Ion permeation through a narrow cavity constriction in KCNQ1 channels: Mechanism and implications for pathogenic variants.

KCNQ1 potassium channels play a pivotal role in the physiology and pathophysiology of several human excitable and epithelial tissues. The latest cryo-electron microscopy (cryo-EM) structures provide unique insights into channel function and pharmacology, opening avenues for different therapeutic strategies against human diseases associated with KCNQ1 mutations. However, these structures also raise fundamental questions about the mechanisms of ion permeation. Cryo-EM structures thought to represent the open state of the channel feature a cavity region not wide enough for accommodation of hydrated K. To understand how K passes through the cavity constriction, we utilized microsecond-scale molecular dynamics (MD) simulations using the KCNQ1/KCNE3 cryo-EM structure, characterized mutants at the G345 residue situated at the narrowest point of the cavity, and recorded single channels. The findings indicate that ions become partially dehydrated at the constriction, which enables permeation. MD simulations demonstrate that the constriction can impede the flow of ions through the channel's pore, a finding that is corroborated by mutational screening and single-channel recordings. Reduced channel conductance is the key mechanism underlying reported pathological KCNQ1 mutations at or near the constriction site.