In both prokaryotes and eukaryotes, transport systems of organic solutes can be classified as passive transporters, such as channels and facilitated transporters, and active transporters, which utilize diverse energy-coupling mechanisms. In the past decade, our understanding of the biochemistry and molecular biology of transporters from Escherichia coli has progressed significantly, whereas the analysis of mammalian transporters has initially been limited by the ability to purify membrane proteins. The recent development of methods to detect the activity of recombinant proteins in individual cells, however, has led to the cloning of several novel mammalian transporter cDNAs. One of the most useful expression cloning systems is Xenopus oocytes in conjunction with uptake studies and electrophysiological experiments. Overall, the sequence information and the functional data derived from many transporters has revealed unifying designs, similar energy-coupling mechanisms and common evolutionary origins. Here, I will provide a general survey of the known transport systems in bacteria, yeast, plants, insects and vertebrates and illustrate the different types of transport systems in mammals by discussing transporters recently studied in our laboratory.

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