Substrate recognition by UDP-galactose and CMP-sialic acid transporters. Different sets of transmembrane helices are utilized for the specific recognition of UDP-galactose and CMP-sialic acid.
Human UDP-galactose transporter (hUGT1) and CMP-sialic acid transporter (hCST) are related Golgi membrane proteins with 10 transmembrane helices. We have constructed chimeras between these proteins in order to identify submolecular regions responsible for the determination of substrate specificity. To assess the UGT and CST activities, chimeric cDNAs were transiently expressed in either UGT-deficient mutant Lec8 cells or CST-deficient mutant Lec2 cells, and the binding of plant lectins, GS-II or PNA, respectively, to these cells was examined. During the course of analysis of various chimeric transporters, we found that chimeras whose submolecular regions contained helices 1, 8, 9, and 10, and helices 2, 3, and 7 derived from hUGT1 and hCST sequences, respectively, exhibited both UGT and CST activities. The dual substrate specificity for UDP-galactose and CMP-sialic acid of one such representative chimera was directly confirmed by in vitro measurement of the nucleotide sugar transport activity using a heterologous expression system in the yeast Saccharomyces cerevisiae. These findings indicated that the regions which are critical for determining the substrate specificity of UGT and CST resided in different submolecular sites in the two transporters, and that these different determinants could be present within one protein without interfering with each other's function.
Department of Physiological Chemistry, The Tokyo Metropolitan Institute of Medical Science, 3-18-22 Honkomagome, Bunkyo-ku, Tokyo 113-8613, Japan.
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Aoki
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Ishida
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Kawakita
D000595:Amino Acid Sequence; D000818:Animals; D016466:CHO Cells; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism*; D002460:Cell Line; D006224:Cricetinae; D003569:Cytidine Monophosphate N-Acetylneuraminic Acid / Q000378:metabolism; D018076:DNA, Complementary; D006056:Golgi Apparatus / Q000378:metabolism*; D006801:Humans; D007425:Intracellular Membranes / Q000378:metabolism; D007700:Kinetics; D037102:Lectins / Q000378:metabolism; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism*; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins / Q000737:chemistry* / Q000378:metabolism*; D033704:Nucleotide Transport Proteins; D017433:Protein Structure, Secondary; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity; D014162:Transfection; D014531:Uridine Diphosphate Galactose / Q000378:metabolism; D000099237:UDP-Galactose Translocators
D002352:Carrier Proteins; D003569:Cytidine Monophosphate N-Acetylneuraminic Acid; D018076:DNA, Complementary; D037102:Lectins; D008565:Membrane Proteins; D009004:Monosaccharide Transport Proteins; D033704:Nucleotide Transport Proteins; D011993:Recombinant Fusion Proteins; D014531:Uridine Diphosphate Galactose; D000099237:UDP-Galactose Translocators; C491081:SLC35A1 protein, human