M. Handford, C. Rodriguez-Furlán and A. Orellana. Nucleotide-sugar transporters: structure, function and roles in vivo. Braz J Med Biol Res 2006; 39: 1149-1158.  

Figure 1. Model for the topology of glycosylation reactions in the Golgi/endoplasmic reticulum (ER) apparatus. Nucleotide-sugars are imported into the lumen from the cytosol via specific transporter proteins. Glycosyltransferase enzymes polymerize the addition of the sugar portion to glycoconjugates. The resulting NDP is cleaved by a luminal NDPase forming inorganic phosphate (Pi) and NMP, the latter of which exits in antiport with the incoming nucleotide-sugar. The precise exit mechanism of inorganic phosphate is unknown.