The mechanism of sodium and substrate release from the binding pocket of vSGLT

Department of Physiology, University of California, Los Angeles, Los Angeles, California 90095-1759, USA.
Nature (Impact Factor: 41.46). 12/2010; 468(7326):988-91. DOI: 10.1038/nature09580
Source: PubMed


Membrane co-transport proteins that use a five-helix inverted repeat motif have recently emerged as one of the largest structural classes of secondary active transporters. However, despite many structural advances there is no clear evidence of how ion and substrate transport are coupled. Here we report a comprehensive study of the sodium/galactose transporter from Vibrio parahaemolyticus (vSGLT), consisting of molecular dynamics simulations, biochemical characterization and a new crystal structure of the inward-open conformation at a resolution of 2.7 Å. Our data show that sodium exit causes a reorientation of transmembrane helix 1 that opens an inner gate required for substrate exit, and also triggers minor rigid-body movements in two sets of transmembrane helical bundles. This cascade of events, initiated by sodium release, ensures proper timing of ion and substrate release. Once set in motion, these molecular changes weaken substrate binding to the transporter and allow galactose readily to enter the intracellular space. Additionally, we identify an allosteric pathway between the sodium-binding sites, the unwound portion of transmembrane helix 1 and the substrate-binding site that is essential in the coupling of co-transport.

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    • "Modeling Homology modeling was performed with the Modeler 9.10 software (Sali and Blundell, 1993) using the previously described alignment to vSGLT (Olkhova et al., 2011) and vSGLT crystal structures with PDB identifiers 3DH4 (Faham et al., 2008) and 2XQ2 (Watanabe et al., 2010). Secondary structure was restrained to helical structure for all residues in transmembrane domains of vSGLT that were assigned as a helical by DSSP (Kabsch and Sander, 1983). "
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