Structure of the KcsA channel intracellular gate in the open state

Ion channels catalyze the selective transfer of ions across the membrane in response to a variety of stimuli. These channels gate by controlling the a ccess of ions to a centrally located water-filled pore. The crystal structu re of the Streptomyces lividans potassium channel (KcsA) has allowed a mole cular exploration of this mechanism. Electron paramagnetic resonance (EPR) studies have uncovered significant conformational changes at the intracellu lar end of the second transmembrane helix (TM2) upon gating. We have used s ite-directed spin labeling (SDSL) and EPR spectroscopy in an attempt to qua ntify the structural rearrangements of the KcsA TM2 bundle underlying the t ransition from the closed to the open state. Under conditions favoring the closed and open conformations, 10 intersubunit distances were obtained acro ss TM2 segments from tandem dimer constructs. Analysis of these data points to a mechanism in which each TM2 helix tilts away from the permeation path way, towards the membrane plane, and rotates about its helical axis, suppor ting a scissoring-type motion with a pivot point near residues 107-108. The se movements are accompanied by a large increase in the diameter of the ves tibule below the central water-filled cavity.