Decoupling between Voltage Sensor Movement and Pore Opening of Kv2.1 Channels

Abstract

The relationship between membrane potential and the conductance of Kv2-type voltage gated K+ channels is modulated by cells. To begin studies into the mechanistic underpinnings of coupling between voltage sensing and K+ conductance, we are developing transition state theory kinetic models of coupling between voltage sensor movement and pore opening. We have constrained these gating models with voltage-clamp recordings of the dynamics of rat Kv2.1 channels expressed in CHO-K1 cells. Consistent with prior reports, our models suggest that Kv2.1 gating consists of fast voltage sensor movement, and a slower pore opening step that has little inherent voltage dependence. We found that the activation kinetics of Kv2.1 conductance are influenced by the probability of voltage sensor activation, and a concerted pore opening step is rate limiting. The degree of sigmoid delay of activation kinetics in response to voltage steps predicts the kinetics of a fast component of gating current. A relatively slow pore closing step with little voltage dependence leads to voltage insensitive deactivation kinetics over a range of membrane potentials. In response to sufficiently negative membrane potentials, deactivation voltage dependence steepens. This change of voltage dependence requires an alternate route of exit from the open state. Modeling predicts that open channels subject to sufficiently negative voltages enter a short-lived state where voltage sensors have deactivated before the concerted pore closing conformational change occurs. This transient decoupling of voltage sensor movement from pore closing allowed estimation of the coupling energy between voltage sensor and pore conformational changes in Kv2.1 channels.