KdpFABC reconstituted in E. coli lipid vesicles : substrate dependence of the transport rate

Abstract

KdpFABC complexes were reconstituted in Escherichia coli lipid vesicles, and ion pumping was activated by addition of ATP to the external medium which corresponds to the cytoplasm under physiological conditions. ATP-driven potassium extrusion was studied in the presence of various substrates potentially influencing transport rate. The pump current was detected as a decrease of the membrane potential by the voltage-sensitive dye DiSC3(5). The results indicate that high cytoplasmic K+ concentrations have an inhibitory effect on the KdpFABC complex. The pump current decreased to ∼25% of the maximal value at 140 mM K+ and minimal Mg2+concentrations. This effect could be counteracted with increased Mg2+ concentrations on the cytoplasmic side. This observation may be explained by the Gouy–Chapman effect of two Mg2+ ions probably bound with a K1/2 of 0.8 mM close to the entrance of the access channel to the binding sites. This factor ensures that under physiological conditions the rate-limiting effect of K+ release is significantly reduced. Also both ADP and inorganic phosphate are able to reduce the turnover rate of the pump by reversing the phosphorylation step (Ki of 151 μM) and the dephosphorylation step (Ki of 268 μM), respectively. In the case of the DDM-solubilized KdpFABC complex, activation energy under turnover conditions was previously found to be 55 kJ/mol, and the o-vanadate inhibition constant is shown here to be ∼1 μM, which is in agreement with values reported for other P-type ATPases. In the case of the reconstituted enzyme, however, significant differences were observed that have to be assigned to effects of the lipid bilayer environment. The activation energy was increased by a factor of 2, whereas the inhibition by o-vanadate became reduced in a way that only ∼66% of the enzyme could be inhibited and the inhibition constant was increased to a value of ∼60 μM.