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(Received for publication, June 10, 1996, and in revised form, July 23, 1996)
From the The effects of changing Glu-779, located in the
fifth transmembrane segment of the Na,K-ATPase
Volume 271, Number 40,
Issue of October 4, 1996
pp. 24610-24616
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.
Subunit Removes Voltage Dependence of Ion Transport
,
,
and
Department of Molecular Genetics,
Biochemistry and Microbiology, College of Medicine, University of
Cincinnati, Cincinnati, Ohio 45267-0524 and the ¶ Bockus Research
Institute, Graduate Hospital,
Philadelphia, Pennsylvania 19146
subunit, on the
phosphorylation characteristics and ion transport properties of the
enzyme were investigated. HeLa cells were transfected with cDNA
coding the E779A substitution in an ouabain-resistant sheep 1
subunit (RD). Steady state phosphorylation stimulated by
Na+ concentrations less than 20 mM or by
imidazole were similar for RD and E779A enzymes, an indication that
phosphorylation and Na+ occlusion were not altered by
this mutation. With E779A enzyme, higher Na+ concentrations
reduced the level of phosphoenzyme and stimulated Na-ATPase activity in
the absence of K+. These effects were a consequence of
Na+ increasing the rate of protein dephosphorylation. In
voltage-clamped HeLa cells expressing E779A enzyme, a prominent
electrogenic Na+-Na+ exchange was observed in
the absence of extracellular K+. Thus, increased Na-ATPase
activity and Na+-dependent dephosphorylation
result from Na+ acting as a K+ congener with
low affinity at extracellular binding sites. These data suggest that
E779A does not directly participate in ion binding but does affect the
connection between extracellular ion binding and intracellular enzyme
dephosphorylation. In cells expressing control RD enzyme, Na,K-pump
current was dependent on membrane potential and extracellular
K+ concentration. However, Na,K-pump current in cells
expressing E779A enzyme was voltage independent at all extracellular
K+ tested. These results indicate that Glu-779 may be
part of the access channel determining the voltage dependence of ion
transport by the Na,K-ATPase.
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