Na+/K+-pump ligands modulate gating of palytoxin-induced ion channels.
The Na+/K+ pump is a ubiquitous P-type ATPase that binds three cytoplasmic Na+ ions deep within its core where they are temporarily occluded before being released to the extracellular surface. The 3Na+/2K+ -exchange transport cycle is completed when two extracellular K+ ions bind and become temporarily occluded within the protein and subsequently released to the cytoplasm. Coupling of Na+ -ion occlusion to phosphorylation of the pump by ATP and of K+ -ion occlusion to its dephosphorylation ensure the vectorial nature of net transport. The occluded-ion conformations, with binding sites inaccessible from either side, represent intermediate states in these alternating-access descriptions of transport. They afford protection against potentially catastrophic effects of inadvertently allowing simultaneous access from both membrane sides. The marine toxin, palytoxin, converts Na+/K+ pumps into nonselective cation channels, possibly by disrupting the normal strict coupling between opening of one access pathway in the Na+/K+ ATPase and closing of the other. We show here that gating of the channels in palytoxin-bound Na+/K+ pumps in excised membrane patches is modulated by the pump's physiological ligands: cytoplasmic application of ATP promotes opening of the channels, and extracellular replacement of Na+ ions by K+ ions promotes closing of the channels. This suggests that, despite the presence of bound palytoxin, certain partial reactions of the normal Na+/K+ -transport cycle persist and remain capable of effecting the conformational changes that control access to the pump's cation-binding sites. These findings affirm the alternating-access model of ion pumps and offer the possibility of examining ion occlusion/deocclusion reactions in single pump molecules.
Artigas
Pablo
P
Laboratory of Cardiac/Membrane Physiology, The Rockefeller University, 1230 York Avenue, New York, NY 10021-6399, USA.
Gadsby
David C
DC
D000178:Acrylamides / Q000494:pharmacology*; D000255:Adenosine Triphosphate / Q000494:pharmacology; D002460:Cell Line; D002586:Cesium / Q000494:pharmacology; D003064:Cnidarian Venoms; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects*; D017136:Ion Transport; D010766:Phosphorylation; D011188:Potassium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000187:drug effects* / Q000378:metabolism; D000095702:Polyether Toxins
D000178:Acrylamides; D000255:Adenosine Triphosphate; D002586:Cesium; D003064:Cnidarian Venoms; D011188:Potassium; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; C010272:palytoxin; D000095702:Polyether Toxins