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Alohamora! The Opening Mechanism of an ATP-releasing Plasma Membrane Channel

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Abstract

Pannexin 1 (Panx1) is an ATP-releasing channel expressed in virtually all vertebrate tissues. In addition to roles in blood pressure regulation and cancer metastasis, Panx1 plays an important role in apoptosis. During cell death, Panx1 releases ATP as a “find-me” signal to recruit phagocytes for cell clearance, preventing unnecessary inflammation. Under normal conditions, the carboxyl-terminal tails act as a “plug” inside the channel pore, keeping it closed. During apoptosis, proteases cleave these tails, opening the channel for ATP release. Perplexingly, however, if a longer portion of the C-terminus is cleaved, the channel remains closed despite the removal of the pore plug. Our lab recently identified a crucial amino acid sequence, termed the C-terminal activating domain (CAD), which must remain intact in the C-terminus to enable channel opening. Using cryo-electron microscopy, we visualized the Panx1 channel with and without the CAD and discovered that this critical region is freed upon C-terminal cleavage. This process forces the N-termini to reside inside the pore, priming the channel for opening. By analyzing the electrostatic free energy of these conformations, we found that electrostatic forces, rather than steric forces, regulate the channel’s opening and closing. This comprehensive study is the first to provide a complete mechanistic understanding of how the Panx1 channel facilitates ATP release during apoptosis.