Box Extension 12.1

Evolution and Molecular Function of Voltage-Gated Channels

Voltage-gated channels are amazing molecular mechanisms that make possible the functions of nervous systems. Recent molecular and genomic studies have suggested a sequence of steps in the evolution of voltage-gated channels, and have largely clarified the structural basis of their action. Box Extension 12.1 shows how voltage-gated channels are thought to have evolved, and how their critical features—ion selectivity and voltage gating—work at the molecular level.

Evolution of voltage-gated channels

Voltage-gated channels have striking sequence homology and overall structural similarity. Genomic studies show their evolutionary relationship and suggest the sequence of their evolutionary divergence. There are many members of the voltage-gated channel superfamily, a few of which we consider here (Figure A).

Figure A Hypothetical sequence in the evolution of voltage-gated channels 2TM, two transmembrane segments; 6TM, six transmembrane segments. (After Kandel et al. 2000.)

K+ channels are thought to have evolved first. Primitive ion channels may have resembled bacterial K+ channels, which consist of four subunits, each with two transmembrane segments (2TM; see Part 1 of the figure). Similar 2TM K+ channels occur widely, from bacteria to mammals, and are not voltage-gated.

K+ channels with six transmembrane segments (6TM; see Part 2) are activated by depolarization (i.e., are voltage-gated) and are present in animals, plants, fungi, and protists. The 2TM K+ subunit corresponds to segments 5 and 6 and the ion-selective P loop of the 6TM protein. The smaller K+ channel protein may have combined with another protein that contained the segment-4 gating region to make the channel voltage-dependent.

Ca2+ and Na+ channels are thought to have evolved from the 6TM K+ channel protein by gene duplications and successive mutations, leading to the 4 × 6TM structure of these channels (see Part 3). Other channel proteins apparently evolved from 6TM K+ subunits as well.

Some K+ channels can be inactivated, and their subunits have an extra portion at the NH2 terminal that acts as a “ball on a string” to close the channel’s inner end (see Part 4). Other K+ channels are activated by intracellular Ca2+ ions and have a Ca2+-binding site near the COOH terminal (see Part 5). Cyclic nucleotide–activated cation channels have a binding site for cyclic nucleotides (see Part 6), and mutations at the P loop make the channels unselective among cations.

Each of the channel proteins described is actually a family of channel proteins, coded by a family of genes that varies both within and among species. (For example, humans appear to have more than 100 gene members in the superfamily.) Increasing genomic information has allowed investigators to construct a tree of the probable evolutionary history of the many members of the voltage-gated channel superfamily (Figure B).

Figure B Molecular relationships in the voltage-gated channel superfamily The greatest diversity is among K+ channels, shown in red. These are composed of tetramers of four subunits, each with two to six membrane-spanning subunits (see Figure A). The smaller families of voltage-gated Na+ and Ca2+ channels are shown in blue. Cation channels of the TRP family, shown in green, play important roles in sensory transduction, as discussed in Chapter 14.  CNG—cyclic nucleotide-gated channel, HCN—hyperpolarization-activated cyclic nucleotide-gated channel, K2P—two-pore (leakage) potassium channel, Kir—inward rectifying potassium channel, TPC—two-pore channel.

How do voltage-gated channels work?

Two critical functions of voltage-gated channels are ion selectivity and voltage gating. Recent studies have substantially clarified how these functions work at the molecular level.

What is the structural basis of the ion selectivity of the channels? The ions Na+ and K+ are chemically similar monovalent cations (of the same electromotive series of the periodic table). How can one kind of channel be selectively permeable to Na+ and another be selectively permeable to K+? Actually, neither the Na+ channel nor the K+ channel is absolutely selective; both channels are slightly permeable to the “wrong” ion and are also permeable to other ions that are not normally present in organisms. The three-dimensional structure of the protein channels, determined by X-ray diffraction, reveals structural elements of the channels that determine their ion-selective properties.

Recall that ions in solution are normally hydrated—that is, surrounded by a shell of water molecules. The water molecules around a cation have their electronegative oxygen atoms facing inward toward the ion, held to it by charge attraction. Fully hydrated ions cannot pass through the pore, but the walls of the pore are lined by oxygen atoms that compete with the water molecules of the hydrated ion, minimizing the energy barrier to permeation.

The ion selectivity of K+ channels is more fully understood (Figure C). X-ray diffraction studies have clarified the three-dimensional structure of K+ channels and show that K+ ions completely lose their water of hydration in passing through the narrow pore of the K+ channel’s selectivity filter. There are four K+ sites in the selectivity filter, but typically only alternate sites are occupied. Entry of a K+ ion from the cytoplasm into the inner chamber of the channel displaces other ions by charge repulsion (see Box Figure C3c), “knocking” ions to the next site. Although the basis of Na+ exclusion is not completely clear, these studies are elucidating the molecular basis of an ion channel’s ability to be both highly permeable and highly selective.

Voltage-gated Na+ channels have a wider pore that K+ channels. The pore also contains a lining of oxygen atoms that can interact with Na+ ions, but the oxygen atoms are associated with different amino acids than in the K+ channel. Evidently Na+ ions lose some but not all of their inner shell of water molecules in passing through the pore of a Na+ channel, and the ion interaction with the channel favors Na+ rather than K+. Selectivity, then, depends on the pore being the right size with the right properties to interact with a specific ion species, allowing the ion channel to be both highly permeable and highly selective.

Figure C Ion permeation through K+ channels (a) (1) The structure of a bacterial K+ channel as determined by X-ray diffraction. The subunits correspond to membrane-spanning segments 5 and 6 and the P loop of voltage-gated channels (see Box Figure 12.1). The narrow ion-selective pore is lined by the P loop of each of the four subunits (yellow; only two of the four are visible here), and it has four sites that can be occupied by K+ ions (green spheres). An additional K+ ion can occupy an inner cavity below the selectivity filter. (2) Sites occupied by K+ ions in and near the selectivity filter. K+ ions are normally surrounded by polar water molecules, but in the pore of a potassium channel, oxygen atoms lining the pore compete with water molecules to attract the cation. The image shows a K+ ion in the inner chamber with eight water molecules around it (bottom), four unhydrated K+ ions at the selectivity filter sites, a K+ ion at the outer face of the pore, and a K+ ion with a partial shell of four water molecules (top). (3) A model of K+ ion permeation. This chain reaction allows the channel to be both highly selective and highly permeable to K+. (1 from Morais-Cabral et al. 2001; 2 from Zhou et al. 2001; 3 after Miller 2001.)

Finally, how does the voltage-gating mechanism work to open and close a channel? We noted earlier that membrane-spanning segment 4 is the gating portion of a voltage-gated channel. It contains four positively charged amino acids that respond to membrane depolarization, probably by sliding outward and inducing comformation changes in the adjacent segments 5 and 6 that line the pore. Structures of K+ and Na+ channels, analyzed by X-ray diffraction and other molecular techniques, suggest that the inner portion of segment 6 blocks the inner end of the channel in the closed conformation, and is bent or rotated to open the channel in response to depolarization. The animation, Box Figure D, presents the clearest contemporary model of the gating process.

Click to open animation in a new window.

Figure D Animation of a molecular model of opening and then closing of a voltage-gated Na+ channel A side view of part of the channel is shown, with the portion outside the cell at the top and the cytoplasmic side at the bottom. Helical portions of the channel are embedded in the cell membrane; only the pore region (membrane-spanning segments 5 and 6 of each of the four subunits) is shown. The four subunit portions are shown in different colors and are viewed first from the membrane bilayer, and then rotated 90° for a view from the intracellular surface. The channel is a prokaryotic voltage-gated Na+ channel, with substantial sequence homology to the channels of eukaryotic excitable cells.

References

Bagnéris, C., Naylor, Claire E., McCusker, Emily C., and B.A. Wallace. 2015. Structural model of the open–closed–inactivated cycle of prokaryotic voltage-gated sodium channels. J Gen Physiol. 145:5–16.

Catterall, W. A. 2012. Voltage-gated sodium channels at 60; structure, function, and pathophysiology. J. Physiol. 590: 2577–2589.

Miller, C. 2001. See potassium run. Nature 414: 23–24.

Pathak, M. M., V. Yarov-Yarovoy, G. Agarwal, B. Roux, P. Barth, S. Kohout, F. Tombola, and E. Y. Isacoff. 2007. Closing in on the resting state of the shaker K+ channel. Neuron 56: 124–140.

Yu, F. H., V. Yarov-Yarovoy, G. A. Gutman, and W. A. Catterall. 2005. Overview of molecular relationships in the voltage-gated channel superfamily. Pharmacol. Rev. 57: 387–395.

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