Evidence for Na+ influx via the NtpJ protein of the KtrII K+ uptake systemin Enterococcus hirae

The ntpJ gene, a cistron located at the tail end of the vacuolar-type Na+ - ATPase (ntp) operon of Enterococcus hirae, encodes a transporter of the Ktr II K+ uptake system. We found that K+ accumulation in the ntpJ-disrupted mu tant JEM2 was markedly enhanced by addition of valinomycin at pH 10, Studie s of the membrane potential (Delta psi; inside negative) by 3,3'-dihexyloxa carbocyanine iodide fluorescence revealed that the Delta psi was hyperpolar ized at pH 10 in JEM2; the Delta psi values of the parent strain ATCC 9790 and JEM2, estimated by determining the equilibrium distribution of K+ or Rb + in the presence of valinomycin, were -118 and -160 mV, respectively. Delt a psi generation at pH 10 was accomplished by an electrogenic Na+ efflux vi a the Na+-ATPase, whose levels in the two strains were quite similar. Na+ u ptake driven by an artificially imposed Delta psi (inside negative) was mis sing in JEM2, suggesting that NtpJ mediates Na+ movement in addition to Kmovement. Finally, the growth of JEM2 arrested in K+-Iimited high-Na+ mediu m at pH 10 was restored by addition of valinomycin. These results suggest t hat NtpJ mediates electrogenic transport of K+ as well as Na+, that it like ly mediates K+ and Na+ cotransport, and that Na+ movement via NtpJ is the m ajor Na+ reentry pathway at high pH values.