85
6
Α-scorpion-, β-scorpion- and some spider-toxins constitute a group of peptide neurotoxins. They contain 60–70 amino acids and bind to extracellular loops of the channel
(sites 3 and 4, . Fig. 4.2b). This changes the voltage-dependent gating of the channels
and slows down their inactivation. Lipid-soluble toxins, including brevetoxins and
ciguatoxins, bind to transmembrane segments of the channel (site 5). They cause a shift
in activation gating and lead to repeated firing of neurons. These polyether compounds
are produced by dinoflagellates (Karenia spec and Chattonella) and also contribute to
neurotoxic shellfish poisoning during Karenia blooms, e.g. in the Gulf of Mexico
(Watkins et al. 2008).
VOCs are also affected by a number of animal toxins. The P/Q type channel is sensitive
to Ω-agatoxin, a poison of the spider Agelenopsis that paralyses the spiders pray (American
grass spider). N-type channels are blocked by ω-conotoxin, a poison produced by deepwater snails (Conus geographicus). R-type channels are expressed in brain regions responsible
for pain transmission and in the amygdala, a brain region involved in fear response. They
are sensitive to some tarantula venoms, e.g. SNX-482, a peptide with 41 amino acids from
Hysterocrates gigas, the giant baboon spider.
The tarantula toxin guangxitoxin (GxTX) belongs to a class of spider venoms comprising small peptides with a characteristic cysteine knot fold that is stabilized by disulphide bridges. They act on voltage-gated Ca 2+ -, K + - and Na + -channels. Their affinity
depends on the channel conformation displaying either open or closed states. This varies
from 2 nM for the inactive (closed) channel to 200 nM for the activated (open) channel
conformation. GxTX specifically targets K v 2.1-potassium channels. Thus, fluorescently
labelled derivatives of GxTX have been used to report receptor activity of a K v 2.1potassium channel. Fluorescence was measured when the toxin was bound to a closed
channel. It decayed when the channel opened as the toxin lost affinity and dissociated.
This was reversible, after repolarization of the cell, fluorescence could be recovered
(Tilley et al. 2014).
6.2 Nicotinic Acetylcholine Receptor (NAchR): Nicotine
The first ligand-gated ion channel investigated at the molecular level was the NAchR. It
belongs to a large family of pentameric ligand-gated ion channels (pLICs) (Lynagh and
Pless 2014). These channel proteins are composed of five identical or homologous subunits. Each subunit has four transmembrane domains. The N-terminal and C-terminal
domains are extracellular located. The N-terminus forms a large extracellular domain that
is stabilized by a conserved cysteine bridge. Therefore, these receptors are also referred to
as Cysteine-loop receptors (CLRs), see . Fig. 6.1. When ligands interact with specific
extracellular binding sites, the channels open. The extracellular domain provides a large
basin, the so-called vestibule. Due to the size of this structure, it is able to take hydrated
ions, allowing very fast ion entry. After that the ions have to pass the selectivity filter,
which is formed by two of the transmembrane helices (M2 and M4). The movement of
ions through this narrow part of the pore requires loss of the hydrate shell. The energy for
this process is provided by transient interactions of the passing ions with amino acid
residues lining the pore, and with water molecules.
6.2 · Nicotinic Acetylcholine Receptor (NAchR): Nicotine
6
Α-scorpion-, β-scorpion- and some spider-toxins constitute a group of peptide neurotoxins. They contain 60–70 amino acids and bind to extracellular loops of the channel
(sites 3 and 4, . Fig. 4.2b). This changes the voltage-dependent gating of the channels
and slows down their inactivation. Lipid-soluble toxins, including brevetoxins and
ciguatoxins, bind to transmembrane segments of the channel (site 5). They cause a shift
in activation gating and lead to repeated firing of neurons. These polyether compounds
are produced by dinoflagellates (Karenia spec and Chattonella) and also contribute to
neurotoxic shellfish poisoning during Karenia blooms, e.g. in the Gulf of Mexico
(Watkins et al. 2008).
VOCs are also affected by a number of animal toxins. The P/Q type channel is sensitive
to Ω-agatoxin, a poison of the spider Agelenopsis that paralyses the spiders pray (American
grass spider). N-type channels are blocked by ω-conotoxin, a poison produced by deepwater snails (Conus geographicus). R-type channels are expressed in brain regions responsible
for pain transmission and in the amygdala, a brain region involved in fear response. They
are sensitive to some tarantula venoms, e.g. SNX-482, a peptide with 41 amino acids from
Hysterocrates gigas, the giant baboon spider.
The tarantula toxin guangxitoxin (GxTX) belongs to a class of spider venoms comprising small peptides with a characteristic cysteine knot fold that is stabilized by disulphide bridges. They act on voltage-gated Ca 2+ -, K + - and Na + -channels. Their affinity
depends on the channel conformation displaying either open or closed states. This varies
from 2 nM for the inactive (closed) channel to 200 nM for the activated (open) channel
conformation. GxTX specifically targets K v 2.1-potassium channels. Thus, fluorescently
labelled derivatives of GxTX have been used to report receptor activity of a K v 2.1potassium channel. Fluorescence was measured when the toxin was bound to a closed
channel. It decayed when the channel opened as the toxin lost affinity and dissociated.
This was reversible, after repolarization of the cell, fluorescence could be recovered
(Tilley et al. 2014).
6.2 Nicotinic Acetylcholine Receptor (NAchR): Nicotine
The first ligand-gated ion channel investigated at the molecular level was the NAchR. It
belongs to a large family of pentameric ligand-gated ion channels (pLICs) (Lynagh and
Pless 2014). These channel proteins are composed of five identical or homologous subunits. Each subunit has four transmembrane domains. The N-terminal and C-terminal
domains are extracellular located. The N-terminus forms a large extracellular domain that
is stabilized by a conserved cysteine bridge. Therefore, these receptors are also referred to
as Cysteine-loop receptors (CLRs), see . Fig. 6.1. When ligands interact with specific
extracellular binding sites, the channels open. The extracellular domain provides a large
basin, the so-called vestibule. Due to the size of this structure, it is able to take hydrated
ions, allowing very fast ion entry. After that the ions have to pass the selectivity filter,
which is formed by two of the transmembrane helices (M2 and M4). The movement of
ions through this narrow part of the pore requires loss of the hydrate shell. The energy for
this process is provided by transient interactions of the passing ions with amino acid
residues lining the pore, and with water molecules.
6.2 · Nicotinic Acetylcholine Receptor (NAchR): Nicotine