Electron microscopic analysis of KvAP voltage-dependent K+ channels in an open conformation

Voltage-dependent ion channels serve as field-effect transistors by opening a gate in response to membrane voltage changes(1). The gate's response to voltage is mediated by voltage sensors(2), which are arginine-containing structures that must move with respect to the membrane electric field. We have analysed by electron microscopy a voltage-dependent K+ channel from Aeropyrum pernix (KvAP)(3). Fab fragments were attached to 'voltage sensor paddles' and identified in the electron microscopy map at 10.5 Angstrom resolution. The extracellular surface location of the Fab fragments in the map is consistent with the membrane-depolarized, open conformation of the channel in electrophysiological experiments. Comparison of the map with a crystal structure(4) demonstrates that the voltage sensor paddles are 'up' ( that is, near the channel's extracellular surface) and situated at the protein - lipid interface. This finding supports the hypothesis that in response to changes in voltage the sensors move at the protein - lipid interface(5) rather than in a gating pore surrounded by protein(6,7).