From the Cover: Na+/K+-pump ligands modulate gating of palytoxin-induced ion channels

摘要

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.