Topology of glycosyltransferases

Glycosyltransferases show a great structural diversity. They can be as small as 200 amino acids or as large as 1500 amino acids long. They can be soluble or membrane-bound. As membrane proteins, glycosyltransferases can have a single transmembrane domain or be completely embedded in membranes through a dozen of transmembrane domains. In eukaryotes, most glycosyltransferases have a type-II transmembrane topology, with a short N-terminal cytosolic domain, a single transmembrane domain and a large luminal catalytic domain. The exceptions to the rule are ER-localized glycosyltransferases utilizing Dol-P based donor substrates, such as the enzymes involved in the assembly of lipid-linked oligosaccharides. The short cytosolic and single transmembrane domains include signal motifs for the proper localization of glycosyltransferases. Recent proteomic studies have revealed that several glycosyltransferases are phosphorylated either at their cytosolic or luminal domains. This discovery suggests that glycosyltransferase activities could be regulated by phosphorylation like signaling proteins.

Figure 25. Domain organization of typical Golgi glycosyltransferase (type-II tranmembrane topology).

Only few glycosyltransferase structures have been elucidated by crystallography to date. The three-dimensional structures determined show two major structural folds, which are often called GT-A and GT-B. Both folds include two Rossmann domains, which are often found in nucleotide-binding proteins. The typical Rossmann fold represents a series of three parallel β-strands intervened by two α-helices. For example, the β1-2 N-acetylglucosaminyltransferase MGAT1 and the β1-4 galactosyltransferase B4GALT1 have a GT-A fold topology while fucosyltransferases have a GT-B fold. Glycosyltransferases that utilize polyprenol-P or Dol-P linked substrates do not follow the GT-A or GT-B fold organization.