Catalytic properties of Na+-translocating V-ATPase in Enterococcus hirae

AbstractV-ATPases make up a family of proton pumps distributed widely from bacteria to higher organisms. We found a variant of this family, a Na+-translocating ATPase, in a Gram-positive bacterium, Enterococcus hirae. The Na+-ATPase was encoded by nine ntp genes from F to D in an ntp operon (ntpFIKECGABDHJ): the ntpJ gene encoded a K+ transporter independent of the Na+-ATPase. Expression of this operon, encoding two transport systems for Na+ and K+ ions, was regulated at the transcriptional level by intracellular Na+ as the signal. Structural aspects and catalytic properties of purified Na+-ATPase closely resembled those of other V-type H+-ATPases. Interestingly, the E. hirae enzyme showed a very high affinity for Na+ at catalytic reaction. This property enabled the measurement of ion binding to this ATPase for the first time in the study of V- and F-ATPases. Properties of Na+ binding to V-ATPase were consistent with the model that V-ATPase proteolipids form a rotor ring consisting of hexamers, each having one cation binding site. We propose here a structure model of Na+ binding sites of the enzyme