A unique F-type H+-ATPase from Streptococcus mutans: An active H+ pump at acidic pH

We have shown previously that the Streptococcus mutans F-type H+-ATPase (FOF1) c subunit gene could complement Escherichia coli defective in the corresponding gene, particularly at acidic pH (Araki et al., (2013) [14]). In this study, the entire S. mutans FOF1 was functionally assembled in the E. coli plasma membrane (SFOF1). Membrane SFOF1 ATPase showed optimum activity at pH 7, essentially the same as that of the S. mutans, although the activity of E. coli FOF1 (EFOF1) was optimum at pH ⩾ 9. The membranes showed detectable ATP-dependent H+-translocation at pH 5.5-6.5, but not at neutral conditions (pH ⩾ 7), consistent with the role of S. mutans FOF1 to pump H+ out of the acidic cytoplasm. A hybrid FOF1, consisting of membrane-integrated FO and -peripheral F1 sectors from S. mutans and E. coli (SFOEF1), respectively, essentially showed the same pH profile as that of EFOF1 ATPase. However, ATP-driven H+-transport was similar to that by SFOF1, with activity at acidic pH. Replacement of the conserved c subunit Glu53 in SFOF1 abolished H+-transport at pH 6 or 7, suggesting its role in H+ transport. Mutations in the SFOF1c subunit, Ser17Ala or Glu20Ile, changed the pH dependency of H+-transport, and the FO could transport H+ at pH 7, as the membranes with EFOF1. Ser17, Glu20, and their vicinity were suggested to be involved in H+-transport in S. mutans at acidic pH.