49
4
sive to voltage-gated ion channels, but they have also been found in other proteins that
respond to changes in membrane potential, e.g. voltage-sensitive phosphatases. Two
recent structural studies have elucidated the movement of helix S4 in response to changes
in the membrane potential in two different isolated voltage-sensing domains, one from
the Ciona intestinalis voltage-sensing phosphatase and one from a voltage-gated potassium channel (Li et al. 2014; Nozaki et al. 2016). In both cases, helix S4, which typically
contains positively charged amino acid residues (Lys and Arg), moves towards the exterior
side of the membrane in response to depolarization. During movement, it also rotates
around its own axis. This change in the position of helix S4 is suggested to induce pore
opening in channel proteins.
4.3.2 Voltage-Gated Na + -Channels
Voltage-gated Na + -channels are overall similarly structured to voltage-gated K + -channels.
However, instead of being assembled by four independent subunits, they are formed by one
large protein that has four repetitive subdomains, which are structurally and functionally
equivalent to the monomers of the K + -channels. Each subdomain is thus composed of six
transmembrane helices (S1–S6). Large intracellular loops are formed between the four
subdomains. The single inactivation segment for these channels is constituted by a patch of
hydrophobic amino acids localized in the third intracellular loop, just behind helix S6 of
the third subdomain. The lining of the channels is made by helices S5 and S6 of each subdomain, and their intervening sequences constitute the selectivity pore. These regions
show extended sequence similarity with the domains that form the pore in non- gated K + -
channels. Structural investigation of voltage-gated Na + -channels had been difficult because
of the large size of these molecules. Their structure has been deduced by comparison with
voltage-gated potassium channels and from functional and mutational analysis of neurotoxin binding, including tetrodotoxin and saxitoxin, which plug the pores of these channels. In this way also the S4 helix has been indicated as voltage sensor in Na + -channels, and
experimental evidence from mutagenesis studies has supported this function (see later)
(Catterall 2000). In 2017, a cryo electron microscopic structure of a putative voltage-gated
sodium channel from the American cockroach became available (Shen et al. 2017). In this
study, the impact of the voltage-dependent conformational change of helix S4 within the
voltage-sensing domain on interactions with pore-opening regions of the molecule is demonstrated. They indicate coupling of the voltage-sensing mechanism to the move of poreforming helices in between adjacent repeats of the channel protein (see . Fig. 4.2b).
The proteins described above constitute the α-subunits of Na + -channels that associate
with one or two β-subunits in mammalian cells. The α-subunits are sufficient to form the
functional voltage-responsive ion channel, and β-subunits are involved in modulation of
channel parameters. In humans, nine Na + -channel α-subunits are known, named as
Na v 1.1–Na v 1.9 and Na x . They are expressed abundantly in the central and peripheral nervous system, in dorsal root ganglia, Schwann cells and astrocytes. Some heritable human
disorders, so-called channelopathies, are associated with these proteins. These include
several kinds of epilepsy, familial hemiplegic migraine, autism spectrum disorder
and channelopathy-associated insensitivity to pain. Na v 1.4 is specifically expressed in the
skeletal muscle, and mutations in this gene are connected with potassium-aggravated
myotonia. Na v 1.5 is, among other organs, expressed in gastrointestinal smooth muscle
cells and associated with irritable bowel syndrome (Huang et al. 2017).
4.3 · Voltage-Gated Ion Channels
4
sive to voltage-gated ion channels, but they have also been found in other proteins that
respond to changes in membrane potential, e.g. voltage-sensitive phosphatases. Two
recent structural studies have elucidated the movement of helix S4 in response to changes
in the membrane potential in two different isolated voltage-sensing domains, one from
the Ciona intestinalis voltage-sensing phosphatase and one from a voltage-gated potassium channel (Li et al. 2014; Nozaki et al. 2016). In both cases, helix S4, which typically
contains positively charged amino acid residues (Lys and Arg), moves towards the exterior
side of the membrane in response to depolarization. During movement, it also rotates
around its own axis. This change in the position of helix S4 is suggested to induce pore
opening in channel proteins.
4.3.2 Voltage-Gated Na + -Channels
Voltage-gated Na + -channels are overall similarly structured to voltage-gated K + -channels.
However, instead of being assembled by four independent subunits, they are formed by one
large protein that has four repetitive subdomains, which are structurally and functionally
equivalent to the monomers of the K + -channels. Each subdomain is thus composed of six
transmembrane helices (S1–S6). Large intracellular loops are formed between the four
subdomains. The single inactivation segment for these channels is constituted by a patch of
hydrophobic amino acids localized in the third intracellular loop, just behind helix S6 of
the third subdomain. The lining of the channels is made by helices S5 and S6 of each subdomain, and their intervening sequences constitute the selectivity pore. These regions
show extended sequence similarity with the domains that form the pore in non- gated K + -
channels. Structural investigation of voltage-gated Na + -channels had been difficult because
of the large size of these molecules. Their structure has been deduced by comparison with
voltage-gated potassium channels and from functional and mutational analysis of neurotoxin binding, including tetrodotoxin and saxitoxin, which plug the pores of these channels. In this way also the S4 helix has been indicated as voltage sensor in Na + -channels, and
experimental evidence from mutagenesis studies has supported this function (see later)
(Catterall 2000). In 2017, a cryo electron microscopic structure of a putative voltage-gated
sodium channel from the American cockroach became available (Shen et al. 2017). In this
study, the impact of the voltage-dependent conformational change of helix S4 within the
voltage-sensing domain on interactions with pore-opening regions of the molecule is demonstrated. They indicate coupling of the voltage-sensing mechanism to the move of poreforming helices in between adjacent repeats of the channel protein (see . Fig. 4.2b).
The proteins described above constitute the α-subunits of Na + -channels that associate
with one or two β-subunits in mammalian cells. The α-subunits are sufficient to form the
functional voltage-responsive ion channel, and β-subunits are involved in modulation of
channel parameters. In humans, nine Na + -channel α-subunits are known, named as
Na v 1.1–Na v 1.9 and Na x . They are expressed abundantly in the central and peripheral nervous system, in dorsal root ganglia, Schwann cells and astrocytes. Some heritable human
disorders, so-called channelopathies, are associated with these proteins. These include
several kinds of epilepsy, familial hemiplegic migraine, autism spectrum disorder
and channelopathy-associated insensitivity to pain. Na v 1.4 is specifically expressed in the
skeletal muscle, and mutations in this gene are connected with potassium-aggravated
myotonia. Na v 1.5 is, among other organs, expressed in gastrointestinal smooth muscle
cells and associated with irritable bowel syndrome (Huang et al. 2017).
4.3 · Voltage-Gated Ion Channels