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: 42.35). 12/2010; 468(7326):988-91. DOI: 10.1038/nature09580
Source: PubMed

ABSTRACT 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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Available from: John M. Rosenberg, Aug 01, 2015
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    • "Based on these crystal structures for the LeuT fold a number of modeling studies were performed (Forrest et al. 2008; Watanabe et al. 2010; Zhao et al. 2010; Shi and Weinstein 2010; Adelman et al. 2011, Koldsø et al. 2011), and the transport mechanism was discussed in general (Abramson and Wright 2009; Forrest et al. 2011, Rudnick 2011). However, as the crystal structures are static snapshots of a dynamic process, they do not imply a unique mechanistic interpretation. "
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    • "In the light of the open inward-facing state observed for BetP-G153D, hereafter, we will refer to the betaine-bound structure (PDB entry 2WIT) as occluded inward-facing conformation. Superimposition of both structures (Figure 4C) supports a rigid-body movement of the bundle domain (first two helices of each repeat) relative to the scaffold of adjacent helices (helices 3 and 4 of each repeat) that is comparable to the conformational changes described very recently for vSGLT (Supplementary Table SI) (Watanabe et al, 2010). The intracellular halves of TM3 and TM8 are displaced by 61 and 51, respectively (Figure 4C). "
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