Identification of Nucleotide Binding Sites in V-Type Na⁺-ATPase from Enterococcus hirae

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A and B subunits of the V-type Na⁺-ATPase from Enterococcus hirae were suggested to possess nucleotide binding sites (Murata, T. et al., J. Biochem., 132, 789–794 (2002)), although the B subunit did not have the consensus sequence for nucleotide binding. To further characterize the binding sites in the V-ATPase, we did the photoaffinity labeling study using 8-azido-[α-³²P]ATP. A and B subunits were labeled with 8-azido-[α-³²P]ATP when analysed with SDS polyacrylamide gel electrophoresis. The peptide fragment of A subunit obtained by lysyl endopeptidase digestion after labeling showed a molecular size of 9 kDa and its amino acid sequencing revealed that it corresponded to residues Arg⁴²³-Lys⁴⁹⁴. The peptide fragment from B subunit after photoaffinity labeling and lysyl endopeptidase digestion showed the size of 5 kDa and corresponded to residues Phe⁴⁰⁴-Lys⁴⁴³. In our structure model, these peptides were close to the adenine ring of ATP. We suggest that non-catalytic B subunit of E. hirae V-ATPase has a nucleotide binding site, similarly to eukaryotic V-ATPases and F-ATPases.