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JBC, Vol. 252, Issue 4, 1394-1401, Feb, 1977
D. B. Rhoads and W. Epstein
Energy coupling for three K+ transport systems of Escherichia coli K-12 was
studied by examining effects of selected energy sources and inhibitors in
strains with either a wild type or a defective (Ca2+, Mg2+)-stimulated
ATPase. This approach allows discrimination between transport systems
coupled to the proton motive force from those coupled to the hydrolysis of
a high energy phosphate compound (ATP-driven). The three K+ transport
systems here studied are: (a) the Kdp system, a repressible high affinity
(Km=2 muM) system probably coded for by four linked Kdp genes; (b) the Trka
system, a constitutive system with high rate and modest affinity (Km=1.5
mM) defined by mutations in the single trkA gene; and (c) the TrkF system,
a nonsaturable system with a low rate of uptake (Rhoads, D.B., Waters,
F.B., and Epstein, W. (1976) J. Gen. Physiol. 67, 325-341). Each of these
systems has a different mode of energy coupling: (a) the Kdp system is
ATP-driven and has a periplasmic protein component; (b) the TrkF system is
proton motive force-driven; and (c) the TrkA system is unique among
bacterial transport systems described to date in requiring both the proton
motive force and ATP for activity. We suggest that this dual requirement
represents energy fueling by ATP and regulation by the proton motive force.
Absence of ATP-driven systems in membrane vesicles is usually attributed to
the requirement of such systems for a periplasmic protein. This cannot
explain the failure to demonstrate the TrkA system in vesicles, since this
system does not require a periplasmic protein. Our findings indicate that
membrane vesicles cannot couple energy to ATP-driven transport systems.
Since vesicles can generate a proton motive force, the inability of
vesicles to generate ATP or couple ATP to transport (or both) must be
invoked to explain the absence of TrkA in vesicles. The TrkF system should
function in vesicles, but its very low rate may make it difficult to
identify.
Energy coupling to net K+ transport in Escherichia coli K-12
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