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Modulation of ClC-3 gating and proton/anion exchange by internal and external
protons and the anion selectivity filter.
Rohrbough J, Nguyen HN, Lamb FS
Submitted Externally on 9/10/2018
The Journal of physiology
Volume : Pages
596 : 4091 - 4119
The ClC-3 2Cl- /1H+ exchanger modulates endosome pH and Cl- concentration. We
investigated the relationships between ClC-3-mediated ion transport
(steady-state transport current, ISS ), gating charge (Q) and cytoplasmic
alkalization. ClC-3 transport is functionally unidirectional. ClC-5 and ClC-3
display indistinguishable exchange ratios, but ClC-3 cycling is less
"efficient", as reflected by a large Q/ISS . An M531A mutation predicted to
increase water-wire stability and cytoplasmic proton supply improves efficiency.
Protonation (pH 5.0) of the outer glutamate gate (Gluext ; E224) reduces Q,
inhibits transport, and weakens coupling. Removal of the central tyrosine anion
gate (Y572S) greatly increases uncoupled anion current. Tyrosine -OH removal
(Y572F) alters anion selectivity and impairs coupling. E224 and Y572 act as
anion barriers, and contribute to gating. The Y572 side chain and -OH regulate Q
movement kinetics and voltage dependence. E224 and Y572 interact to create a
"closed" inner gate conformation that maintains coupling during cycling., We
utilized plasma membrane-localized ClC-3 to investigate relationships between
steady-state transport current (ISS ), gating charge (Q) movement, and
cytoplasmic alkalization rate. ClC-3 exhibited lower transport efficiency than
ClC-5, as reflected by a larger Q/ISS ratio, but an indistinguishable Cl- /H+
coupling ratio. External SCN- reduced H+ transport rate and uncoupled anion/H+
exchange by 80-90%. Removal of the external gating glutamate ("Gluext ") (E224A
mutation) reduced Q and abolished H+ transport. We hypothesized that Methionine
531 (M531) impedes "water wire" H+ transfer from the cytoplasm to E224.
Accordingly, an M531A mutation decreased the Q/ISS ratio by 50% and enhanced H+
transport. External protons (pH 5.0) inhibited ISS and markedly reduced Q while
shifting the Q-voltage (V) relationship positively. The Cl- /H+ coupling ratio
at pH 5.0 was significantly increased, consistent with externally protonated
Gluext adopting an outward/open position. Internal "anion gate" removal (Y572S)
dramatically increased ISS and impaired coupling, without slowing H+ transport
rate. Loss of both gates (Y572S/E224A) resulted in a large "open pore"
conductance. Y572F (removing only the phenolic hydroxide) and Y572S shortened Q
duration similarly, resulting in faster Q kinetics at all voltages. These data
reveal a complex relationship between Q and ion transport. Q/ISS must be
assessed together with coupling ratio to properly interpret efficiency. Coupling
and transport rate are influenced by the anion, internal proton supply and
external protons. Y572 regulates H+ coupling as well as anion selectivity, and
interacts directly with E224. Disruption of this "closed gate" conformation by
internal protons may represent a critical step in the ClC-3 transport cycle.
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