39 research outputs found

    Cation stoichiometry and cation pathway in the Na,K-ATPase and nongastric H,K-ATPase

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    The mechanism of cation translocation by the Na,K-ATPase was investigated by cysteine scanning mutagenesis and measurements of accessibility through exposure to cysteine reagents. In the native protein, accessible residues were found only at the most extracellular residues of the 5th and 6th transmembrane segments (TMS) and the short loop between them. However, after modification by palytoxin a number of residues became accessible along the whole length of the 5th TMS and in the outer half of the 6th TMS, showing the contribution of each of these segments to the "channel" formed by the palytoxin-transformed Na,K-pump. Assuming that this structure is similar in the native and the palytoxin-transformed pump, our data allow us to determine the residues lining the cation pathway from the extracellular solution to their binding sites. A critical position in the 5th TMS contains a lysine conserved in all known nonelectrogenic H,K-ATPases, and a serine in all known electrogenic Na,K-ATPase sequences. Wild-type or mutant Na,K-or H,K-ATPase a subunits were coinjected with the Bufo beta2 subunit in Xenopus oocytes and Rb(86) uptake and electrophysiological measurements were performed. An electrogenic activity was recorded for the H,K-ATPase mutants in which the positively charged lysine had been replaced by neutral or negatively charged residues, while nonelectrogenic transport was observed with the S(782)R mutant of the Na,K-ATPase. The presence or the absence of a positively charged residue at the S(782) position appears to be critical for the stoichiometry of cation exchange

    Electrogenicity of Na,K- and H,K-ATPase activity and presence of a positively charged amino acid in the fifth transmembrane segment.

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    The transport activity of the Na,K-ATPase (a 3 Na+ for 2 K+ ion exchange) is electrogenic, whereas the closely related gastric and non-gastric H,K-ATPases perform electroneutral cation exchange. We have studied the role of a highly conserved serine residue in the fifth transmembrane segment of the Na,K-ATPase, which is replaced with a lysine in all known H,K-ATPases. Ouabain-sensitive 86Rb uptake and K+-activated currents were measured in Xenopus oocytes expressing the Bufo bladder H,K-ATPase or the Bufo Na,K-ATPase in which these residues, Lys800 and Ser782, respectively, were mutated. Mutants K800A and K800E of the H,K-ATPase showed K+-stimulated and ouabain-sensitive electrogenic transport. In contrast, when the positive charge was conserved (K800R), no K+-induced outward current could be measured, even though rubidium transport activity was present. Conversely, the S782R mutant of the Na,K-ATPase had non-electrogenic transport activity, whereas the S782A mutant was electrogenic. The K800S mutant of the H,K-ATPase had a more complex behavior, with electrogenic transport only in the absence of extracellular Na+. Thus, a single positively charged residue in the fifth transmembrane segment of the alpha-subunit can determine the electrogenicity and therefore the stoichiometry of cation transport by these ATPases

    Bufo marinus bladder H-K-ATPase carries out electroneutral ion transport

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    Bufo marinus bladder H-K-ATPase belongs to the Na-K-ATPase and H-K-ATPase subfamily of oligomeric P-type ATPases and is closely related to rat and human nongastric H-K-ATPases. It has been demonstrated that this ATPase transports K(+) into the cell in exchange for protons and sodium ions, but the stoichiometry of this cation exchange is not yet known. We studied the electrogenic properties of B. marinus bladder H-K-ATPase expressed in Xenopus laevis oocytes. In a HEPES-buffered solution, K(+) activation of the H-K-ATPase induced a slow-onset inward current that reached an amplitude of approximately 20 nA after 1-2 min. When measurements were performed in a solution containing 25 mM HCO at a PCO(2) of 40 Torr, the negative current activated by K(+) was reduced. In noninjected oocytes, intracellular alkalization activated an inward current similar to that due to B. marinus H-K-ATPase. We conclude that the transport activity of the nongastric B. marinus H-K-ATPase is not intrinsically electrogenic but that the inward current observed in oocytes expressing this ion pump is secondary to intracellular alkalization induced by proton transport

    Effect of prestressing on the behaviour of CFRP under gamma irradiation

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    SIGLEAvailable from British Library Document Supply Centre- DSC:9091.9(AEA-InTec--0848) / BLDSC - British Library Document Supply CentreGBUnited Kingdo

    Assessing possibilities for enhanced EU-South Korea cooperation on chemical regulation

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