Nucleotide Sugar Transporter Genes and Their Functional Analysis

Abstract

Carbohydrate structures on glycoproteins and glycolipids play important roles in various biological processes, such as morphogenesis/organ development, viral and bacterial infections/immune response, and cancer invasion. Nucleotide sugar transporters (NSTs) are crucial components in the synthesis of glycoconjugates (Fig. 1) (Caffaro and Hirschberg 2006). Glycosylation can be performed by various types of glycosyltransferase in the lumens of the endoplasmic reticulum and the Golgi apparatus. All glycosyltransferases require donor sugars activated by the addition of a nucleoside mono- or diphosphate (UDP, GDP, or CMP), that is, nucleotide sugars. Nucleotide sugars are synthesized in the cytosol (or in the nucleus in the case of CMP-sialic acid). The translocation of nucleotide sugars from the cytosol into the lumen compartment is mediated by specific NSTs. NSTs are multiple-membrane-spanning proteins that transport nucleotide sugars in coupling with the antiport of nucleoside monophosphate (NMP), which is produced as the result of a glycosyltransferase reaction and a subsequent luminal nucleoside diphosphatase (NDPase) reaction. Recently, NSTs have been suggested to be possible crucial players in the synthesis of glycoconjugates (Suda et al. 2004; Caffaro and Hirschberg 2006).