Deck 5: Structure of Ion Channels

Question 1

A nicotinic acetylcholine receptor is activated by

A) ACh.
B) muscarine.
C) intracellular messenger systems.
D) -bungarotoxin.
E) serotonin.

Answer 1

A

Question 2

ACh, the ligand, only binds to at -subunits, which are 2 of the 5 subunits that comprise a nAChR yet conformational changes occur throughout its tertiary and quaternary structure. Why?

A) Ligands span between αγ and αγ-subunits so in fact 4 of the 5 subunits are affected.
B) Subunits interact via amino acid chains connecting their intramembrane regions, so movement in one subunit influences the others too.
C) β-subunits drive movement for all other subunits exclusively via M2-M3 connecting loops.
D) Via subunits tilting away from the axis of the receptor.
E) Regardless of how many ligands bind, all subunits have M2 helices that move within the membrane.

Answer 2

B

Question 3

Which modifications reduce receptor conductance?

A) substitution of 2′ threonine (T) with a smaller hydrophilic residue
B) substitution of 2′ threonine (T) with a larger hydrophilic residue
C) substitution of 2′ threonine (T) with a smaller hydrophobic residue
D) substitution of 2′ threonine (T) with any polar residue
E) substitution of 2′ threonine (T) with a larger hydrophobic residue

Answer 3

E

Question 4

Charged amino acid residues located at cytoplasmic and extracellular surfaces of the receptor

A) enable homomultimeric α7 channels even when other subunit isotypes are not present.
B) influence affinity for the bound ligand.
C) influence channel open time.
D) govern cation or anion selectivity of the channel.
E) govern ligand selectivity (e.g., serotonin, glutamate, γ-aminobutyric acid, glycine, or zinc).

Question 5

Which type of information below helped determine the subunit composition and stoichiometry of nAChRs?

A) Position of ligand binding sites at α-γ and α-δ interfaces
B) Affinity for α-bungarotoxin
C) Electron imaging and other physical techniques
D) Molecular weight of the purified Torpedo nAChR and its constituent subunits
E) More diameter and relative selectivity of large cations

Question 6

Hydrophobicity of amino acid residues informs which property of nAChRs?

A) Their primary structure, i.e., amino acid sequence
B) Secondary and tertiary structure: membrane insertion of alpha helices
C) Secondary and tertiary structure: configuration of intracellular loops
D) Secondary and tertiary structure: ligand-binding site(s)
E) Quaternary structure: arrangement of subunits

Question 7

Which subunits from vertebrate nAChRs, when their mRNA is expressed in oocytes, may be form homomultimeric channels?

A) Any α₂-α₁₀
B) Any α₂-α₆, α₁₀
C) Any α₇, α₈, or α₉
D) Any β₂-β₄
E) Any α₂-α₆, α₁₀ with any of β₂, β₃, or β₄

Question 8

A sequence of 24 hydrophobic amino acids at the amino tail of the nAChR subunit ensures

A) that ligand binding occurs principally on the α-subunit (and not β, γ, or δ).
B) M1-M4 transmembrane domains align in parallel.
C) that nAChRs can dimerize.
D) that the first transmembrane domain (M1) forms an helix as required for intercalation in the membrane.
E) that ligand-binding sites reside extracellularly.

Question 9

Why was the electric organ of the ray Torpedo key in the structural characterization of nAChRs?

A) It provided sufficient quantities for biochemical isolation of the receptor.
B) Its receptors have unique affinity for α-bungarotoxin.
C) Compared to vertebrates, Torpedo receptors have a greater diversity of subunits that comprise the receptor and thus provide a wider spectrum of possible tertiary structures.
D) Its receptors are uniquely nicotinic whereas vertebrate receptors which can be nicotinic or muscarinic.
E) The tertiary structure of its subunits is equivalent to that of vertebrate receptors.

Question 10

Why do we refer to the nAChR as a receptor rather than a channel?

A) To distinguish it from muscarinic receptors, which do not form a channel
B) Because it is present in skeletal muscle and electric organs (a modified form of muscle) as well as neurons
C) Because its principal function is reception of signals from presynaptic partners
D) To emphasize its commonality with other Cys-loop receptors and dissimilarity compared to voltage-activated ion channels
E) Because its ligand-binding properties more substantively contributed to its characterization than the conduction properties of its channel

Question 11

Positively charged arginines within the long intracellular loop connecting M3 and M4 dramatically influence channel conductance of which Cys-loop receptor?

A) The glutamate (GluCl) receptor
B) The serotonin (5-HT3) receptor
C) The γ-aminobutyric acid (GABA_A) receptor
D) The glycine receptor
E) ZAC, the zinc receptor

Question 12

Which of the following is an invertebrate anion channel?

A) MOD-1 receptor
B) EXP-1 receptor
C) GluCl receptor
D) GABA_A receptor
E) Glycine receptor

Question 13

Which of the following ligands gates inhibitory synaptic transmission in vertebrate and invertebrate nervous systems?

A) Serotonin
B) γ-aminobutyric acid
C) Glycine
D) γ-aminobutyric acid and glycine
E) Zinc

Question 14

5-HT₃ receptors and nAChRs share which of these properties?

A) The ligand that binds to them
B) Which ion(s) they conduct
C) Heterogeneity of subunits comprising them
D) Their expression in both vertebrate and invertebrate nervous systems
E) Their expression in both skeletal muscle and the nervous system cells

Question 15

Modifying the primary structure of a receptor encapsulates which of the following techniques?

A) Site-directed mutagenesis
B) Receptor expression in Xenopus oocytes
C) Electrical recording of single-channel or whole-cell receptor currents
D) Ligand binding affinity and pharmacology
E) Replacement of polar amino acids with nonpolar ones

Question 16

Modification of the primary structure of a receptor encapsulates which of the following?

A) Site-directed mutagenesis
B) Receptor expression and subunit assembly in Xenopus oocytes
C) Electrical recording of single-channel or whole-cell receptor currents
D) Ligand binding affinity and pharmacology
E) Replacement of polar amino acids with nonpolar ones

Question 17

What technique allowed scientists to view nAChRs in their open state?

A) Biochemical purification of the receptor from Torpedo
B) Plunge freezing
C) Site-directed mutagenesis
D) Photo-labeling (i.e., ultraviolet flashes) in the presence of Tritiated chloropromazine.
E) Cysteine screening followed by MTSEA exposure

Question 18

Maximal channel current in nAChRs depends on

A) leucine and valine at 9′, 13′, and 16′ in the M2 helix.
B) hydrophobic isoleucine adjacent to lipid or other parts of the protein's secondary or tertiary structure.
C) substitution of serine at 6′ in the M2 helix with a more hydrophilic amino acid.
D) substitution of serine at 6′ in the M2 helix with a more hydrophobic amino acid.
E) hydrophilic serine at 6′ in the M2 helix.

Question 19

With respect to the structure and function of nAChRs, cysteine scanning

A) provided binding sites for tritiated chlorpromazine in the M2 helix.
B) refers to scanning for radioactivity of tritiated chlorpromazine after it reacts with cysteines in the M2 helix.
C) provided novel binding sites for QX222 in the open channel compared to existing QX222 biding sites in the native receptor.
D) revealed that amino acids at 2′, 3′, 6′, 8′, 9′, 10′, 13′, and 16′ lined the pore-forming side of the M2 helix.
E) revealed that amino acids at 2′, 6′, and 9′ lined the pore-forming side of the M2 helix.

Question 20

Which of the following voltage-activated sodium channels is expressed in cells of both the peripheral and central nervous system?

A) Na_V 1.3
B) Na_V 1.4
C) Na_V 1.5
D) Na_V 1.6
E) Na_V 1.7

Question 21

Why were electric eels used to discover nAChRs and voltage-activated sodium channels but not voltage-activated potassium channels?

A) nAChRs and voltage-activated sodium channels in eels are similar in structure and function to vertebrate channels so they make a good model to understand channels across phylogeny.
B) Electric eels provide an abundance of channels for biochemical purification prior to sequencing and cloning, but voltage-activated potassium channels were cloned using a different strategy.
C) A-type voltage-activated potassium channels are not expressed in electric eels, whereas those A-type channels are expressed in invertebrate Drosophila (fruit flies).
D) Electric eels are aquatic organisms and thus sensitive to toxins like α-bungarotoxin, TTX, and STX, which assist in biochemical purification.
E) Shaker-type mutants were a fortuitous discovery that facilitated biochemical purification of voltage-activated potassium channels thus obviating the need an eel-like model system with abundant channels.

Question 22

The α-subunit of the voltage-activated potassium channel resembles

A) the α-subunit of the voltage-activated sodium channel.
B) the P-loop of the voltage-activated sodium channel.
C) domain IV of the voltage-activated sodium channel.
D) the α-subunit of the voltage-activated calcium channel.
E) the α-subunit typical of a Cys-loop receptor.

Question 23

Which of the following characteristics are similar for Cys-Loop and voltage-activated sodium channels?

A) Relative size and molecular mass of their -subunits
B) Relative size and molecular mass of their β-subunits
C) The total number of membrane-spanning segments per subunit or domain
D) Structure of Cys-Loop receptor subunits and sodium channel domains I-IV
E) Primary structure of M2 helices in Cys-Loop receptors and S4 regions of sodium channels

Question 24

Which of the following characteristics are similar for voltage-activated sodium channels and voltage-activated calcium channels?

A) Selectivity of their pore regions
B) Voltage sensitivity and persistence of activation
C) Their position and function of their associated β-subunits
D) Prevalence of glycosylation sites in functional channels
E) The primary sequence similarity of their α-subunit and equivalent structure of domains I-IV

Question 25

What senses membrane potential in voltage-activated channels?

A) Arginine and lysine residues on the S4 transmembrane segment
B) Glycosylation sites ubiquitous on the α-subunit
C) Auxiliary β-subunits
D) The moderately hydrophobic P-loop region
E) Positively charged residues of the S6 transmembrane segment

Question 26

m₁ and m₂ of K_CS channels most closely mimic

A) M1 and M2 of nAChRs.
B) S1 and S2 of voltage-activated channels.
C) S4 of voltage-activated channels.
D) S5 and S6 of voltage-activated channels .
E) the extracellular loop connecting S5 and S6 of voltage-activated channels.

Question 27

Opening a voltage-gated channel generally involves which of the following roles for S6?

A) Forming the ion selectivity filter in combination with S5 and the S5-S6 linker region
B) Sensing transmembrane depolarization via its positively charged amino acids
C) Relieving the physical constraints that block the cytoplasmic mouth of the channel
D) Displacement within in the membrane via interactions with voltage-detecting S4
E) Possibly none: a large body of evidence shows that activation of channel currents can occur independently of an S4-coupled gating mechanism, which casts doubt on the role of S6, too

Question 28

Where is the ion selectivity filter located, considering the quaternary structure of a representative, well-studied ion channel like K_CS?

A) The outer vestibule of the channel, which is approximately 2 nm in diameter
B) Adjacent to short helices and their neighboring residues, which form a box-like structure at the channel's extracellular face
C) Within the central pore
D) Within the lower pore
E) By the inner helix, which predominantly forms the narrowest part of the channel structure

Question 29

Which statement summarizes how the 0.3 nm selectivity filter of K_CS channels enables potassium to penetrate but not sodium or lithium?

A) Because of their smaller diameter and charge, sodium and lithium ions interact strongly with carbonyl oxygens and effectively get stuck in the pore region and do not flow through the channel.
B) Because their diameters exceed 0.3 nm, sodium and lithium are excluded on the basis of size.
C) Carbonyl oxygens provide a surrogate environment that keeps potassium ions in aqueous solution while passing through the channel.
D) Carbonyl oxygens provide a surrogate environment for dehydrated potassium ions in the selectivity filter that mimics the hydration energy of ions in solution.
E) Carbonyl oxygens provide a surrogate environment for dehydrated sodium or lithium ions in the selectivity filter that mimics the hydration energy of ions in solution.

Question 30

In voltage-activated channels, preferential ion selectivity for calcium over sodium is governed by which of these factors?

A) Cumulative negative charge of amino acids that line the pore
B) An ensemble mix of negative, positive, and neutral amino acids that line the pore
C) Glutamine residues at four positions that line the pore
D) Aspartate followed by glutamate-that pattern repeated twice-at four key positions that line the pore
E) Characteristic features of the P-loop

Question 31

What is the principal mechanism of inactivation in voltage-activated channels?

A) Occlusion by elements of the ion channel that do not lie within the membrane
B) S6 helix closing off the cytoplasmic mouth of the channel
C) Displacement of S4 back to its "resting" position within the membrane
D) Inactivation, or current "switching off," of the P-loop, possibly involving the selectivity filter
E) Displacement f S6 back to its "resting" position within the membrane

Question 32

Piezo 1 is likely to transduce which of the following stimuli or senses?

A) Sound (pitch)
B) Body orientation (position with respect to gravity)
C) Touch (e.g., pressure applied to the skin)
D) Interoception
E) Heat

Question 33

Deafness is associated with genetic knockout of which gene?

A) Transmembrane Channel-like Protein 1 (TMC1)
B) Transmembrane Channel-like Protein 2 (TMC2)
C) Tetraspan Membrane Protein of Hair Cell Cilia (TMHS)
D) Transmembrane Inner Ear Protein (TMIE)
E) Both TMC1 and TMC2

Question 34

Which structural property is common among glutamate receptors and K_CS potassium channels?

A) Ion selectivity of the pore
B) The number of transmembrane segments that comprise the receptor/channel
C) A small helical region between transmembrane segments that forms the pore
D) Positively charged amino acids located at every third position in one of the transmembrane domains
E) The pore region located on the cytoplasmic side of the receptor/channel

Question 35

Imagine that you discover a new brain nucleus and its constituent neurons excite their downstream postsynaptic neighbors. Which of the following receptors are most likely to be expressed by the postsynaptic neighbors?

A) Nicotinic ACh receptors
B) Serotonin (5-HT3) receptors
C) γ-aminobutyric acid (GABA_A) receptors
D) Glycine receptors
E) AMPA receptors

Question 36

Which of the following factors gates present at the cytoplasmic side of the channel gates outward current flow in K_ir channels?

A) β-subunits
B) A relatively long carboxy tail that inactivates the channel like in voltage-activated sodium channels
C) Calcium
D) Magnesium
E) Intracellular biochemical messengers

Question 37

A mammal senses painful heat on its skin. Which is the most likely channel transducing the stimulus?

A) Piezo 2
B) TRPV1
C) TRPM8
D) K_ir
E) Na_V 1.7

Question 38

A mammal is drinking cold water from a stream Which is the most likely channel transducing the stimulus?

A) Piezo 2
B) TRPV1
C) TRPM8
D) K_ir
E) Na_V 1.7

Question 39

Genetic mutations affecting this channel can cause persistent burning pain sensations?

A) TRPV1
B) K_V 7.1
C) K_V 7.2 and K_V 7.3
D) K_V 7.4
E) Na_V 1.7

Question 40

Mutations of which gene in cochlear hair cells causes congenital deafness?

A) TMC1
B) TMC2
C) KCNQ4
D) TMHS
E) TMIE

Question 41

Replacing a codon for one amino acid with another one prior to translation is an example of

A) gene transcription.
B) glycosylation.
C) RNA editing.
D) alternative splicing.
E) post-translational modification.

Question 42

With regard to ion channel opening, the M1 ligand-binding domain of Cys-loop receptors functionally corresponds to which of the following regions of voltage-activated ion channels?

A) S1 transmembrane domain
B) S1-S4 in its entirety
C) S4 transmembrane domain
D) S5-S6, which form the P-loop
E) S6 transmembrane uniquely

Question 43

Ion selectivity of Cys-loop and voltage-activated ion channels depends

A) inversely on the number of subunits (i.e., fewer subunits means higher ion specificity).
B) directly on the number of subunits (i.e., more subunits means higher ion specificity).
C) on properly spaced charged amino acids in the primary structure of the channel proteins.
D) on auxiliary subunits (e.g., β-subunits), which associate with the main (α) subunit of the channel.
E) on the P-loop.

Question 44

Describe three experimental observations that led to our current understanding of conformational changes of Cys-loop receptors that allow current to flow.

Question 45

Explain the key role of natural high-affinity toxins in discovering and cloning nAChRs and voltage-activated sodium channels? Name the toxins and their targets as a part of your answer.

Question 46

Evaluate the evidence that S4 transmembrane domain of voltage-activated sodium channels moves tangent to the plane of the membrane in response to depolarization.

Question 47

Explain the key role played by blocking molecules in discovering the role of the P-loop in voltage-activated ion channels. Given blocking molecule names as part of your answer.

Question 48

Create a viable explanation for how calcium activated potassium channels can coexist, and even function cooperatively with voltage-activated calcium channels

Question 49

Explain how α and β units differ in their structure and function for voltage-activated sodium channels versus nAChRs?

Question 50

Describe the conceptually different functional roles for Piezo 1 and Piezo 2 in sensory function?

Question 51

Make an argument that TMC in cochlear hair cells is an ion channel.

Question 52

Make an argument for why foods like red peppers are perceived as hot to our sense of taste.

Question 53

Make an argument for how the W897X mutation of Na_V 1.7 (Scn9a) ion channels leads to "painless" persons who show no ability to perceive pain.

Question 54

How do gain of function mutations to Scn9a (Na_V 1.7) lead to hyperalgesia (excessive or spurious pain)?

Question 55

Explain the role of K_V 7.1 in normal cardiac electrophysiology and cardiac function as well as how mutant K_V 7.1 channels with diminished function lead to pathology.

Question 56

If we consider Cys-loop and voltage-activated channels as having three main parts: (1) an extracellular portion, (2) an intramembranous portion, and (3) an intracellular portion; then, provide a general functional attribute for each region and how it functions in both Cys-loop and voltage-activated channels.

Question 57

Explain how -helices and β-sheets contribute to the specific tertiary and quaternary structural features of Cys-loop receptors like nAChR?

Question 58

Evaluate the notion that hydrophilic amino acids line the pore region of Cys-Loop receptors.