Permeation in potassium channels: implications for channel structure.

The SR K+ channel is a single-ion channel with a tunnel that is not very selective, while the DR and CaK channels are both more selective, multi-ion channels. The permeation mechanisms of the three channels are probably most systematically distinguished by the length of their tunnels; the SR has the shortest and the DR the longest. Although different in their mechanisms of activation, the DR and CaK channels have very similar permeation characteristics, down to the details of selectivity and blockade. The longer tunnel and reduced conductance (perhaps a result of the extra tunnel length) of the DR K+ channel are the main differences. The selectivity of the rate-limiting barriers and the binding sites within the channels, however, are strikingly similar. A successful potassium channel must satisfy two criteria: It must let potassium ions through and not much else, and it must let many potassium ions through. To be selective the channel must have a narrow selectivity filter, so that an ion must shed some of its waters of hydration to pass through. Sodium ions are excluded because they are more reluctant to lose their water, and they are not adequately compensated for this loss by interaction with the selectivity filter. To carry a large current the narrow region must be short, with wide antechambers to reduce the diffusional access resistance (48). Energetically, the channel must strike a balance. There must be enough binding energy to compensate the ions for their lost hydration energy, so that the energy barrier to permeation is small. If the channel binds the ion too tightly, however, the ion will not be able to exit, and the current will be small. Some of the shared properties of different potassium channels are probably consequences of these requirements; others may be incidental to function, suggesting a common origin. Barium ions have almost exactly the same radius as potassium ions but twice the charge, so it is perhaps not surprising that barium can block any potassium channel by binding where potassium does, but too tightly. It seems more surprising that blockade by TEA+ and other quaternary ammonium ions is also well conserved. All three of the potassium channels considered here have a mouth that binds QA ions and that has a nearby hydrophobic pocket; the frog DR and the CaK channels also have a TEA+-specific site on the opposite side. The QA site might not be an obligatory feature of potassium channels, but rather a conserved evolutionary vestige.(ABSTRACT TRUNCATED AT 400 WORDS)