Studying Ions in Depth

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Science  17 Aug 2007:
Vol. 317, Issue 5840, pp. 873
DOI: 10.1126/science.317.5840.873c

Detailed understanding of how particular proteins function in human cells can provide the foundation for pathophysiology-based therapies, but it rarely is feasible to study these proteins directly. Instead, bacterial substitutes are usually more tractable, and the application of homology modeling and site-specific mutagenesis of mammalian proteins can yield useful insights. Forrest et al. offer a rigorous example of this approach, starting with a previously published structure of a bacterial amino acid transporter, LeuT, which is representative of transporters that couple the movement of small molecules, such as leucine and serotonin, to the transmembrane Na+ gradient. From an analysis of a structure-based sequence alignment of LeuT with the serotonin transporter (SERT), they find that the carboxylate of a buried glutamate in LeuT, in which leucine transport is Cl-independent, occupies the same location as a chloride ion (coordinated by a serine) in SERT, which exhibits Cl-stimulated serotonin transport. Changing the serine to a glutamate or aspartate had no effect on the basal transport activity of SERT but fully abrogated the stimulation by Cl, and further mutagenesis of other Cl-coordinating residues in other amino acid transporters confirmed the predicted effects on activity. Other modulators of leucine transport by LeuT include the tricyclic antidepressants, as shown by Singh et al. (see also Zhou et al., Science Express, 9 August 2007) — GJC

Proc. Natl. Acad. Sci. U.S.A. 104, 12761 (2007); Nature 448, 10.1038/nature06038 (2007).

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