title: Keeping it simple, transport mechanism and pH regulation in Na+/H+ exchangers.
abstract: Na(+)/H(+) exchangers are essential for regulation of intracellular proton and sodium concentrations in all living organisms. We examined and experimentally verified a kinetic model for Na(+)/H(+) exchangers, where a single binding site is alternatively occupied by Na(+) or one or two H(+) ions. The proposed transport mechanism inherently down-regulates Na(+)/H(+) exchangers at extreme pH, preventing excessive cytoplasmic acidification or alkalinization. As an experimental test system we present the first electrophysiological investigation of an electroneutral Na(+)/H(+) exchanger, NhaP1 from Methanocaldococcus jannaschii (MjNhaP1), a close homologue of the medically important eukaryotic NHE Na(+)/H(+) exchangers. The kinetic model describes the experimentally observed substrate dependences of MjNhaP1, and the transport mechanism explains alkaline down-regulation of MjNhaP1. Because this model also accounts for acidic down-regulation of the electrogenic NhaA Na(+)/H(+) exchanger from Escherichia coli (EcNhaA, shown in a previous publication) we conclude that it applies generally to all Na(+)/H(+) exchangers, electrogenic as well as electroneutral, and elegantly explains their pH regulation. Furthermore, the electrophysiological analysis allows insight into the electrostatic structure of the translocation complex in electroneutral and electrogenic Na(+)/H(+) exchangers.
authors.affiliation: From the Departments of Biophysical Chemistry and.
authors.forename: Octavian
authors.initials: O
authors.lastname: Călinescu
authors.forename: Cristina
authors.initials: C
authors.lastname: Paulino
authors.forename: Werner
authors.initials: W
authors.lastname: Kühlbrandt
authors.forename: Klaus
authors.initials: K
authors.lastname: Fendler
mesh_terms: D019843:Archaeal Proteins / Q000235:genetics / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport / Q000502:physiology; D063747:Methanocaldococcus / Q000235:genetics / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers / Q000235:genetics / Q000378:metabolism*
chemical_list: D019843:Archaeal Proteins; D017923:Sodium-Hydrogen Exchangers
keywords: Archaea; Cation Proton Antiporter; EcNhaA; Electrophysiology; Membrane Transport; MjNhaP1; Sodium Proton Exchange; Solid Supported Membrane; pH Regulation