PDBsum entry 2bob

Immune system/transport protein

PDB id: 2bob

Contents

Protein chains

219 a.a.

212 a.a.

103 a.a.

Ligands

TBA

Metals

_TL

_CO

Waters ×56

References listed in PDB file

Key reference

Title

Structural basis of tea blockade in a model potassium channel.

Authors

M.J.Lenaeus, M.Vamvouka, P.J.Focia, A.Gross.

Ref.

Nat Struct Mol Biol, 2005, 12, 454-459. [DOI no: 10.1038/nsmb929]

PubMed id

15852022

Abstract

Potassium channels catalyze the selective transfer of potassium across the cell membrane and are essential for setting the resting potential in cells, controlling heart rate and modulating the firing pattern in neurons. Tetraethylammonium (TEA) blocks ion conduction through potassium channels in a voltage-dependent manner from both sides of the membrane. Here we show the structural basis of TEA blockade by cocrystallizing the prokaryotic potassium channel KcsA with two selective TEA analogs. TEA binding at both sites alters ion occupancy in the selectivity filter; these findings underlie the mutual destabilization and voltage-dependence of TEA blockade. We propose that TEA blocks potassium channels by acting as a potassium analog at the dehydration transition step during permeation.

Figure 1.
Figure 1. Mechanism of permeation in potassium channels. (a) Two diagonal subunits of KcsA with the selectivity filter highlighted in black (residues 74 -80). Potassium ions are green spheres. (b) Model of permeation29. The selectivity filter is shown schematically with ions numbered 1 -4. Oxygen ligands are red crosses (water) or circles (carbonyl or hydroxyl). The two states of the selectivity filter (1/3 and 2/4) are occupied alternatively during permeation. A dehydration transition site for potassium is observed experimentally on the outside of the channel, but not on the inside (green arrow). (c) Structure of the selectivity filter in low permeant ion concentration. Left, low potassium (PDB entry 1K4D). Residues Thr74 -Asp80 are in stick representation. Oxygen, nitrogen and carbon are red, blue and gray, respectively. Middle, low thallium (PDB entry 1R3K). Thallium ions are purple spheres. Right, schematic representation of the structure observed at low permeant ion concentration with one ion alternating between sites 1 and 4. (d) Structure of the selectivity filter in high permeant ion concentrations. Left, high potassium (PDB entry 1K4C). The residues in the selectivity filter are labeled at the carbonyl level. Middle, high thallium (PDB entry 1R3J). Right, schematic representation with two ions alternating between the 1/3 and 2/4 states. All figures were generated with MolScript42 and Raster3D^43.

Figure 4.
Figure 4. Mechanism of TEA blockade. (a) The selectivity filter in high potassium (PDB entry 1K4C) is shown. In cesium (PDB entry 1R3L), an additional ion-binding site is observed in the cavity32 and a cesium ion is shown as a yellow sphere at this position. Two TEA molecules are shown in the TBA and TEAs positions, respectively. Arrows point to the external dehydration transition site observed in potassium and to the internal dehydration transition site observed in cesium. (b) Model of blockade. The selectivity filter is shown schematically with thallium ions drawn as filled circles and TEA as open circles. Binding of TEA stabilizes the close ion and destabilizes the remote ion. The observed states are highlighted. In the case of internal TEA binding, the external pore collapses to form the proposed inactivated state. (c) Stereo view down the four-fold axis onto the external TEA site. Four TEA molecules and their ligands were extracted from CPS (PDB entry 1A9X) and superimposed by least-squares fitting the TEA molecules. All possible orientations of the TEA-ligand cloud were generated and docked into the TEAs structure by a least-squares fit of the CPS TEA to TEAs. Oxygen ligands of TEA are red, nitrogen ligands are blue and chloride ions are green. KcsA residues of the external pore are shown. Arrows identify carbonyl oxygen atoms and the van der Waals contact between TEA and Tyr82. (d) View up the symmetry axis onto the internal TEA site. The same TEA-ligand structure as in c was docked into the TBA structure by a least-squares fit. Arrows identify carbonyl and hydroxyl oxygen atoms.

The above figures are reprinted by permission from Macmillan Publishers Ltd: Nat Struct Mol Biol (2005, 12, 454-459) copyright 2005.