Nav and Kv channels are voltage-gated ion channels (VGICs), with specificity for sodium and potassium cations, respectively, that are responsible for the propagation of action potentials in neurological signal transmission.  Abnormalities in VGICs are responsible for a number of “channelopathy” diseases and VGICs are targets for anesthetic action.  As a result, they are of substantial biomedical significance.  High-resolution 3-D structures of VGICs have been provided by conventional techniques like X-ray crystallography and cryo-EM, but only in the absence of a transmembrane voltage.  Hence, we still don’t know the mechanism by which VGICs couple conformational changes in their four voltage sensor domains (VSDs)—in response to changes in the transmembrane voltage—to opening or closing the ion channel within their pore domain (PD).  As a result, it has been necessary to develop computational approaches in order to address the 3-D structures of the activated, open state and the deactivated, closed state of VGICs within hydrated phospholipid bilayer membranes, where “activated” and “deactivated” refer to the voltage-dependent state of the VSDs and “open” and “closed” refer to the state of the PD’s ion channel.  But each of the computational approaches is dependent on a particular set of assumptions, and direct experimental validation of the structures for each of these two states of VGICs remains essential.

The cover of the August 20 issue of Biophysical Journal is an artist’s rendition of our experimental approach to this problem.  It depicts a single Kv channel within a phospholipid bilayer membrane between the electrodes of a parallel plate capacitor with application of transmembrane voltages represented by the lightning bolt icon for dramatic effect; two of the four VSDs are shown in red, the PD in turquoise, the phospholipids in white, and the water in light blue.  This image is from a molecular dynamics simulation of the activated, open state of the Kv channel as described in the article by J. K. Blasie and co-workers.  In reality, our experiment utilized an ensemble of vectorially oriented Kv channels at high in-plane density within a hydrated phospholipid bilayer membrane.  Hyperpolarizing and depolarizing transmembrane voltage pulses were synchronized with the incident neutron pulses and applied to the membrane cyclically.  Neutron reflectivity data were collected separately for each voltage and averaged over many cycles to achieve adequate statistics.  The 2-D image of the averaged neutron reflectivity data for only one such voltage is shown with the incident neutrons depicted as coming in from the left of the membrane and the reflected neutrons depicted exiting toward the right and ending at the 2-image of the data.  These data provided the unique profile structure of the membrane, dominated by the Kv channel protein, for the deactivated, closed state and activated, open state of the channel, as well as the profile structure for water within the channel for these two states.  The results of our work demonstrate that, for at least the prokaryotic KvAP channel investigated, a relatively large inward translation of the voltage-sensing S4 helix within the four voltage sensor domains of the channel and a dewetting of the cytoplasmic half of the pore domain upon the transition between the activated, open state and the deactivated, closed state. 

  —Kent Blasie, Valeria Lauter, Andrew Geragotelis and Douglas Tobias

Cover Image credit: Jill Hemman, Oak Ridge National Laboratory.