Palytoxin‐Induced Channel Formation within the Na+/K+‐ATPase does not Require a Catalytically Active Enzyme

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Abstract
It has been demonstrated that palytoxin binds to and forms a channel within the Na+/K+‐ATPase. To investigate whether palytoxin‐induced channel formation within the sodium pump can occur independently of ATP hydrolysis and phosphorylation of the enzyme, an Asp369→Ala mutant of the α1 subunit of the sheep sodium pump was produced and coexpressed with β subunits in the yeast Saccharomyces cerevisiae. This aspartic acid residue, which during ion transport becomes phosphorylated from ATP, is essential for the function of the sodium pump. Therefore, as expected, microsomes isolated from yeast expressing the mutant sodium pump do not exhibit any ouabain sensitive ATPase activity, whereas in microsomes from yeast expressing the wild‐type sodium pump, 60% of the total ATPase activity is ouabain‐sensitive. Ouabain binds to yeast membranes containing either wild‐type or mutant sodium pumps with similar Bmax (1.45±0.05 versus 1.37±0.02 pmol/mg) and Kd values (27.7±0.91 versus 29.57±0.93 nM), thus indicating that the mutant sodium pumps are expressed in the yeast and that the mutation does not considerably affect the conformation of the enzyme. In the presence of phosphate ouabain binds to microsomes containing the wild‐type sodium pump with a Kd of 3.62±0.34 nM, showing that, although not necessary, phosphoenzyme formation enhances binding of the steroid. Phosphate or ATP, however, inhibit binding of ouabain to microsomes containing the mutant sodium pump with IC50 values of 78±3 μM and 3.0±0.4 μM, respectively. Despite these radical changes in the interactions of the mutant enzyme with ouabain, the interactions with palytoxin are not affected by the mutation. Palytoxin causes K+ efflux from yeast cells expressing the wild‐type or mutant sodium pumps with EC50 values of 3.5±0.4 nM and 6.2±0.9 nM, respectively. Palytoxin‐induced efflux from cells expressing wild‐type or mutant sodium pumps occurs with similar t1/2 values of 20.3±2.1 min and 22.2±3.1 min, respectively. Ouabain inhibits K+ efflux from both cell types with IC50 values of 28±2 μM and 210±15 μM, respectively. Cells expressing the Asp369→Ala mutants have an IC50 7.5‐fold higher than that obtained with cells expressing the wild‐type sodium pumps, possibly because ATP or phosphate present in the cytosol of the yeast cells influence and decrease ouabain binding to the mutant sodium pump. Thus, while ouabain binding and the associated inhibition of ion fluxes is promoted by phosphorylation of the wild‐type enzyme by phosphate or ATP, palytoxin‐induced channel formation is independent of phosphorylation and can be separated from the ATPase function of the sodium pump. Since ion fluxes through the sodium pump protein do not depend on ATP hydrolysis, the results suggest that the ionophores of pumps and ion channels might share common structural features.

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