title: Functional asymmetry of bidirectional Ca2+-movements in an archaeal sodium-calcium exchanger (NCX_Mj).
abstract: Dynamic features of Ca(2+) interactions with transport and regulatory sites control the Ca(2+)-fluxes in mammalian Na(+)/Ca(2+)(NCX) exchangers bearing the Ca(2+)-binding regulatory domains on the cytosolic 5L6 loop. The crystal structure of Methanococcus jannaschii NCX (NCX_Mj) may serve as a template for studying ion-transport mechanisms since NCX_Mj does not contain the regulatory domains. The turnover rate of Na(+)/Ca(2+) exchange (kcat=0.5±0.2 s(-1)) in WT-NCX_Mj is 10(3)-10(4) times slower than in mammalian NCX. In NCX_Mj, the intrinsic equilibrium (Kint) for bidirectional Ca(2+) movements (defined as the ratio between the cytosolic and extracellular Km of Ca(2+)/Ca(2+) exchange) is asymmetric, Kint=0.15±0.5. Therefore, the Ca(2+) movement from the cytosol to the extracellular side is ∼7-times faster than in the opposite direction, thereby representing a stabilization of outward-facing (extracellular) access. This intrinsic asymmetry accounts for observed differences in the cytosolic and extracellulr Km values having a physiological relevance. Bidirectional Ca(2+) movements are also asymmetric in mammalian NCX. Thus, the stabilization of the outward-facing access along the transport cycle is a common feature among NCX orthologs despite huge differences in the ion-transport kinetics. Elongation of the cytosolic 5L6 loop in NCX_Mj by 8 or 14 residues accelerates the ion transport rates (kcat) ∼10 fold, while increasing the Kint values 100-250-fold (Kint=15-35). Therefore, 5L6 controls both the intrinsic equilibrium and rates of bidirectional Ca(2+) movements in NCX proteins. Some additional structural elements may shape the kinetic variances among phylogenetically distant NCX variants, although the intrinsic asymmetry (Kint) of bidirectional Ca(2+) movements seems to be comparable among evolutionary diverged NCX variants.
authors.affiliation: Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel.
authors.forename: Lior
authors.initials: L
authors.lastname: Almagor
authors.affiliation: Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel.
authors.forename: Moshe
authors.initials: M
authors.lastname: Giladi
authors.affiliation: Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel.
authors.forename: Liat
authors.initials: L
authors.lastname: van Dijk
authors.affiliation: Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel.
authors.forename: Tal
authors.initials: T
authors.lastname: Buki
authors.affiliation: Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel.
authors.forename: Reuben
authors.initials: R
authors.lastname: Hiller
authors.affiliation: Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel. Electronic address: dhanan@post.tau.ac.il.
authors.forename: Daniel
authors.initials: D
authors.lastname: Khananshvili
mesh_terms: D000818:Animals; D001105:Archaea / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism*; D017136:Ion Transport / Q000502:physiology*; D007477:Ions / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger / Q000737:chemistry / Q000378:metabolism*
chemical_list: D007477:Ions; D019831:Sodium-Calcium Exchanger; D002118:Calcium
keywords: Alternating access; Antiporter; Calcium; Functional asymmetry; NCX