title: Membrane-integral pyrophosphatase subfamily capable of translocating both Na+ and H+.
abstract: 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 (22)Na(+) 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.
authors.affiliation: Department of Biochemistry and Food Chemistry,  University of Turku, FIN-20014 Turku,  Finland.
authors.forename: Heidi H
authors.initials: HH
authors.lastname: Luoto
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Baykov
authors.forename: Reijo
authors.initials: R
authors.lastname: Lahti
authors.forename: Anssi M
authors.initials: AM
authors.lastname: Malinen
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D001439:Bacteroides / Q000201:enzymology / Q000235:genetics; D002462:Cell Membrane / Q000378:metabolism; D007887:Fabaceae / Q000201:enzymology / Q000235:genetics; D006801:Humans; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D043564:Inorganic Pyrophosphatase / Q000737:chemistry / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D017136:Ion Transport; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010802:Phylogeny; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D012964:Sodium / Q000378:metabolism; D020124:Thermotoga maritima / Q000201:enzymology / Q000235:genetics
chemical_list: D001426:Bacterial Proteins; D010940:Plant Proteins; D011994:Recombinant Proteins; D012964:Sodium; D043564:Inorganic Pyrophosphatase