Abstract
Voltage sensors (VSs) initiate the pore opening and closure in voltage-gated ion channels. Here, we propose a technique for estimation of the equilibrium constant of the up- and downward VS movements and rate constants of pore transitions from macroscopic current kinetics. Bell-shaped voltage dependence of the activation/deactivation time constants and Bolzmann distributions of CaV1.2 activation were analyzed in terms of a circular four-state (rest, activated, open, deactivated) channel model: both dependencies uniquely constrain the model parameters. Neutralization of gating charges in IS4 or IIS4 only slightly affects the equilibrium constant of VS transition while affecting simultaneously the rate constants of pore opening and closure. The application of our technique revealed that pore mutations on IS6–IVS6 segments induce pronounced shifts of the VS equilibrium between the resting (down) and activated (up) position. Analyzing a channelopathy mutation highlighted that the leftward shift of the activation curve induced by I781T on IIS6 is only partially (35 %) caused by a destabilization of the channel pore but predominantly (65 %) by a shifted VS equilibrium towards activation. The algorithm proposed for CaV1.2 may be applicable for calculating rate constants from macroscopic current kinetics in other voltage-gated ion channels.Electronic supplementary materialThe online version of this article (doi:10.1007/s00424-013-1319-8) contains supplementary material, which is available to authorized users.
Highlights
Hodgkin and Huxley [17] hypothesized that changes in the membrane voltages move charged particles, leading to channel opening
Our analysis revealed that mutations in the channel pore shift the Voltage sensors (VSs) distribution from rest to the activated state
The entire molecule dwells in 2×2=4 states: the pore is closed and the voltage-sensing mechanism locks the pore (R); the voltage-sensing mechanism is activated and releases the pore (A), which, remains closed; the pore is open (O); and the deactivated voltage-sensing mechanism is in the down position while the pore is still open (D)
Summary
Hodgkin and Huxley [17] hypothesized that changes in the membrane voltages move charged particles (gating particles), leading to channel opening. This charge movement was later directly measured as gating current in several ion channels [1, 7]. In analogy to other voltage-gated channels, it is assumed that the movement of charge-carrying S4 segments initiates the opening and closure of the activation gates, which are formed by the Pflugers Arch - Eur J Physiol (2014) 466:265–274 intracellular thirds of the S6 segments [11, 19]. A fundamental and still unanswered question is how these movements of charged residues are mechanically coupled to opening and closure of the channel pore
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