Single-channel kinetics of BK (Slo1) channels

Abstract

Single-channel kinetics has proven a powerful tool to reveal information about the gating mechanisms that control the opening and closing of ion channels. This introductory review focuses on the gating of large conductance Ca2+- and voltage-activated K+ (BK or Slo1) channels at the single-channel level. It starts with single-channel current records and progresses to presentation and analysis of single-channel data and the development of gating mechanisms in terms of discrete state Markov (DSM) models. The DSM models are formulated in terms of the tetrameric modular structure of BK channels, consisting of a central transmembrane pore-gate domain (PGD) attached to four surrounding transmembrane voltage sensing domains (VSD) and a large intracellular cytosolic domain (CTD), also referred to as the gating ring. The modular structure and data analysis shows that the Ca2+ and voltage dependent gating considered separately can each be approximated by 10-state two-tiered models with five closed states on the upper tier and five open states on the lower tier. The modular structure and joint Ca2+ and voltage dependent gating are consistent with a 50 state two-tiered model with 25 closed states on the upper tier and 25 open states on the lower tier. Adding an additional tier of brief closed (flicker states) to the 10-state or 50-state models improved the description of the gating. For fixed experimental conditions a channel would gate in only a subset of the potential number of states. The detected number of states and the correlations between adjacent interval durations are consistent with the tiered models. The examined models can account for the single-channel kinetics and the bursting behavior of gating. Ca2+ and voltage activate BK channels by predominantly increasing the effective opening rate of the channel with a smaller decrease in the effective closing rate. Ca2+ and depolarization thus activate by mainly destabilizing the closed states.

BK channels: multiple sensors, one activation gate

“…The VSD and CTD in BK channels sense voltage and intracellular signaling molecules and open the channel gate in the PGD by electromechanical and chemomechanical couplings between the PGD and the VSD and CTD, respectively (Figure 1C ). Voltage and Ca 2+ activate BK channels mainly by destabilizing the closed state with small effects on stabilizing the activated state of the PGD (Geng and Magleby, 2014 ). Various allosteric models have been developed to describe Ca 2+ - and voltage-dependent BK channel gating based on the analysis of single channel kinetics and macroscopic current (McManus and Magleby, 1991 ; Cox et al, 1997 ; Horrigan et al, 1999 ; Rothberg and Magleby, 1999 , 2000 ; Cui and Aldrich, 2000 ; Horrigan and Aldrich, 2002 ; Shelley et al, 2010 ).…”

Coupling of Ca2+and voltage activation in BK channels through the αB helix/voltage sensor interface

“…Large conductance Ca 2+ -and voltage-activated K + channels (BK, Slo1, Maxi-K, KCa1.1, KCNMA1 gene) are widely distributed and opened by the synergistic action of depolarization and intracellular Ca 2+ (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17), with intracellular Mg 2+ modulating the activation (18,19). Open BK channels allow the outward movement of intracellular K + , which drives the membrane potential in a negative direction, decreasing the excitability of cells by deactivating voltage dependent Na + and Ca 2+ channels.…”

“…Defects in BK channels can lead to epilepsy and dyskinesia (24,25), hypertension (23), and contribute to human obesity (26). The voltage-and Ca 2+ -dependent gating of BK channels can be described by two-tiered allosteric gating mechanisms formulated as discrete state models (5,11), and also by more general models (6), including the modulation by Mg 2+ (18,27,28). However, less is understood about the specific allosteric pathways involved in voltage and Ca 2+ activation of the channel.…”

“…It is known that the BK channel is of modular design, with the voltage sensors and pore gate forming the transmembrane domain of the channel (TMD), and the large cytosolic domain (CTD) containing the Ca 2+ sensors (3,4,(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17). Removing the CTD removes all Ca 2+ sensitivity (29), leaving a voltage gated channel (29,30).…”

Ion-channel mechanisms revealed

“…Slo1 channels pass through multiple states over time during gating. These kinetic states are typically attributed to the time that the channel spends in different structural conformations 10 . Hite et al observed four different structures for Slo1 at zero millivolts and without Ca 2+ or Mg 2+ -conditions in which the channels would be expected to be closed, but transitioning between a subset of conformational states.…”