Study on ion-coupled subunit interaction of Na-HransbcatingV-ATPase

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(i) The membrane rotor ring from the vacuolar-type (V-type) sodium ion-pumping adenosine triphosphatase (Na+-ATPase) from Enterococcus hirae consists of 10 NtpK subunits, which are homologs of the 16-kilodalton and 8-kilodalton proteolipids found in other V-ATPases and in FIFo-or F-ATPases, respectively. Each NtpK subunit has four transmembrane alpha helices, with a sodium ion bound between helices 2 and 4 at a site buried deeply in the membrane that includes the essential residue glutamate-139. This site is probably connected to the membrane surface by two half-channels in subunit NtpI, against which the ring rotates. Symmetry mismatch between the rotor and catalytic domains appears to be an intrinsic feature of both V-and F-ATPases.(ii) By biochemical analysis of V-ATPase complex formation with the ntp gene-deleted mutant strains, we found that the VI portion is composed of subunit A, B, C, D, E, F, G and V0 portion of subunit I and K. Subunit B is not necessary to form the ACDIK complex, and subunit A plays a key role for V0V1 complex formation. All these results are important for understanding the molecular interaction of V-ATPase subunits during ion translocation.