The chemical basis for electrical signaling

Abstract

Electrical signals generated by minute currents of ions moving across cell membranes are central to all rapid processes in biology. Initiation and propagation of electrical signals requires voltage-gated sodium (Na_V) and calcium (Ca_V) channels. These channels contain a tetramer of membrane-bound subunits or domains comprising a voltage sensor and a pore module. Voltage-dependent activation occurs as membrane depolarization drives outward movements of positive gating changes in the voltage sensor via a sliding-helix mechanism, which leads to a conformational change in the pore module that opens its intracellular activation gate. A unique negatively charged site in the selectivity filter conducts hydrated Na⁺ or Ca²⁺ rapidly and selectively. Ion conductance is terminated by voltage-dependent inactivation, which causes asymmetric pore collapse. This Review focuses on recent advances in structure and function of Na_V and Ca_V channels that expand our current understanding of the chemical basis for electrical signaling mechanisms conserved from bacteria to humans.

Figure 1. Overall architecture of voltage-gated sodium, calcium, and potassium channels

a, Model of representative Na_V, Ca_V, and K_V channels in a lipid membrane. From left to right: bacterial Na_VAb (cyan), mammalian Ca_V1.1 complex with auxiliary subunits (yellow), and mammalian K_V1.2/2.1 chimera complex with cytoplasmic β subunits (green). b, Structure of the bacterial sodium channel Na_VAb single subunit. The structure comprises the voltage-sensing module (S1–S4) connected to the pore module (S5–S6) via the S4–S5 linker. The P loop connects the S5 and S6 segments and contains the ion selectivity filter motif (Fig. 5). c, Na_VAb homotetramer with domain swapping illustrated by different color for each subunit. The pore is located at the center of the tetramer with the voltage-sensing module interacting with the pore module of the neighboring subunit. d, Single particle cryo-EM reconstruction of Ca_V1.1 with Na_VAb-like core α1 subunit highlighted in yellow and the α1 C-terminal domain in pink. Auxiliary α2δ, β, and γ subunits are colored in green, cyan, and magenta, respectively. Black lines depict membrane boundaries. The C-terminal domain of Ca_V1 channels is exceptionally large, and only partially resolved in the cryo-EM structure (Fig. 1d). The structure reveals an unexpected interaction between the intracellular domain III–IV linker and the intracellular C-terminal domain of the α1 subunit. The domain III–IV linker serves as the fast-inactivation gate in eukaryotic sodium channels (see below), and structure-function studies suggest that it also interacts with the C-terminal domain^,. The functional significance of this conserved interaction in Ca_V1 channels is unknown.

Figure 2. Structure of the voltage-sensing module of NavAb

The TM helices are colored from S1 to S4 segments in blue to red spectrum. Side chains of the extracellular and intracellular negative charge cluster (ENC and INC) amino acids are highlighted in red and side chains of the positive gating charge Arg residues (R1–R4) in blue. The hydrophobic constriction site (HCS) is shown in green. The aqueous cleft can be seen between the S1–S2 and S3–S4 hairpins from the overlaying semi-transparent surface.

Figure 3. Structural models of resting and activated states of voltage-sensing modules

a, Structural models of voltage-sensing module of NaChBac from Rosetta Membrane computational modeling. The Arg gating charges on S4 moves outward from the most resting state (Resting 1) to the most activated state (Activated 3) via several intermediates, passing through the HCS and exchanging their interactions with different INC and ENC side chains. b, Structures of voltage-sensing modules from available X-ray crystallographic structures of voltage-sensitive ion channels and enzyme. From left to right: domain 2 (resting state) of plant TPC1; resting state of Ci-VSP; activated state of Ci-VSP; activated state of Na_VAb; inactivated state of Na_VRh. Same color scheme as Fig. 2 is used.

Figure 4. Structural model of conformational changes during channel activation and pore opening

a, Pore-opening conformational change between the closed pore of Na_VAb and the open pore of Na_VMs. Na_VAb structure (left, cyan) contains two pore constriction sites (CS1 and CS2) with CS2 site completely sealing the pore. Na_VMs structure (right, orange) contains one pore constriction site (CS1) that remains open to allow hydrated sodium ion to pass through. b, Superposition of Na_VAb and Na_VMs pores viewed from the intracellular side. A counterclockwise twisting motion of the S6 segment from the closed pore of Na_VAb to the open pore of Na_VMs shifts the end of S6 helix outward to dilate the pore diameter. c, Channel activation involves clockwise rotation of the voltage sensor around the pore. Top left: Superposition of TPC1 (light purple for activated state domain I (DI) and dark purple for resting state domain II (DII)) and Na_VAb (cyan) structures. Top right: Superposition of Na_VRh (yellow) and Na_VAb (cyan) structures. Bottom: Overlay of voltage-sensing modules in TPC1, Na_VAb, and Na_VRh as in the top panel but with the pore modules of TPC1 and Na_VRh omitted for clarity. The voltage-sensing module progressively rotates around the pore from the most resting state in TPC1 DII to increasingly activated states in Na_VAb, TPC1 DI, and Na_VRh.

Figure 5. Chemical mechanism of ion permeation and selectivity of Na_V and Ca_V channels with structural models of their ion selectivity filters with ions bound

a, Na⁺ selectivity filter (TLESWSM) in Na_VAb. b, Representative conformations of Na⁺ selectivity filter from molecular dynamic simulations of sodium permeation in Na_VAb. Conformational dunking of Glu side chain of the high-field strength site allows direct coordination of Na⁺ ions. c, Ca²⁺ selectivity filter (TLDDWSN) in Ca_VAb. d, Hydrated Ca²⁺ bound in the Ca_VAb selectivity filter. e, Ca²⁺ selectivity filter of Ca_V1.1 from Domains I (TMEGWTD) and III (TFEGWPQ). f, Ca²⁺ selectivity filter of Ca_V1.1 from Domains II (TGEDWNS) and IV (TGEAWQE). Na⁺ (purple) and Ca²⁺ (green) ions are shown with semi-transparent ionic sphere. Dash lines indicate network of interactions among coordinated water molecules with the ions and protein atoms from high-filed strength site (Glu in Na_VAb and Ca_V1.1, and Asp in Ca_VAb, underlined) and backbone carbonyls of Leu and Thr. For clarity, only two opposing subunits in the tetramer are shown. Of note, a distantly related non-voltage-gated Ca²⁺ channel has a different architecture of its outer pore with Asp residues from each subunit directly binding dehydrated Ca2+ in a closed state structure. The significance of this binding mode in ion conductance is unknown.

Figure 6. Conformational changes in the pore associated with slow inactivation

a, An extracellular view of the pore through the selectivity filter. The selectivity filter collapses from a four-fold symmetric shape in pre-open Na_VAb (left) to an oval shape in inactivated Na_VAb (middle) to a completely closed pore in inactivated Na_VRh (right). b, An intracellular view of the pore at the C-termini of the S6 segments. The pore distorts from a square shape in the pre-open Na_VAb structure to a parallelogram shape in the inactivated Na_VAb and Na_VRh structures.