Towards the molecular mechanism of Na(+)/solute symport in prokaryotes.
The Na(+)/solute symporter family (SSF, TC No. 2.A.21) contains more than 40 members of pro- and eukaryotic origin. Besides their sequence similarity, the transporters share the capability to utilize the free energy stored in electrochemical Na(+) gradients for the accumulation of solutes. As part of catabolic pathways most of the transporters are most probably involved in the acquisition of nutrients. Some transporters play a role in osmoadaptation. With a high resolution structure still missing, a combination of genetic, protein chemical and spectroscopic methods has been used to gain new insights into the structure and molecular mechanism of action of the transport proteins. The studies suggest a common 13-helix motif for all members of the SSF according to which the N-terminus is located in the periplasm and the C-terminus is directed into the cytoplasm (except for proteins containing a N- or C-terminal extension). Furthermore, an amino acid substitution analysis of the Na(+)/proline transporter (PutP) of Escherichia coli, a member of the SSF, has identified regions of particular functional importance. For example, amino acids of TM II of PutP proved to be critical for high affinity binding of Na(+) and proline. In addition, it was shown that ligand binding induces widespread conformational alterations in the transport protein. Taken together, the studies substantiate the common idea that Na(+)/solute symport is the result of a series of ligand-induced structural changes.
Universität Osnabrück, Fachbereich Biologie/Chemie, Abteilung Mikrobiologie, D-49069, Osnabrück, Germany. jung_h@biologie.uni-osnabrueck.de
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D000595:Amino Acid Sequence; D026921:Amino Acid Transport Systems, Neutral; D001426:Bacterial Proteins / Q000378:metabolism*; D001665:Binding Sites; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002414:Cations, Monovalent; D003593:Cytoplasm / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism; D029968:Escherichia coli Proteins; D008024:Ligands; D008956:Models, Chemical; D008969:Molecular Sequence Data; D019897:Periplasm / Q000378:metabolism; D011392:Proline / Q000378:metabolism; D011487:Protein Conformation; D012964:Sodium / Q000378:metabolism*; D027981:Symporters
D026921:Amino Acid Transport Systems, Neutral; D001426:Bacterial Proteins; D002352:Carrier Proteins; D002414:Cations, Monovalent; D029968:Escherichia coli Proteins; D008024:Ligands; D027981:Symporters; C060609:PutP protein, E coli; D011392:Proline; D012964:Sodium