Nucleotide sugar transporters: Structure and function, and physiological regulation

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Several nucleotide sugar transporters including human UDP-GlcNAc transporter, human CMP -sialic acid transporter, human UDP-GlcA/UDP-GalNAc/UDP-GlcNAc multi-specific transporter (hUGTrel7), Drosophila multi-specific UDP-sugar transporter (UST74C) were cloned and characterized. Substrate specificity of these transporters were convincingly defined by their functional expression in Saccharomyces cerevisiae Golgi membranes. HUGTrel7 is localized to the ER membranes and may be involved in glucuronidation of xenobiotics and the biosynthesis of proteoglycans. UST74C is the product of Drosophila frc gene whose defect results in phenotypes resembling wingless and Notch. Analysis of the phenotypes of frc mutants suggested that change s in nucleotide-sugar levels could differently affect these signaling pathways.Molecular chimeras between UDP-Gal transporter and CMP-sialic acid transporter were constructed and analyzed in order to identify submolecular regions responsible for the determination of … More substrate specificity. We found that chimeras that contain transmembrane helices 2,3 and 7 derived from CMP-sialic acid transporter in the UDP-Gal transporter background could transport both UDP-Gal and CMP-sialic acid. This indicates that the regions which are critical for determining the substrate specificity of these two transporters resided in different submolecular sites in the two transporters, and that these different determinants could be present within one protein without interfering each other's function.The amount of mRNA for UDP-Gal transporter was significantly increased in colon cancer tissues compared with nonmalignant mucosa tissues. The increase was more prominent in patients with advanced colorectal cancer of Duke's stages C and D. Transfection of UDP-Gal transporter cDNA to cultured colon cancer cells led to increased expression of Thomsen-Freidenreich antigen and of sialyl Lewis A and sialyl Lewis X determinants in transfected cells, which resulted in markedly enhanced cell adhesion to vascular E-selectin. These findings suggest that nucleotide sugar transporters can act as one of the regulatory elements of cellular behavior through their effects on the structure of cell surface glycoconjugates. Less