Independent and simultaneous translocation of two substrates by a nucleotide sugar transporter.

Nucleotide sugar transporters play an essential role in protein and lipid glycosylation, and mutations can result in developmental phenotypes. We have characterized a transporter of UDP-N-acetylglucosamine and UDP-N-acetylgalactosamine encoded by the Caenorhabditis elegans gene C03H5.2. Surprisingly, translocation of these substrates occurs in an independent and simultaneous manner that is neither a competitive nor a symport transport. Incubations of Golgi apparatus vesicles of Saccharomyces cerevisiae expressing C03H5.2 protein with these nucleotide sugars labeled with (3)H and (14)C in their sugars showed that both substrates enter the lumen to the same extent, whether or not they are incubated alone or in the presence of a 10-fold excess of the other nucleotide sugar. Vesicles containing a deletion mutant of the C03H5.2 protein transport UDP-N-acetylglucosamine at rates comparable with that of wild-type transporter, whereas transport of UDP-N-acetylgalactosamine was decreased by 85-90%, resulting in an asymmetrical loss of substrate transport.