Transport, sialic acid

Sialin was first identified as the product of the gene defective in sialidosis, a lysosomal storage disorder. The transporter mediates the movement of sialic acid out of lysosomes by coupling to the proton electrochemical gradient across the lysosomal membrane. Unlike the vesicular neurotransmitter transporters which are antiporters, sialin is a sympoiter with sialic acid and protons both moving out of the lysosome. 

Misaki, R., Fujiyama, K. and Seki, T. (2006) Expression of human CMP-V-acetylneuraminic acid synthetase and CMP-sialic acid transporter in tobacco suspension-cultured cell. Biochemical and Biophysical Research Communications, 339 (4), 1184—1189. 

All the nearest-neighbor interactions between sialic acid or Neu5Ac2en and the protein are with totally conserved amino acids. Thus an inhibitor designed to bind only to the conserved active-site residues of neuraminidase would inhibit neuraminidase activity across all strains of influenza. This would enable the development of an antiviral drug that would affect the spread of viral replication potentially in three ways, i.e., transport through the protective mucosal layer, desialyation of freshly synthesized viral glycoproteins, and elution of progeny virions from infected cells. 

Three different rare genetic metabolic defects in sialic acid metabolism are known, as indicated in Fig. 4.3.2 [3, 21] (1) free sialic acid storage disease (SASD Online Mendelian Inheritance in Man, OMIM 604369, 269920), a lysosomal membrane transporter defect (2) sialuria (OMIM 269921), a feedback inhibition defect in sialic acid biosynthesis (3) sialidosis (OMIM 256550), a breakdown defect of sialyloli-gosaccharides caused by a defect of lysosomal sialidase. In all these genetic defects, an increased amount of sialic acid can be found in tissues and or body fluids, either bound to OGSs as in (3), or in its free state as in (1) and (2). 

All clinical forms of the disease are due to a defect in the lysosomal membrane transporter for sialic acid necessary for the export of sialic acid out of the lysosome [11]. The gene coding for this transporter, SLC17A5, contains 11 exons and encodes a 495-amino-acid transmembrane protein, sialin [20]. 

Mancini GM, Beerens CE, Aula PP, Verheijen FW (1991) Sialic acid storage diseases. A multiple lysosomal transport defect for acidic monosaccharides. J Clin Invest 87 1329-1335... 

Verheijen FW, Verbeek E, Aula N, Beerens CE, Havelaar AC, Joosse M, Peltonen L, Aula P, Galjaard H, van der Spek PJ, Mancini GM (1999) A new gene, encoding an anion transporter, is mutated in sialic acid storage diseases. Nat Genet 23 462-465... 

Verheijen FW, Mancini GM (2003) Lysosomal sialic acid transporter sialin (SLC17A5) sialic acid storage disease (SASD). In Broer S, Wagner CA (eds) Membrane Transporter Diseases. Kluwer Academic/Plenum, New York, pp 233-239... 

This intracellular concentration of UDP-GlcNAc has also been shown to be important in another respect. An elevated UDP-GlcNAc appears to cause a decrease in sialylation, which may be explained metabolically by the inhibition of CMP-sialic acid transport (Pels Rijcken et al., 1995). This may be the mechanism for the observed decrease in sialylation that has been shown to accompany high levels of ammonia in culture (Yang and Butler, 2000). 

Nucleotide sugar donors, such as UDP-GalNAc, UDP-GlcNAc, GDP-fucose, and CMP-sialic acid, serve as substrates for the glycosyltransferases and are the source of the sugars that are added to substrate proteins (Table 1). Nucleotide sugar donors are synthesized in the cytoplasm and imported into the secretory pathway by membrane-resident transporters (1). 

N-acetylagalactosamine 1-phosphtransferase lysosomal hydrolases Mucolipin 1, Ca++/cation ion channel Sialic acid transporter... 

However, the augmentation of intracellular sialic acid pools by the introduction of ManNAc was found to be ineffective in increasing the overall level of sialylation of human tissue inhibitor of metalloproteinases 1 (TIMP-1) produced by CHO cells and mouse NSO cells [3]. Therefore, the overexpression of nucleotide sugar transporters, particularly the CMP-sialic acid transporter (CMP-SAT), was investigated as a means of improving the sialylation process in CHO cells. Increasing cellular CMP-SAT levels by recombinant means results in increased transport of CMP-sialic acid into the Golgi, and subsequent increase in the CMP-sialic acid intra-lumenal pool affords increased sialylation of the proteins expressed [4]. 

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