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1 Cell Biology Program, Hospital for Sick Children, Toronto M5G 1X8, and 2 Department of Surgery, Toronto Hospital and University of Toronto, Toronto, Ontario M5G 1L7; and 3 Department of Physiology, McGill University, Montreal, Quebec, Canada H3G 1Y6
Na+/H+ exchanger (NHE)
activity is exquisitely dependent on the intra- and extracellular
concentrations of Na+ and H+. In addition,
Cl
ions have been suggested to modulate NHE activity, but
little is known about the underlying mechanism, and the
Cl
sensitivity of the individual isoforms has not been
established. To explore their Cl
sensitivity, types 1, 2, and 3 Na+/H+ exchangers (NHE1, NHE2, and NHE3)
were heterologously expressed in antiport-deficient cells. Bilateral
replacement of Cl
with nitrate or thiocyanate inhibited
the activity of all isoforms. Cl
depletion did not affect
cell volume or the cellular ATP content, which could have indirectly
altered NHE activity. The number of plasmalemmal exchangers was
unaffected by Cl
removal, implying that inhibition was
due to a decrease in the intrinsic activity of individual exchangers.
Analysis of truncated mutants of NHE1 revealed that the anion
sensitivity resides, at least in part, in the COOH-terminal domain of
the exchanger. Moreover, readdition of Cl
into the
extracellular medium failed to restore normal transport, suggesting
that intracellular Cl
is critical for activity. Thus
interaction of intracellular Cl
with the COOH terminus of
NHE1 or with an associated protein is essential for optimal activity.
antiport; type 1 Na+/H+ exchanger; anion dependence; osmotic activation; volume regulation
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