Journal
JOURNAL OF BIOLOGICAL CHEMISTRY
Volume 291, Issue 38, Pages 19786-19799Publisher
AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC
DOI: 10.1074/jbc.M116.731455
Keywords
conformational change; membrane protein; molecular dynamics; monoamine transporter; neurotransmitter transport
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Funding
- Texas Advanced Computing Center at the University of Texas at Austin [TG-MCB120008]
- Office of Science of the U.S. Department of Energy [DE-AC05-00OR22725, PSCA14026P]
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Ions play key mechanistic roles in the gating dynamics of neurotransmitter:sodium symporters (NSSs). In recent microsecond scale molecular dynamics simulations of a complete model of the dopamine transporter, a NSS protein, we observed a partitioning of K+ ions from the intracellular side toward the unoccupied Na2 site of dopamine transporter following the release of the Na2-bound Na+. Here we evaluate with computational simulations and experimental measurements of ion affinities under corresponding conditions, the consequences of K+ binding in the Na2 site of LeuT, a bacterial homolog of NSS, when both Na+ ions and substrate have left, and the transporter prepares for a new cycle. We compare the results with the consequences of binding Na+ in the same apo system. Analysis of >50-s atomistic molecular dynamics and enhanced sampling trajectories of constructs with Glu(290), either charged or neutral, point to the Glu(290) protonation state as a main determinant in the structural reconfiguration of the extracellular vestibule of LeuT in which a water gate opens through coordinated motions of residues Leu(25), Tyr(108), and Phe(253). The resulting water channel enables the binding/dissociation of the Na+ and K+ ions that are prevalent, respectively, in the extracellular and intracellular environments.
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