domains not found in other galactosyltransferases. Surprisingly, a β3-N-acetylglucosaminyltransferase enzyme as well as proteins homologous to the Drosophila signaling proteins Brainiac and Fringe were also identified among the β3GalT-related proteins. β3GalTs participate in the shaping of several oligosaccharide structures in O-glycans, N-glycans and glycolipids. This review summarizes the properties of three β3GalT enzymes that direct the formation of type-1 chains, the support of Le^a and Le^b antigens.