Molecular diversity of Ion Channels

Question 1

Where are the voltage sensors located?

Answer 1

on the periphery of the channel, at the protein-lipid interface

Question 2

Describe the process of “N-type” inactivation or the “ball &chain” mechanism.

Answer 2

• inactivation can be eliminated by enzymatic cleavage (trypsin) or artificial deletion (ΔN) of the N terminus
• inactivation of the N-terminally shortened channels can be restored by exposure to the isolated N-terminal peptide
• in some K+ channels an associated β subunit provides the ball & chain

Question 3

What is a disease associated with voltage gated K+ channels?

Answer 3

mutation: prolongedventricular action potential (long QT syndrome)

⇒ventricular fibrillation, sudden death

Question 4

What is the structure of an ATP- sensitive K+ channel?

Answer 4

octamer built from 4 pore-forming K+ channel subunits and 4 sulfonylurea receptor (SUR) subunits

Question 5

What does the SUR contain?

Answer 5

3 TMDs

2 intracellular NBDs

Question 6

What ensures the correct assembly of the octamer?

Answer 6

ER retention signals

Question 7

The ATP-sensitive K+ channel pore shows _____

Answer 7

inward rectification

Question 8

What is the ionic current flowing in the outward durection of a ATP-sensitive K+ channel blocked by?

Answer 8

intracellular Mg²⁺

polyamines

Question 9

When are ATP-sensitive K+ channels inactive?

Answer 9

in resting living cells

Question 10

When are ATP-sensitive K+ channels active?

Answer 10

excised patches

Question 11

How is the channel inhibited?

Answer 11

ATP superfused from the cytosolic side inhibits channel activity with high affinity

Question 12

How are ATP-sensitive K⁺ channels activated?

Answer 12

• from the cytosolic side by superfusion with ADP
  
  • even in the presence of ATP ⇒ physiological activation
• K+ channel openers (e.g. diazoxide) stimulare similarly to ADP

Question 13

What is the function of sulfonylureas?

Answer 13

inhibit the channel

Question 14

What subunit does ATP acts on?

Answer 14

Kir6.2 subunit

Question 15

What subunit does ADP, sulfonylureas , and K+ channel openers act on?

Answer 15

SUR subunit

Question 16

What is the location of Kir6.2 + SUR1?

Answer 16

pancreatic β- cells

Question 17

What is the role of Kir6.2 + SUR1?

Answer 17

ATP/ADP ratio ↑ ⇒ K_ATP channels shut

depolarization

↓

insulin secretion

Question 18

What are mutations of Kir6.2 + SUR1?

Answer 18

persistent hyperinsulinaemic hypoglycaemia (PHHI)

⇒ some patients respond to diazoxide

Question 19

In type II. diabetes mellitus what can be used as an oral antidiabetic?

Answer 19

sulfonylureas

Question 20

What is the structure of prototype Torpedo Clc-0 channel?

Answer 20

double barreled shotgun with common lock:

• two independently gated pores
• common (slow gate)

Question 21

What is the structure of a bacterial Clc protein?

Answer 21

in the Clc channel pore a negatively charged Cl^- ion is stabalized by helix dipoles

Question 22

Medical implications of Clc proteins: Clc-1

Question 23

What protein family does the CFTR Cl^- belong to?

Answer 23

ABC (ATP Binding Cassette)

Question 24

How many human ABC proteins are there? Examples?

Answer 24

48

SUR (K_ATP channels)

P-glycoprotein (multidrug resistance)

TAP (antigen presentation)

Question 25

What is the structure of the CFTR Cl- channel?

Answer 25

consists of a single polipeptide chain:

2 transmembrane domains (TMD)

2 intracellular nucleotide binding domains (NBDs)

regulatory (R-) domain

Question 26

What are the most highly conserved modules of ABC proteins?

Answer 26

NBDs

Question 27

How is the CFTR Cl- channel activated?

Answer 27

phosphorylated by cAMP-dependent protein kinase

Question 28

In relation to CFTR Cl- channel; what does the gating of phosphorylated channels require?

Answer 28

presence of intracellular ATP

Question 29

PKA phosphorylates at least ___ individual _______ in the regulatory ___ domain.

Answer 29

10

serines

R

Question 30

What is the relationship between channel activity and phosphorylation?

Answer 30

channel activity incrementally increases with phosphorylation

Question 31

What drives gating of phosphorylated channels? How?

Answer 31

ATP, by binding to the two cytosolic NBDs

Question 32

What activates the CFTR channel?

Answer 32

ATP, with very high affinity

⇒ but the physiological regulator is phosphorylation, rather than ATP

Question 33

What is the active conformation of ABC proteins?

Answer 33

head-to-tail NBD1- NBD2 heterodimer with 2 ATP molecules sandwhiched at the interface

Question 34

CFTR gating cycle: How does the pore close?

Answer 34

The NBD2 “head” hydrolyzes ATP

↓

dimer dissociates

Question 35

What is a common mutation in the CFTR channel?

Answer 35

Cystic fibrosis

• autosomal recessive
• protein folding/ processing defects
• gating or permeation

Question 36

What are the symptoms of cystic fibrosis?

Answer 36

• viscous alveolar mucus (→lung infections)
• pancreatic insufficiency
• meconium ileus
• high-salt sweat

Question 37

How is the Nicotinic Ach receptor channel (nAchR) activated?

Answer 37

binding of two ligand molecules

• prolonges stimulation (or high affinity agonist) will lead to desensitization

Question 38

What is the structure of an Ach binding protein?

Answer 38

• homo- pentamer, perfect 5x rotational symmetry
• aromatic ligand binding pockets in intersubunit clefts

Question 39

What is a mutation of the nAchR channel?

Answer 39

congenital myasthenia

(slow channel syndrome)

Question 40

What is a medical significance of the nAchR channel?

Answer 40

autoantibody against MIR: myasthenia gravis