Mechanism of the Golgi membrane GDP-fucose antiporter. GDP-fucose is synthesized in the cytosol from either GDP-mannose (probably the major pathway) or from free fucose. The nucleotide sugar is then transported via the GDP-fucose transporter into the lumen of the Golgi apparatus, where specific fucosyltransferases add fucose to a variety of glycoproteins and glycolipids. GDP, the other reaction product, is converted by a lumenal nucleoside diphosphatase to GMP and inorganic phosphate. The former is exported to the cytoplasm through an antiport mechanism that is coupled with GDP-fucose transport, whereas the latter is postulated to leave the Golgi lumen via a specific channel, GOLAC (23). Fucosyl-T, fucosyltransferases; Pi, inorganic phosphate.