title: Identification and function of a cytoplasmic K+ site of the Na+, K+ -ATPase.
abstract: A cytoplasmic nontransport K(+)/Rb(+) site in the P-domain of the Na(+), K(+)-ATPase has been identified by anomalous difference Fourier map analysis of crystals of the [Rb(2)].E(2).MgF(4)(2-) form of the enzyme. The functional roles of this third K(+)/Rb(+) binding site were studied by site-directed mutagenesis, replacing the side chain of Asp(742) donating oxygen ligand(s) to the site with alanine, glutamate, and lysine. Unlike the wild-type Na(+), K(+)-ATPase, the mutants display a biphasic K(+) concentration dependence of E(2)P dephosphorylation, indicating that the cytoplasmic K(+) site is involved in activation of dephosphorylation. The affinity of the site is lowered significantly (30-200-fold) by the mutations, the lysine mutation being most disruptive. Moreover, the mutations accelerate the E(2) to E(1) conformational transition, again with the lysine substitution resulting in the largest effect. Hence, occupation of the cytoplasmic K(+)/Rb(+) site not only enhances E(2)P dephosphorylation but also stabilizes the E(2) dephosphoenzyme. These characteristics of the previously unrecognized nontransport site make it possible to account for the hitherto poorly understood trans-effects of cytoplasmic K(+) by the consecutive transport model, without implicating a simultaneous exposure of the transport sites toward the cytoplasmic and extracellular sides of the membrane. The cytoplasmic K(+)/Rb(+) site appears to be conserved among Na(+), K(+)-ATPases and P-type ATPases in general, and its mode of operation may be associated with stabilizing the loop structure at the C-terminal end of the P6 helix of the P-domain, thereby affecting the function of highly conserved catalytic residues and promoting helix-helix interactions between the P- and A-domains in the E(2) state.
authors.affiliation: Centre for Membrane Pumps in Cells and Disease (PUMPKIN), Danish National Research Foundation, University of Aarhus, DK-8000 Aarhus C, Denmark; Institute of Physiology and Biophysics, University of Aarhus, DK-8000 Aarhus C, Denmark.
authors.forename: Vivien Rodacker
authors.initials: VR
authors.lastname: Schack
authors.affiliation: Centre for Membrane Pumps in Cells and Disease (PUMPKIN), Danish National Research Foundation, University of Aarhus, DK-8000 Aarhus C, Denmark; Department of Molecular Biology, University of Aarhus, DK-8000 Aarhus C, Denmark.
authors.forename: Jens Preben
authors.initials: JP
authors.lastname: Morth
authors.affiliation: Centre for Membrane Pumps in Cells and Disease (PUMPKIN), Danish National Research Foundation, University of Aarhus, DK-8000 Aarhus C, Denmark; Institute of Physiology and Biophysics, University of Aarhus, DK-8000 Aarhus C, Denmark.
authors.forename: Mads S
authors.initials: MS
authors.lastname: Toustrup-Jensen
authors.affiliation: Centre for Membrane Pumps in Cells and Disease (PUMPKIN), Danish National Research Foundation, University of Aarhus, DK-8000 Aarhus C, Denmark; Institute of Physiology and Biophysics, University of Aarhus, DK-8000 Aarhus C, Denmark.
authors.forename: Anne Nyholm
authors.initials: AN
authors.lastname: Anthonisen
authors.affiliation: Centre for Membrane Pumps in Cells and Disease (PUMPKIN), Danish National Research Foundation, University of Aarhus, DK-8000 Aarhus C, Denmark; Department of Molecular Biology, University of Aarhus, DK-8000 Aarhus C, Denmark.
authors.forename: Poul
authors.initials: P
authors.lastname: Nissen
authors.affiliation: Centre for Membrane Pumps in Cells and Disease (PUMPKIN), Danish National Research Foundation, University of Aarhus, DK-8000 Aarhus C, Denmark; Institute of Physiology and Biophysics, University of Aarhus, DK-8000 Aarhus C, Denmark.
authors.forename: Jens Peter
authors.initials: JP
authors.lastname: Andersen
authors.affiliation: Centre for Membrane Pumps in Cells and Disease (PUMPKIN), Danish National Research Foundation, University of Aarhus, DK-8000 Aarhus C, Denmark; Institute of Physiology and Biophysics, University of Aarhus, DK-8000 Aarhus C, Denmark. Electronic address: bv@fi.au.dk.
authors.forename: Bente
authors.initials: B
authors.lastname: Vilsen
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D001665:Binding Sites / Q000502:physiology; D018360:Crystallography, X-Ray; D003593:Cytoplasm / Q000201:enzymology* / Q000235:genetics; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D020125:Mutation, Missense; D010766:Phosphorylation; D011188:Potassium / Q000737:chemistry / Q000378:metabolism; D017433:Protein Structure, Secondary / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000502:physiology; D051381:Rats; D012413:Rubidium / Q000737:chemistry / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
chemical_list: C499075:Atp1a1 protein, rat; D000254:Sodium-Potassium-Exchanging ATPase; D012413:Rubidium; D011188:Potassium