TCB Publications - Abstract
Wei Han, Ricky C. Cheng, Merritt C. Maduke, and Emad Tajkhorshid. Water access points and hydration pathways in CLC H+/Cl− transporters. Proceedings of the National Academy of Sciences, USA, 111:1819-1824, 2014. (PMC: PMC3918786)
has been established, the pathway of H
translocation
remains obscure. Through a highly concerted computational and experimental approach,
we characterize microscopic details essential to understanding H-translocation. An
extended (0.4 
s) equilibrium molecular dynamics simulation of membrane-
embedded, dimeric ClC-ec1, a CLC from Escherichia coli, reveals transient but
frequent hydration of the central hydrophobic region by water molecules from the
intracellular bulk phase via the interface between the two subunits. We characterize a
portal region lined by E202, E203, and A404 as the main gateway for hydration. Supporting
this mechanism, site-specific mutagenesis experiments show that ClC-ec1 ion transport
rates decrease as the size of the portal residue at position 404 is increased. Beyond the
portal, water wires form spontaneously and repeatedly to span the 15-Åhydrophobic
region between the two known H transport sites [E148 (Glu
) and E203
(Glu
)]. Our finding that the formation of these water wires requires the
presence of Cl
explains the previously mystifying fact that Cl occupancy
correlates with the ability to transport protons. To further validate the idea that these water
wires are central to the H transport mechanism, we identified I109 as the residue that
exhibits the greatest conformational coupling to water wire formation and experimentally
tested the effects of mutating this residue. The results, by providing a detailed microscopic
view of the dynamics of water wire formation and confirming the involvement of specific
protein residues, offer a mechanism for the coupled transport of H and Cl ions
in CLC transporters.
