How palytoxin transforms the Na⁺,K⁺ pump into a cation channel

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Physiology

Background:

• Palytoxin (PTX) is a potent marine toxin known to alter the function of Na+,K+-ATPase (NKA), an essential ion pump.
• Previous hypotheses suggested PTX controls NKA's gating mechanisms, potentially creating a nonselective cation channel.
• Understanding PTX's interaction mechanism is crucial for elucidating NKA's ion transport and regulation.

Purpose of the Study:

• To investigate the structural basis of how palytoxin (PTX) interacts with Na+,K+-ATPase (NKA).
• To determine whether PTX controls existing gates or forms a new pathway in NKA.
• To elucidate the dynamic architecture of the NKA ion pathway.

Main Methods:

• Determined high-resolution structures of NKA in complex with PTX in three distinct functional states using cryo-electron microscopy.
• Utilized a stable transition state analog (AlF$_{x}$) to mimic phosphate binding and study reaction cycle progression.
• Analyzed the binding site and conformational changes induced by PTX and ligands (Na+, ATP/ADP).

Main Results:

• PTX binds to NKA within the E2P state, specifically within the physiological Na+ exit pathway, similar to istaroxime.
• The NKA·PTX complex forms an open channel across the membrane upon addition of Na+ and ATP/ADP.
• PTX binding prevents the closure of the extracellular ion pathway, revealing a dynamic NKA architecture distinct from a simple two-gate model.

Conclusions:

• NKA's ion pathway architecture is dynamic, composed of segments that reconfigure during ion transport.
• PTX does not merely control existing gates but fundamentally alters NKA's structure, creating a persistent channel.
• These findings challenge the traditional 'two-gate' model and offer new insights into ion pump mechanism and toxin interaction.