Membrane-integral pyrophosphatase subfamily capable of translocating both Na+ and H+



: Luoto HH, Baykov AA, Lahti R, Malinen AM

Publisher: NATL ACAD SCIENCES

: 2013

: Proceedings of the National Academy of Sciences of the United States of America

: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA

: PROC NATL ACAD SCI USA

: 4

: 110

: 4

: 1255

: 1260

: 6

: 1091-6490

DOI: https://doi.org/10.1073/pnas.1217816110


One of the strategies used by organisms to adapt to life under conditions of short energy supply is to use the by-product pyrophosphate to support cation gradients in membranes. Transport reactions are catalyzed by membrane-integral pyrophosphatases (PPases), which are classified into two homologous subfamilies: H+-transporting (found in prokaryotes, protists, and plants) and Na+-transporting (found in prokaryotes). Transport activities have been believed to require specific machinery for each ion, in accordance with the prevailing paradigm in membrane transport. However, experiments using a fluorescent pH probe and Na-22(+) measurements in the current study revealed that five bacterial PPases expressed in Escherichia coli have the ability to simultaneously translocate H+ and Na+ into inverted membrane vesicles under physiological conditions. Consistent with data from phylogenetic analyses, our results support the existence of a third, dual-specificity bacterial Na+, H+-PPase subfamily, which apparently evolved from Na+-PPases. Interestingly, genes for Na+, H+-PPase have been found in the major microbes colonizing the human gastrointestinal tract. The Na+, H+-PPases require Na+ for hydrolytic and transport activities and are further activated by K+. Based on ionophore effects, we conclude that the Na+ and H+ transport reactions are electrogenic and do not result from secondary antiport effects. Sequence comparisons further disclosed four Na+, H+-PPase signature residues located outside the ion conductance channel identified earlier in PPases using X-ray crystallography. Our results collectively support the emerging paradigm that both Na+ and H+ can be transported via the same mechanism, with switching between Na+ and H+ specificities requiring only subtle changes in the transporter structure.



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