Voltage-gated calcium channels control key functions of excitable cells, like synaptic transmission in neurons and the contraction of heart and skeletal muscles. To accomplish such diverse functions, different calcium channels activate at different voltages and with distinct kinetics. To identify the molecular mechanisms governing specific voltage sensing properties, we combined structure modeling, mutagenesis, and electrophysiology to analyze the structures, free energy, and transition kinetics of the activated and resting states of two functionally distinct voltage sensing domains (VSDs) of the eukaryotic calcium channel CaV1.1. Both VSDs displayed the typical features of the sliding helix model; however, they greatly differed in ion-pair formation of the outer gating charges. Specifically, stabilization of the activated state enhanced the voltage dependence of activation, while stabilization of resting states slowed the kinetics. This mechanism provides a mechanistic model explaining how specific ion-pair formation in separate VSDs can realize the characteristic gating properties of voltage-gated cation channels.
Keywords: CaV1.1; molecular biophysics; molecular dynamics simulation; mouse; neuroscience; resting state structure; structural biology; voltage sensing; voltage-gated calcium channel.
Communication in our body runs on electricity. Between the exterior and interior of every living cell, there is a difference in electrical charge, or voltage. Rapid changes in this so-called membrane potential activate vital biological processes, ranging from muscle contraction to communication between nerve cells. Ion channels are cellular structures that maintain membrane potential and help ‘excitable’ cells like nerve and muscle cells produce electrical impulses. They are specialized proteins that form highly specific conduction pores in the cell surface. When open, these channels let charged particles (such as calcium ions) through, rapidly altering the electrical potential between the inside and outside the cell. To ensure proper control over this process, most ion channels open in response to specific stimuli, which is known as ‘gating’. For example, voltage-gated calcium channels contain charge-sensing domains that change shape and allow the channel to open once the membrane potential reaches a certain threshold. These channels play important roles in many tissues and, when mutated, can cause severe brain or muscle disease. Although the basic principle of voltage gating is well-known, the properties of individual voltage-gated calcium channels still vary. Different family members open at different voltage levels and at different speeds. Such fine-tuning is thought to be key to their diverse roles in various parts of the body, but the underlying mechanisms are still poorly understood. Here, Fernández-Quintero, El Ghaleb et al. set out to determine how this variation is achieved. The first step was to create a dynamic computer simulation showing the detailed structure of a mammalian voltage-gated calcium channel, called CaV1.1. The simulation was then used to predict the movements of the voltage sensing regions while the channel opened. The computer modelling experiments showed that although the voltage sensors looked superficially similar, they acted differently. The first of the four voltage sensors of the studied calcium channel controlled opening speed. This was driven by shifts in its configuration that caused oppositely charged parts of the protein to sequentially form and break molecular bonds; a process that takes time. In contrast, the fourth sensor, which set the voltage threshold at which the channel opened, did not form these sequential bonds and accordingly reacted fast. Experimental tests in muscle cells that had been engineered to produce channels with mutations in the sensors, confirmed these results. These findings shed new light on the molecular mechanisms that shape the activity of voltage-gated calcium channels. This knowledge will help us understand better how ion channels work, both in healthy tissue and in human disease.
© 2021, Fernández-Quintero et al.