KvAP, a voltage-dependent K+ channel
Aeropyrum pernix (archaea)
When nerve or muscle cells receive an electric stimulus, the voltages across the cell membranes change from negative to positive, but soon return to their initial values. These voltage changes are called action potentials and also generate responses in adjacent areas of the membrane. Thus action potentials quickly propagate along the membrane surfaces, and their arrival at particular positions causes the initiation of various cell processes. Voltage-dependent cation channels play a central role in the generation of action potentials, controlling the permeation of cations through the cell membranes. These channels sense electrical stimuli such as voltage changes and open and close the gates of the pores through which ions pass. It is known that the opening and closing of the gates is caused by the movement of charged amino acids called 'gating charges' through the protein core of the channels. This displacement of the charges is measured as the transient electric current with respect to the gating, separately from the ion movement.
The structure of the voltage-dependent K+ channel, a member of the family of voltage-dependent cation channels, has suggested the mechanism for pore gating. The channel is a homotetramer and each of its subunit contains six segments (S1-S6). The S5 and S6 segments affect the ion selectivity. The second helices in the S3, S3b, and S4 segments form a helix-turn-helix structure on the outer perimeter of the channel, packing tightly against each other, which the authors call voltage-sensor paddles. The first four Arg residues of the paddles correspond to the gating charges. The conformation of the S4 segments of the paddles changes during the gating and, therefore, a new model for the movement of the gating charges has been suggested. When the channel senses the voltage changes, the gating charges are carried through the membrane from inside to outside of the cell by the movement of the voltage-sensor paddles on the membrane surface. As a result of the movement of the gating charges, the gate of the pore opens.
Protein Data Bank (PDB)
Jiang, Y. Lee, A. Chen, J. Ruta, V. Cadene, M. Chait, B.T. MacKinnon, R.; "X-ray structure of a voltage-dependent K+ channel"; Nature; (2003) 423:33-41 PubMed:12721618.
author: Yuko Tsuchiya