Inward rectifier potassium channel KirBac1.1
Burkholderia pseudomallei (bacteria)
On the depolarized side of membranes, the flow of ions through the channel proteins is restricted, because magnesium ions and polyamines in the cell block the ion flows. This phenomenon is called the inward-rectification, and the channel proteins are called inward-rectifier K+ cannels, belonging to the inward-rectifier family of potassium channels.
The structure of the inward-rectifier K+ channel protein, prokaryotic KirBac1.1 derived from Burkholderia pseudomallei, in the closed state has been determined. This protein is a homotetramer. Each subunit is divided into two regions, the transmembrane alpha-helix region and the intracellular domain consisting of beta-sheets. At the center of the tetramer, the ion pathway is formed. The eight negatively charged residues, Glu187 and Glu258 from all the four subunits on the pathway contribute to form a negatively charged double ring, which enhances the potassium ion flow and is also involved in blocking it by magnesium ions and polyamines. The blocking generates the inward-rectification, but does not close the ion pathway. The four Phe146s on the intracellular membrane portion are involved in the closing and opening of the ion pathway. The KirBac1.1 structure has suggested the gating mechanism described below. The rotation of the intracellular domains draws the bottoms of the inner helices which construct the surface of the ion pathway in the transmembrane region. It results in the movement of the flexible helices which connect to the outer helices in the transmembrane region, and then of the outer helices away from the ion pathway. These movements create the space for the inner helices to bend. By the bending of the inner helices opposite to the center, the Phe146s gate of the pathway is opened.
Protein Data Bank (PDB)
Kuo, A. Gulbis, J.M. Antcliff, J.F. Rahman, T. Lowe, E.D. Zimmer, J. Cuthbertson, J. Ashcroft, F.M. Ezaki, T. Doyle, D.A.; "Crystal structure of the potassium channel KirBac1.1 in the closed state."; Science; (2003) 300:1922-1926 PubMed:12738871.
author: Yuko Tsuchiya