Linker-Gating Ring Complex as Passive Spring and Ca2+-Dependent Machine for a Voltage- and Ca2+-Activated Potassium Channel

Abstract

Ion channels are proteins that control the flux of ions across cell membranes by opening and closing (gating) their pores. It has been proposed that channels gated by internal agonists have an intracellular gating ring that extracts free energy from agonist binding to open the gates using linkers that directly connect the gating ring to the gates. Here we find for a voltage- and Ca(2+)-activated K+ (BK) channel that shortening the linkers increases channel activity and lengthening the linkers decreases channel activity, both in the presence and absence of intracellular Ca2+. These observations are consistent with a mechanical model in which the linker-gating ring complex forms a passive spring that applies force to the gates in the absence of Ca2+ to modulate the voltage-dependent gating. Adding Ca2+ then changes the force to further activate the channel. Both the passive and Ca(2+)-induced forces contribute to the gating of the channel.

Tuning the mechanosensitivity of a BK channel by changing the linker length

“…These results provide evidence consistent with the mechanical model recently proposed for the BK channel gating by Niu et al [27]. In this model, eight RCK domains form the gating ring, and the linker that connects the S6 gate with RCK1 domain acts as a passive spring, transducing the force to the Figure 7 Regulation of BK channel gating by membrane stretch based on the spring model proposed for BK channel gating by Niu et al [27]. The membrane stretch (Fm) was conveyed by a membrane-associated protein and STREX, and then transferred to the gate by the linker connecting the transmembrane segment 6 (S6) gate.…”

“…Furthermore, the SAKCaC was less sensitive to membrane stretch in the presence of relatively high intracellular Ca 2+ concentrations or membrane depolarization. These results provide evidence consistent with the mechanical model recently proposed for the BK channel gating by Niu et al [27]. In this model, eight RCK domains form the gating ring, and the linker that connects the S6 gate with RCK1 domain acts as a passive spring, transducing the force to the Figure 7 Regulation of BK channel gating by membrane stretch based on the spring model proposed for BK channel gating by Niu et al [27].…”

“…It is obvious that the P o was greatly increased by shortening the linker (-1), but decreased by lengthening the linker (+3). Similar results were reported previously in the Slo BK channels and the interpretation is that the linker-gating ring complex forms a passive spring to exert the force on the gate [27]. We also generated mutant subunits in which more than one residue was deleted or more than three residues were inserted in the linker.…”

“…Addition of amino acids into the linker leads to changes of linker length (∆L). gating ring may increase the P o by expanding the gating ring to pull the channel open [26,27,30]. Therefore, we conclude that the force generated by Ca 2+ or membrane stretch is transferred to the gate via a common pathway.…”

“…In the model based on the MthK channel structure, the pore-forming a subunit contains two regulator of K + conductance (RCK) domains in the intracellular C-terminus, and eight RCK domains from the tetrameric channel form a gating ring [25,26]. The linker that connects the S6 gate to the RCK domain and the gating ring form the linker-gating complex, which functions as a passive spring and transduces the force generated by intracellular Ca 2+ to the channel gate [27]. The STREX domain is located between RCK1 and RCK2 domains in the long intracellular C-terminus of SAKCaC (Figure 1) [19].…”