Transporters of Nucleotide Sugars, Atp, and Nucleotide Sulfate in the Endoplasmic Reticulum and Golgi Apparatus

Abstract

The lumens of the endoplasmic reticulum and Golgi apparatus are the subcellular sites where glycosylation, sulfation, and phosphorylation of secretory and membrane-bound proteins, proteoglycans, and lipids occur. Nucleotide sugars, nucleotide sulfate, and ATP are substrates for these reactions. ATP is also used as an energy source in the lumen of the endoplasmic reticulum during protein folding and degradation. The above nucleotide derivatives and ATP must first be translocated across the membrane of the endoplasmic reticulum and/or Golgi apparatus before they can serve as substrates in the above lumenal reactions. Translocation of the above solutes is mediated for highly specific transporters, which are antiporters with the corresponding nucleoside monophosphates as shown by biochemical and genetic approaches. Mutants in mammals, yeast, and protozoa showed that a defect in a specific translocator activity results in selective impairments of the above posttranslational modifications, including loss of virulence of pathogenic protozoa. Several of these transporters have been purified and cloned. Experiments with yeast and mammalian cells demonstrate that these transporters play a regulatory role in the above reactions. Future studies will address the structure of the above proteins, how they are targeted to different organelles, 49 0066-4154/98/0701-0049$08.00 Annu. Rev. Biochem. 1998.67:49-69

Characterization of AtNST‐KT1, a novel UDP‐galactose transporter from Arabidopsis thaliana

“…To analyze the substrate affinities in more detail, the apparent kinetic constants of AtNST-KT1 for the transport of UMP and the apparent inhibition constants for UDP-Gal and UDP-Glc were determined. The K M for UMP is 4.2 ± 1.7 lM, while the apparent K i values for the competitive inhibition of UMP transport are 5.1 ± 1.7 lM for UDP-Gal and 12.2 ± 0.4 lM for UDP-Glc which is in the same range shown for other NSTs [5]. The low K i for UDP-Glc indicates that it could bind to the protein without being transported.…”

“…localization of the corresponding protein Most of the NSTs are located in Golgi membranes while some are in addition or exclusively located in ER membranes [5]. To analyze the intracellular localization of AtNST-KT1, the coding region of the cDNA was translationally fused to the GFP cDNA and the construct was introduced into tobacco BY2 cells.…”

“…Therefore, the nucleotide sugars must be transported from the cytosol into the lumen of the ER and Golgi apparatus. While most, but not all of the sugars are transported across the ER membranes attached to dolichol, the transport into the Golgi lumen is mediated by nucleotide sugar transporters (NSTs) [5].…”

“…Both the size of the NST and pPT proteins and the number of membrane spanning regions are similar. The pPTs and NSTs consist of 320-340 amino acids and possess 8-10 transmembrane domains [5,11,15]. Both groups of transporters function as antiporters.…”

Glycoproteins from Insect Cells: Sialylated or Not?

“…Translocation of donor substrates into the lumen of the Golgi apparatus requires a specific CMP-Neu5Ac/CMP antiporter, which is well characterized in mammals (Eckhardt et al, 1996, Hirschberg et al, 1998.…”