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Wang Z, Wong NC, Cheng Y, Kehl SJ, Fedida D: Control of voltage-gated K+ channel permeability to NMDG+ by a residue at the outer pore. J Gen Physiol. 2009 Apr;133(4):361-74.
Crystal structures of potassium (K (+)) channels reveal that the selectivity filter, the narrow portion of the pore, is only approximately 3-A wide and buttressed from behind, so that its ability to expand is highly constrained, and the permeation of molecules larger than Rb (+) (2.96 A in diameter) is prevented. N-methyl-d-glucamine (NMDG (+)), an organic monovalent cation, is thought to be a blocker of Kv channels, as it is much larger (approximately 7.3 A in mean diameter) than K (+) (2.66 A in diameter). However, in the absence of K (+), significant NMDG (+) currents could be recorded from human embryonic kidney cells expressing Kv3.1 or Kv3.2b channels and Kv1.5 R487Y/V, but not wild-type channels. Inward currents were much larger than outward currents due to the presence of intracellular Mg (2+) (1 mM), which blocked the outward NMDG (+) current, resulting in a strong inward rectification. The NMDG (+) current was inhibited by extracellular 4-aminopyridine (5 mM) or tetraethylammonium (10 mM), and largely eliminated in Kv3.2b by an S6 mutation that prevents the channel from opening (P468W) and by a pore helix mutation in Kv1.5 R487Y (W472F) that inactivates the channel at rest. These data indicate that NMDG (+) passes through the open ion-conducting pore and suggest a very flexible nature of the selectivity filter itself. 0.3 or 1 mM K (+) added to the external NMDG (+) solution positively shifted the reversal potential by approximately 16 or 31 mV, respectively, giving a permeability ratio for K (+) over NMDG (+) (P (K)(+)/P (NMDG)(+)) of approximately 240. Reversal potential shifts in mixtures of K (+) and NMDG (+) are in accordance with P (K)(+)/P (NMDG)(+), indicating that the ions compete for permeation and suggesting that NMDG (+) passes through the open state. Comparison of the outer pore regions of Kv3 and Kv1.5 channels identified an Arg residue in Kv1.5 that is replaced by a Tyr in Kv3 channels. Substituting R with Y or V allowed Kv1.5 channels to conduct NMDG (+), suggesting a regulation by this outer pore residue of Kv channel flexibility and, as a result, permeability. |
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