title: A third Na+-binding site in the sodium pump.
abstract: The sodium pump, or Na,K-ATPase, exports three intracellular sodium ions in exchange for two extracellular potassium ions. In the high resolution structure of the related calcium pump, two cation-binding sites have been identified. The two corresponding sites in the sodium pump are expected to be alternatively occupied by sodium and potassium. The position of a third sodium-specific site is still hypothetical. Here, we report the large effects of single residue substitutions on the voltage-dependent kinetics of the release of sodium to the extracellular side of the membrane. These mutations also alter the apparent affinity for intracellular sodium while one of them does not affect the intrinsic affinity for potassium. These results enable us to locate the third sodium-specific site of the sodium pump in a space between the fifth, sixth, and ninth transmembrane helices of the alpha-subunit and provide an experimental validation of the model proposed by Ogawa and Toyoshima [Ogawa, H. & Toyoshima, C. (2002) Proc. Natl. Acad. Sci. USA 99, 15977-15982].
authors.affiliation: Department of Pharmacology and Toxicology, Faculty of Biology and Medicine, University of Lausanne, Bugnon 27, CH-1005 Lausanne, Switzerland.
authors.forename: Ciming
authors.initials: C
authors.lastname: Li
authors.forename: Oihana
authors.initials: O
authors.lastname: Capendeguy
authors.forename: Käthi
authors.initials: K
authors.lastname: Geering
authors.forename: Jean-Daniel
authors.initials: JD
authors.lastname: Horisberger
mesh_terms: D000818:Animals; D001665:Binding Sites; D004553:Electric Conductivity; D018698:Glutamic Acid / Q000235:genetics / Q000378:metabolism; D008954:Models, Biological; D009154:Mutation / Q000235:genetics; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D051381:Rats; D015203:Reproducibility of Results; D012964:Sodium / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D014981:Xenopus
chemical_list: D018698:Glutamic Acid; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase