Mechanistic basis of the dynamic response of TWIK1 ionic selectivity to pH

Abstract

At neutral pH, the TWIK1 channel is highly selective for K+. When exposed to acidification, it becomes permeable to Na+. This change occurs within minutes and is reversible. By combining pKa calculations, molecular dynamics (MD) simulations, mutagenesis and electrophysiology, we identified a network of residues involved in this unique property. MD simulations captured crucial features associated with channel gating and previously observed by cryogenic electron microscopy (cryo-EM) at pH7.4 and pH5, such as the pH-dependent orientation of the lateral side chain of the proton sensor His122 and the elongation of the entire pore structure upon acidification. Between the closed and open states of TWIK1 observed by cryo-EM, MD simulations show that the channel undergoes additional conformational changes between pH 7.5 to 6 that involves the His122, Glu235, Lys246 and Phe109 residues. A complex network of interactions surrounding the selectivity filter at high pH transforms into a simple set of stronger interactions at low pH. In particular, His122 protonated by acidification moves away from Lys246 and engages in a salt bridge with Glu235. In addition, stacking interactions between Phe109 and His122, which stabilize the selectivity filter in its K+-selective state at high pH, disappear upon acidification. This causes dissociation of the Phe109 aromatic side chain from this network, ultimately leading to the Na+-permeable conformation of the channel.