Conformational transitions of the sodium-dependent sugar transporter, vSGLT

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 115; no. 12; pp. E2742 - E2751
• Main Authors: Paz, Aviv, Claxton, Derek P., Kumar, Jay Prakash, Kazmier, Kelli, Bisignano, Paola, Sharma, Shruti, Nolte, Shannon A., Liwag, Terrin M., Nayak, Vinod, Wright, Ernest M., Grabe, Michael, Mchaourab, Hassane S., Abramson, Jeff
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 20-03-2018
• Series: PNAS Plus
• Subjects: Bacteria; Bacterial Proteins - chemistry; Bacterial Proteins - metabolism; Binding; Biological Sciences; Biological Transport, Active; Conformation; Cysteine - genetics; Electrochemical potential; Electrochemistry; Electron Spin Resonance Spectroscopy - methods; Electrons; Galactose - metabolism; Gating; Inverted repeat; Ions; Ligands; Membrane Lipids - chemistry; Membrane Lipids - metabolism; Membrane potential; Micelles; Models, Molecular; Molecular dynamics; Molecular Dynamics Simulation; Molecules; Mutation; PNAS Plus; Protein Conformation; Sodium; Sodium - metabolism; Sodium-Glucose Transport Proteins - chemistry; Sodium-Glucose Transport Proteins - metabolism; Spectroscopy; Sugar; Vibrio parahaemolyticus - chemistry; Waterborne diseases; DEER; conformation; transport; sugar; vSGLT
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1718451115

Sodium-dependent transporters couple the flow of Na⁺ ions down their electrochemical potential gradient to the uphill transport of various ligands. Many of these transporters share a common core structure composed of a five-helix inverted repeat and deliver their cargo utilizing an alternating-access mechanism. A detailed characterization of inward-facing conformations of the Na⁺-dependent sugar transporter from Vibrio parahaemolyticus (vSGLT) has previously been reported, but structural details on additional conformations and on how Na⁺ and ligand influence the equilibrium between other states remains unknown. Here, double electron–electron resonance spectroscopy, structural modeling, and molecular dynamics are utilized to deduce ligand-dependent equilibria shifts of vSGLT in micelles. In the absence and presence of saturating amounts of Na⁺, vSGLT favors an inward-facing conformation. Upon binding both Na⁺ and sugar, the equilibrium shifts toward either an outward-facing or occluded conformation. While Na⁺ alone does not stabilize the outward-facing state, gating charge calculations together with a kinetic model of transport suggest that the resting negative membrane potential of the cell, absent in detergent-solubilized samples, may stabilize vSGLT in an outward-open conformation where it is poised for binding external sugars. In total, these findings provide insights into ligand-induced conformational selection and delineate the transport cycle of vSGLT.