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Proc Natl Acad Sci U S A. 2009 Feb 10;106(6):1778-83. doi: 10.1073/pnas.0809979106. Epub 2009 Jan 28.

The substrate-binding protein imposes directionality on an electrochemical sodium gradient-driven TRAP transporter.

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Department of Biology (Area 10), University of York, P.O. Box 373, York YO10 5YW, United Kingdom.

Erratum in

  • Proc Natl Acad Sci U S A. 2009 May 19;106(20):8398.


Substrate-binding protein-dependent secondary transporters are widespread in prokaryotes and are represented most frequently by members of the tripartite ATP-independent periplasmic (TRAP) transporter family. Here, we report the membrane reconstitution of a TRAP transporter, the sialic acid-specific SiaPQM system from Haemophilus influenzae, and elucidate its mechanism of energy coupling. Uptake of sialic acid via membrane-reconstituted SiaQM depends on the presence of the sialic acid-binding protein, SiaP, and is driven by the electrochemical sodium gradient. The interaction between SiaP and SiaQM is specific as transport is not reconstituted using the orthologous sialic acid-binding protein VC1779. Importantly, the binding protein also confers directionality on the transporter, and reversal of sialic acid transport from import to export is only possible in the presence of an excess of unliganded SiaP.

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