Forces and energetics of the canonical tetrameric cation channel gating

The canonical gating mechanism of tetrameric cation channels involves the spreading of the pore-lining helices at the so-called bundle-crossing gate. Despite a wealth of structural information, we lack a physical description of the gating process. Here, I took advantage of an entropic polymer stretching physical model and MthK structures to derive the forces and energies involved in pore-domain gating. In MthK, the Ca²⁺-induced conformational change in the RCK domain alone opens the bundle-crossing gate through pulling via unfolded linkers. In the open conformation, the linkers serve as entropic springs between the RCK domain and bundle-crossing gate that store an elastic potential energy of 3.6k_BT and exert 9.8 pN (piconewton) radial pulling force to keep the gate open. I further derive that the work to load the linkers to prime the channel for opening is up to 3.8k_BT, exerting up to 15.5 pN to pull the bundle-crossing open. Opening of the bundle-crossing leads to a release of 3.3k_BT spring potential energy. Thus, the closed/RCK-apo and the open/RCK-Ca²⁺ conformations are separated by a barrier of several k_BT. I discuss how these findings relate to the functional properties of MthK and suggest that given the architectural conservation of the helix-pore-loop-helix pore-domain among all tetrameric cation channels, these physical parameters might be quite general.