Abstract
Allosteric interactions between the voltage-sensing domain (VSD), the Ca2+-binding sites, and the pore domain govern the mammalian Ca2+- and voltage-activated K+ (BK) channel opening. However, the functional relevance of the crosstalk between the Ca2+- and voltage-sensing mechanisms on BK channel gating is still debated. We examined the energetic interaction between Ca2+ binding and VSD activation by investigating the effects of internal Ca2+ on BK channel gating currents. Our results indicate that Ca2+ sensor occupancy has a strong impact on VSD activation through a coordinated interaction mechanism in which Ca2+ binding to a single α-subunit affects all VSDs equally. Moreover, the two distinct high-affinity Ca2+-binding sites contained in the C-terminus domains, RCK1 and RCK2, contribute equally to decrease the free energy necessary to activate the VSD. We conclude that voltage-dependent gating and pore opening in BK channels is modulated to a great extent by the interaction between Ca2+ sensors and VSDs.
Highlights
Diverse cellular events involve calcium ions as a primary mediator in the signal transduction pathways triggering, among other signaling processes, Ca2+-activated conductances
We characterized the effects of Ca2+-binding on voltage sensor activation in BK channels by analyzing the gating currents measured in inside-out patches of Xenopus laevis oocyte membrane
Based on the functional independence of the distinct structural domains involved (PD, C-terminal domain (CTD), and voltage-sensing domain (VSD)), the energetic relationship between the sensory modules can be directly defined by comparing the change in the equilibrium of the voltage sensor under two extreme Ca2+ conditions: unliganded and saturated (Horrigan and Aldrich, 2002)
Summary
Diverse cellular events involve calcium ions as a primary mediator in the signal transduction pathways triggering, among other signaling processes, Ca2+-activated conductances. Since the BK channels are regulated by cytosolic Ca2+ and depolarizing voltages (Marty, 1981; Pallotta et al, 1981; Latorre et al, 1982), they are integrators of physiological stimuli involving intracellular Ca2+ elevation and membrane excitability. Functional BK channels are formed by homotetramers of a-subunits (Shen et al, 1994), each comprising a transmembrane voltage-sensing domain (VSD) and an intracellular Ca2+-sensing C-terminal domain (CTD) that can independently modulate the ion conduction gate in the pore domain (PD) (Latorre et al, 2017). The CTDs consist of two non-identical regulators of the conductance of K+ domains (RCK1 and RCK2) arranged in a ring-like tetrameric structure dubbed the gating ring (Wu et al, 2010; Yuan et al, 2010; Yuan et al, 2012; Hite et al., 2017; Tao et al, 2017). Each RCK domain contains distinct ligand-binding sites capable of detecting Ca2+ in the micromolar range (Schreiber and Salkoff, 1997; Bao et al, 2002; Xia et al, 2002)
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