Transporters

The transporters for nucleotide-activated sugars are actually antiporters, which import nucleotide-activated sugars and export the corresponding nucleotide-monophosphates into the cytosol. Most glycosyltransferase reactions yield a nucleotide-diphosphate, meaning that a phosphatase reaction is required to generate nucleotide-monophosphates. Most transporters are specific for a given nucleotide-activated sugar but some transporters enable the import of multiple substrates such as UDP-Gal, UDP-GalNAc and UDP-GlcA. The majority of transporters are embedded in the Golgi membrane but can be localized at the ER membrane in some cases. Data from the last three years have shown that some nucleotide-activated sugars, e.g. GDP-Fuc, are imported in the ER and in the Golgi through different transporters. For example, the inactivation of the Golgi GDP-Fuc transporter leads to a defect of terminal fucosylation in N- and O-glycans, whereas core O-fucosylation, which takes place in the ER, is not affected.

The substrate for glycan sulfation, 3'-phosphoadenosine 5'-phosphosulfate (PAPS), is also imported into the Golgi apparatus through an antiporter, which exports back AMP into the cytosol. A phosphate transporter ensuring the export of the phosphate anion to the cytosol is likely to exist, although such a transporter has not been identified yet.

Figure 21. Cellular distribution of nucleotide-activated sugar transporters. Distinct transporters can mediate the import of a nucleotide-activated sugar in the ER and Golgi apparatus (e.g. GDP-Fuc).