BK channels: multiple sensors, one activation gate

Abstract

Ion transport across cell membranes is essential to cell communication and signaling. Passive ion transport is mediated by ion channels, membrane proteins that create ion conducting pores across cell membrane to allow ion flux down electrochemical gradient. Under physiological conditions, majority of ion channel pores are not constitutively open. Instead, structural region(s) within these pores breaks the continuity of the aqueous ion pathway, thereby serves as activation gate(s) to control ions flow in and out. To achieve spatially and temporally regulated ion flux in cells, many ion channels have evolved sensors to detect various environmental stimuli or the metabolic states of the cell and trigger global conformational changes, thereby dynamically operate the opening and closing of their activation gate. The sensors of ion channels can be broadly categorized as chemical sensors and physical sensors to respond to chemical (such as neural transmitters, nucleotides and ions) and physical (such as voltage, mechanical force and temperature) signals, respectively. With the rapidly growing structural and functional information of different types of ion channels, it is now critical to understand how ion channel sensors dynamically control their gates at molecular and atomic level. The voltage and Ca(2+) activated BK channels, a K(+) channel with an electrical sensor and multiple chemical sensors, provide a unique model system for us to understand how physical and chemical energy synergistically operate its activation gate.

Keywords: BK channels; allosteric gating; calcium binding proteins; ion channel gating; ion permeation; magnesium binding; modular organization; voltage sensor domain.

Figure 1

Structural domains of the large-conductance, Ca²⁺- and voltage-activated BK channels and their allosteric interactions during channel gating. (A) A BK channel can be divided into three structure domains: the pore-gate domain (PGD), the voltage sensor domain (VSD) and the cytosolic tail domain (CTD). Major elements for voltage and ion sensing (see text) are illustrated. (B) A homology model of BK channels based on the CTD structure of the zebra fish BK channel (PDB ID: 3U6N) and the membrane spanning domain of the Kv1.2–Kv2.1 chimera channel structure (PDB ID: 2R9R) by superimposing to the corresponding conserved regions of the MthK channel structure (PDB ID: 1LNQ) using UCSF Chimera software. Different structural domains are shown in colors as in A. (C) A general allosteric gating mechanism including allosteric interactions among three structure domains. C and O: closed and open conformations of PGD, respectively; L: the equilibrium constant for the C–O transition in the absence of voltage sensor activation and Ca²⁺ binding; R and A: resting and activated states of VSD; J: the equilibrium constant for VSD activation; K_C: equilibrium constant for ligand binding to closed channels; D, C, and E: allosteric factors describing the interaction between PGD-VSD, PGD-CTD, and VSD-CTD, respectively. (D) Macroscopic ionic current of BK channels in the absence and presence of 100 μM [Ca²⁺]_i. (E) Increasing [Ca²⁺]_i shifts the conductance-voltage (G–V) relation to more negative voltages. (F,G) In the absence of Ca²⁺, VSD can move in response to membrane voltage changes. (F) Gating current traces. Gating currents are generated due to the movement of the voltage sensor in the electric field across the membrane. (G) The voltage dependence of gating charge movement, the Q–V relation.

Figure 2

Ca²⁺ and Mg²⁺-dependent activation of BK channels. (A) The putative Ca²⁺ binding pocket in the RCK1 site (PDB ID: 3NAF). (B) The Ca²⁺ binding pocket in the Ca²⁺ bowl site (PDB ID: 3U6N). (C) Ca²⁺ binding changes the conformation of the cytosolic tail domain (CTD), which pulls the C-linker to open the activation gate of BK channels. The Ca²⁺-free (3NAF) and Ca²⁺ bound to the Ca²⁺ bowl (3U6N) CTD structures are shown in the left and right panels, respectively. One of the most dramatic Ca²⁺-induced conformational changes happens in the AC region (β A–αC, orange). The rest of the RCK1 domain is shown in blue and the RCK2 domain is shown in green. The bound Ca²⁺ in the Ca²⁺ bowl is shown as red dot in the right panel. (D) The low affinity Mg²⁺ binding site is composed of D99 and N172 in the voltage sensory domain (VSD) and E374 and E399 in the CTD. Magenta and cyan of these residues illustrate that D99/N172 and E374/E399 are from neighboring Slo1 subunit. (E) Mg²⁺ binds to the interface of the VSD and the CTD to activate BK channels through electrostatic interaction with the voltage sensor. The red + sign in S4 represents the major voltage sensing residue R213.

Figure 3

The cytosolic tail domain (CTD) serves as chemical sensors for BK channels. The putative binding sites for different ligands are labeled with colors. RCK1 domain and RCK2 domain are shown in dark gray and light gray, respectively. PDB ID: 3U6N.

Figure 4

Assembly of the BK channel structural domains. (A) Sequence alignment of the inner helices and the C-linkers of different K⁺ channels. Residues that can increase peptide flexibility and alter α helix orientation (i.e., Glycine and Proline) are highlighted in yellow. (B) The C-termini of inner helix and the C-linkers of different K⁺ channels may point to different directions. The PGDs of MthK (PDB ID: 1LNQ), Kv1.2 (PDB ID: 2R9R) and GsuK (PDB ID: 4GX5) channels are superimposed at the selectivity filter by using UCSF Chimera. (C) The relative assembly of the VSD and the CTD in BK channels might be different from the homology model shown in Figure 1B with a relative ~90° angular rotation about the central axis between the PGD and CTD.