Inherent Leakage of Hydrophobic Gating in BK Channels

The big potassium (BK) channels remain open with a small limiting probability of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>P</a:mi> <a:mi mathvariant="normal">o</a:mi> </a:msub> <a:mo>∼</a:mo> <a:msup> <a:mrow> <a:mn>10</a:mn> </a:mrow> <a:mrow> <a:mo>−</a:mo> <a:mn>7</a:mn> </a:mrow> </a:msup> </a:mrow> </a:math> at minimal <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mi mathvariant="normal">C</c:mi> <c:msup> <c:mrow> <c:mi mathvariant="normal">a</c:mi> </c:mrow> <c:mrow> <c:mn>2</c:mn> <c:mo>+</c:mo> </c:mrow> </c:msup> </c:mrow> </c:math> and negative voltages below <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mrow> <f:mo>−</f:mo> <f:mn>100</f:mn> <f:mspace width="0.28em"/> <f:mi>mV</f:mi> </f:mrow> </f:math> . The molecular origin of such an “intrinsic opening” is not understood. Here, free-energy analysis of <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:msup> <h:mrow> <h:mi mathvariant="normal">K</h:mi> </h:mrow> <h:mo>+</h:mo> </h:msup> </h:math> permeation shows that the intrinsic opening of BK channels is likely an inherent property of the vapor barrier, generated by hydrophobic dewetting of the inner pore in the <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"> <j:mrow> <j:mi mathvariant="normal">C</j:mi> <j:msup> <j:mrow> <j:mi mathvariant="normal">a</j:mi> </j:mrow> <j:mrow> <j:mn>2</j:mn> <j:mo>+</j:mo> </j:mrow> </j:msup> </j:mrow> </j:math> -free state. The vapor barrier only gives rise to a finite free-energy barrier, of <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mrow> <m:mo>∼</m:mo> <m:mn>8</m:mn> <m:mspace width="0.28em"/> <m:mi>kcal</m:mi> </m:mrow> </m:math> /mol, and it cannot completely shut down <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"> <o:msup> <o:mrow> <o:mi mathvariant="normal">K</o:mi> </o:mrow> <o:mo>+</o:mo> </o:msup> </o:math> flow even when the voltage sensor domains are fully deactivated. The resulting “leaking” currents can be measured at negative voltages as intrinsic opening. The shallow limiting slope of <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"> <q:msub> <q:mi>P</q:mi> <q:mi mathvariant="normal">o</q:mi> </q:msub> </q:math> at negative voltages arises primarily from the electric field effects on the permeating ion through the vapor barrier. We further demonstrate that the vapor barrier can be perturbed by truncation of the cytosolic domains, inner pore mutations, and ligand binding to the pore, leading to predicable changes in limiting slope measurements highly consistent with existing experimental data. Therefore, the intrinsic opening paves the way for direct experimental study of hydrophobic gating in BK and many other ion channels.