Allosteric Gating of a Large Conductance Ca-activated K+ Channel

Abstract

Large-conductance Ca-activated potassium channels (BK channels) are uniquely sensitive to both membrane potential and intracellular Ca2+. Recent work has demonstrated that in the gating of these channels there are voltage-sensitive steps that are separate from Ca2+ binding steps. Based on this result and the macroscopic steady state and kinetic properties of the cloned BK channel mslo, we have recently proposed a general kinetic scheme to describe the interaction between voltage and Ca2+ in the gating of the mslo channel (Cui, J., D.H. Cox, and R.W. Aldrich. 1997. J. Gen. Physiol. In press.). This scheme supposes that the channel exists in two main conformations, closed and open. The conformational change between closed and open is voltage dependent. Ca2+ binds to both the closed and open conformations, but on average binds more tightly to the open conformation and thereby promotes channel opening. Here we describe the basic properties of models of this form and test their ability to mimic mslo macroscopic steady state and kinetic behavior. The simplest form of this scheme corresponds to a voltage-dependent version of the Monod-Wyman-Changeux (MWC) model of allosteric proteins. The success of voltage-dependent MWC models in describing many aspects of mslo gating suggests that these channels may share a common molecular mechanism with other allosteric proteins whose behaviors have been modeled using the MWC formalism. We also demonstrate how this scheme can arise as a simplification of a more complex scheme that is based on the premise that the channel is a homotetramer with a single Ca2+ binding site and a single voltage sensor in each subunit. Aspects of the mslo data not well fitted by the simplified scheme will likely be better accounted for by this more general scheme. The kinetic schemes discussed in this paper may be useful in interpreting the effects of BK channel modifications or mutations.

Calmodulin Mediates Calcium-dependent Activation of the Intermediate Conductance KCa Channel,IKCa1

“…All four subunits must be activated before the channel opens. Although speculative, the kinetic diagram is similar to previous proposals for a variety of K Ca channels based upon single-channel data (43)(44)(45). These schemes predict that the steep Hill coefficient determined in functional measurements of the Ca 2ϩ sensitivity of channel opening arises from the requirement for Ca 2ϩ -induced conformational changes by each of four subunits in BK Ca , SK Ca , and, as proposed here, IK Ca channels.…”

Stepwise contribution of each subunit to the cooperative activation of BK channels by Ca 2+

“…Such information could place constraints on which types of gating mechanisms are consistent with the observed cooperativity and indicate at which steps in the activation process the cooperative interactions occur. Considerable insight into the mechanism of cooperativity has been obtained by detailed kinetic analysis, which has suggested that the activation of BK channels by Ca 2ϩ and by voltage occurs through separate allosteric mechanisms that modulate the channel opening-closing transitions independently of one another (10,17,18,(20)(21)(22)(23)(24)(25). However, because the gating of BK channels is so complex, it has been necessary in these studies to either make simplifying assumptions about the particular types of the cooperative interactions (10,17,18,(20)(21)(22)25) or to analyze simplified models with fewer assumptions (7,23,24).…”

α5β1 Integrin Engagement Increases Large Conductance, Ca2+-activated K+ Channel Current and Ca2+ Sensitivity through c-src-mediated Channel Phosphorylation

“…Currents recorded in 10 and 100 M Ca 2ϩ did not have leak subtraction performed. Typically, at each Ca 2ϩ concentration, two to three families of current traces were recorded consecutively and averaged before analysis (21). All experiments were carried out at room temperature.…”

“…The levels of free Ca 2ϩ in the other solutions were confirmed using a calcium electrode (Orion model 93-20, WPI, Sarasota, FL). 50 M of the divalent cation chelator (ϩ)-18-crown-6-tetracarboxylic acid was present in the bath solution to prevent divalent cation block at high voltages (21,22). The recording chamber (ϳ0.3-ml volume) was perfused at a constant rate of ϳ1 ml/min using a ValveLink TM 16 solenoid bank (AutoMate Scientific, Inc., NY) to switch between various solutions.…”