Coupling between Voltage Sensor Activation, Ca2+ Binding and Channel Opening in Large Conductance (BK) Potassium Channels

Abstract

To determine how intracellular Ca2+ and membrane voltage regulate the gating of large conductance Ca2+-activated K+ (BK) channels, we examined the steady-state and kinetic properties of mSlo1 ionic and gating currents in the presence and absence of Ca2+ over a wide range of voltage. The activation of unliganded mSlo1 channels can be accounted for by allosteric coupling between voltage sensor activation and the closed (C) to open (O) conformational change (Horrigan, F.T., and R.W. Aldrich. 1999. J. Gen. Physiol. 114:305–336; Horrigan, F.T., J. Cui, and R.W. Aldrich. 1999. J. Gen. Physiol. 114:277–304). In 0 Ca2+, the steady-state gating charge-voltage (QSS-V) relationship is shallower and shifted to more negative voltages than the conductance-voltage (GK-V) relationship. Calcium alters the relationship between Q-V and G-V, shifting both to more negative voltages such that they almost superimpose in 70 μM Ca2+. This change reflects a differential effect of Ca2+ on voltage sensor activation and channel opening. Ca2+ has only a small effect on the fast component of ON gating current, indicating that Ca2+ binding has little effect on voltage sensor activation when channels are closed. In contrast, open probability measured at very negative voltages (less than −80 mV) increases more than 1,000-fold in 70 μM Ca2+, demonstrating that Ca2+ increases the C-O equilibrium constant under conditions where voltage sensors are not activated. Thus, Ca2+ binding and voltage sensor activation act almost independently, to enhance channel opening. This dual-allosteric mechanism can reproduce the steady-state behavior of mSlo1 over a wide range of conditions, with the assumption that activation of individual Ca2+ sensors or voltage sensors additively affect the energy of the C-O transition and that a weak interaction between Ca2+ sensors and voltage sensors occurs independent of channel opening. By contrast, macroscopic IK kinetics indicate that Ca2+ and voltage dependencies of C-O transition rates are complex, leading us to propose that the C-O conformational change may be described by a complex energy landscape.

Large conductance Ca2+-activated K+ (BK) channel: Activation by Ca2+ and voltage

“…In the allosteric model described in Figure 6A, for each voltage sensor activated, the equilibrium constant for channel opening, L, is multiply by an allosteric factor D, so the opening process is facilitated as more voltage sensors are activated. The observation that even when all voltage sensor are resting, P o can be increased by augmenting intracellular Ca 2+ (Horrigan and Aldrich, 2002) is the basis for postulating the allosteric kinetic model depicted in Figure 6B under the assumption that there is only one Ca 2+ -binding site per channel subunit. In this case, for each Ca 2+ -binding site occupied the equilibrium constant L is multiply by an allosteric factor C. Figures 5A and B define the key feature of BK channels:…”

“…If some allosteric coupling (E; Fig. 6D) between Ca 2+ binding and voltage sensor movement is included, the model increases to 70 states (Horrigan and Aldrich, 2002) and in several occasions, this has been the model of choice (e.g., Orio and Latorre, 2005). The beauty of the model is that it is possible to set experimental conditions to determine some of the different parameters unequivocally.…”

Linking Exponential Components to Kinetic States in Markov Models for Single-Channel Gating

“…Hence, the standard dogma is that it is not possible to place physical interpretations on the time constants and magnitudes of the exponential components (Colquhoun and Hawkes, 1995b) except in special cases with extreme differences in some of the rate constants (Colquhoun and Hawkes, 1994), although it should be mentioned that some information relating observed exponentials in experimental data to the underlying states can be obtained when the starting state is known, by examining either fi rst latencies to the next opening/shutting interval or the rise times of macroscopic currents following step changes in agonist concentration or voltage ( Edmonds and Colquhoun, 1992 ;Colquhoun et al, 1996;Wyllie et al, 1998 ;Horrigan and Aldrich, 2002 ).…”

The RCK1 high-affinity Ca 2+ sensor confers carbon monoxide sensitivity to Slo1 BK channels

“…Production of CO by HMOXs requires oxygen (1), and this oxygen dependence raises the possibility that changes in cellular oxygen tension regulate the Slo1 BK channel activity indirectly by regulating the availability of CO (16). HMOX-2, one member of the HMOX family, may colocalize with Slo1 to allow efficient modulation of the channel by CO according to the local oxygen level (16).Gating of the Slo1 BK channel involves allosteric interactions among the main pore gate, voltage sensor domains, and cytoplasmic RCK1 and RCK2 domains with multiple divalent cation sensors (17,18). Although increasing evidence suggests that the Slo1 BK channel is directly stimulated by CO without any requirement for a cGMP-dependent signaling pathway (8, 10), how the allosteric gating mechanism of the Slo1 BK channel is directly altered by CO has not been clearly elucidated.…”

“…Gating of the Slo1 BK channel involves allosteric interactions among the main pore gate, voltage sensor domains, and cytoplasmic RCK1 and RCK2 domains with multiple divalent cation sensors (17,18). Although increasing evidence suggests that the Slo1 BK channel is directly stimulated by CO without any requirement for a cGMP-dependent signaling pathway (8, 10), how the allosteric gating mechanism of the Slo1 BK channel is directly altered by CO has not been clearly elucidated.…”