title: Transient formation of water-conducting states in membrane transporters.
abstract: Membrane transporters rely on highly coordinated structural transitions between major conformational states for their function, to prevent simultaneous access of the substrate binding site to both sides of the membrane--a mode of operation known as the alternating access model. Although this mechanism successfully accounts for the efficient exchange of the primary substrate across the membrane, accruing evidence on significant water transport and even uncoupled ion transport mediated by transporters has challenged the concept of perfect mechanical coupling and coordination of the gating mechanism in transporters, which might be expected from the alternating access model. Here, we present a large set of extended equilibrium molecular dynamics simulations performed on several classes of membrane transporters in different conformational states, to test the presence of the phenomenon in diverse transporter classes and to investigate the underlying molecular mechanism of water transport through membrane transporters. The simulations reveal spontaneous formation of transient water-conducting (channel-like) states allowing passive water diffusion through the lumen of the transporters. These channel-like states are permeable to water but occluded to substrate, thereby not hindering the uphill transport of the primary substrate, i.e., the alternating access model remains applicable to the substrate. The rise of such water-conducting states during the large-scale structural transitions of the transporter protein is indicative of imperfections in the coordinated closing and opening motions of the cytoplasmic and extracellular gates. We propose that the observed water-conducting states likely represent a universal phenomenon in membrane transporters, which is consistent with their reliance on large-scale motion for function.
authors.affiliation: Center for Biophysics and Computational Biology, Department of Biochemistry, and Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
authors.forename: Jing
authors.initials: J
authors.lastname: Li
authors.forename: Saher A
authors.initials: SA
authors.lastname: Shaikh
authors.forename: Giray
authors.initials: G
authors.lastname: Enkavi
authors.forename: Po-Chao
authors.initials: PC
authors.lastname: Wen
authors.forename: Zhijian
authors.initials: Z
authors.lastname: Huang
authors.forename: Emad
authors.initials: E
authors.lastname: Tajkhorshid
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000737:chemistry; D001426:Bacterial Proteins / Q000737:chemistry; D001665:Binding Sites; D002462:Cell Membrane / Q000737:chemistry; D003593:Cytoplasm / Q000737:chemistry; D004926:Escherichia coli / Q000737:chemistry; D006801:Humans; D007477:Ions; D026901:Membrane Transport Proteins / Q000737:chemistry*; D056004:Molecular Dynamics Simulation; D050480:Plasma Membrane Neurotransmitter Transport Proteins / Q000737:chemistry; D011487:Protein Conformation; D051247:Sodium-Glucose Transport Proteins / Q000737:chemistry; D012984:Software; D014867:Water / Q000737:chemistry*
chemical_list: D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D007477:Ions; D026901:Membrane Transport Proteins; D050480:Plasma Membrane Neurotransmitter Transport Proteins; D051247:Sodium-Glucose Transport Proteins; D014867:Water
keywords: ABC transporters; LeuT-fold transporters; major facilitator superfamily; neurotransmitter transporters