Nucleotide Sugar Transporters

Abstract

The cellular loci for the production of secreted and membrane bound glycoproteins and glycolipids are the endoplasmic reticulum (ER) and the Golgi apparatus. Both compartments are sealed vesicular structures that require active in-and outward transport of metabolites that are substrates or products of catalytic reactions taking place at the luminal side. Studies on the secretory pathway carried out in the 1960s, together with the delineation of the steps involved in N-glycosylation in the 1970s, made the need for specific transport systems apparent. Transport was demonstrated in vitro in isolated Golgi vesicles, and genetic proof for the existence of specific nucleotide sugar transporters was obtained when Chinese hamster ovary (CHO) cell mutants with asialo-or asialo-agalacto-surfaces were shown to be inactive in translocating CMP-sialic acid and UDP-galactose, respectively, from the cytoplasm into the Golgi lumen. Further biochemical analysis of the transport process demonstrated that nucleotide sugars are specifically recognized by individual translocators that work in a temperature-dependent, and saturatable manner.Nucleotide sugars can be concentrated in the organellar lumen up to 50-fold. The energy required for this active transport results from exchanging luminal nucleoside monophosphates with cytosolic nucleotide sugars. The nucleoside monophosphates thereby follow their concentration gradients. Attempts to purify nucleotide sugar transporters led to highly enriched and functionally active protein fractions, however, homogeneous purification is a very recent accomplishment and is discussed in this Chapter. Cloning of the first nucleotide sugar transporter genes in 1996 identified a family of highly hydrophobic multimembrane spanning proteins, with molecular masses of 34-42 kDa. Ten transmembrane domains have been demonstrated in the CMP-sialic acid transporter and due to similarities in sequence and hydrophobicity, it seems plausible to assume that the membrane topology is the same for most, if not all, nucleotide sugar transporters.Progress in studying nucleotide sugar transporters over the last few years stimuCarbohydrates in Chemistry and Biology

Loss of srf-3-encoded Nucleotide Sugar Transporter Activity in Caenorhabditis elegans Alters Surface Antigenicity and Prevents Bacterial Adherence

“…In the protozoal parasite Leishmania donovani, which causes visceral leishmaniasis, a mannose-rich lipophosphoglycan (LPG) was shown to be important for virulence; one avirulent mutant, LPG2, has a defect in a GDP-mannose transporter (53). The C. elegans genome contains 16 NST-like proteins, of which only three have been described in detail (54). SQV-7, the null phenotype of which is embryonic lethal, is a transporter with multisubstrate specificity (28,55).…”

Cyclic disulfide analogues of the complement component C3a Synthesis and conformational investigations

“…In contradiction, a recent report has shown that the 21-residue peptide displayed only approximately 100% C3a activity in the ATPrelease assay as well as the ileum contraction assay (16). Previous results from Gerardy-Schahn et al (7) and Ambrosius etal. (8) indicated that flexible linear amphipathic molecules are potent biological agonists (e.g.…”

“…In contrast to Leu75, Leu73 can be replaced by various non-charged amino acids without significant loss of activity (12,13). The replacement of Gly7J by Ala resulted in a two-fold higher activity (7,8). In contrast to former studies, which determined the C-terminal pentapeptide LGLAR (C3a 73-77) as active site, our recent work on very short C3a analogues has shown that the C-terminal tripeptide -LAR is sufficient to trigger ATP release from gp platelets (1 1, 12), if a hydrophobic potentiator (e.g.…”

“…C 3 a -d e~-A r g~~ is totally inactive (9,10). To elucidate the specific role of further amino acids in the C-terminal domain, a series of various C3a analogues with amino acid exchanges and different sequence lengths were synthesized (1 [1][2][3][4][5][6][7][8][9][10][11][12][13]. This work revealed the importance of Leu75 for biological activity.…”

“…Fmoc-Ahx-YRRGRAAALGLAR, 130% C3a activity in ATP-release assay). Fluorescence lifetime experiments gave strong evidence that the hydrophobic part of the peptides interacts with the membrane interior and increases the effective peptide concentration at the cell surface (7,17). Ember et al (18) interpreted similar results with the existence of a "hydrophobic secondary (non-effector) binding site" on the C3a receptor, but up to date there is no significant proof of this hypothesis.…”