Cell signalling 1: ion channels (Prof. Patel)

Question 1

What is signaling ?

Answer 1

The cascade of processes by which an extracellular stimulus (typically a neurotransmitter e.g. Glu or hormone e.g. adrenaline) effects a change in cell function.

Question 2

What are the main types of signaling ?

Answer 2

Signaling through ion channels and receptors.

Question 3

What are the main differences between ion channels and receptor proteins ?

Answer 3

Ion channels form pores through the lipid bilayer to let ions passively through, whereas receptor proteins activate intracellular signaling proteins (when a ligands drives them in their active conformation) which in turn bind to other target molecules such as DNA, metabolic enzymes and cytoskeleton proteins. These can lead to altered gene expression, metabolism or cell shape/movement respectively.

Question 4

What are the main processes regulated by Ca2+ ?

How fast are these ?

Answer 4

Exocytosis (μs) 
Contraction (ms)
Metabolism (s)
Gene transcription (min)
Fertilization, proliferation, hypertrophy (hrs)

Question 5

To what types of molecules is the plasma membrane permeable, semi-permeable and impermeable to ?

Answer 5

Permeable: hydrophobic molecules (02, C02, N2, benzene)
Semi-permeable: small uncharged polar molecules (H20, urea, glycerol) + to a lesser extent, large uncharged polar molcules (sucrose, glucose)
Impermeable: ions

Question 6

What are the 3 ways cells have to transport ions across the membrane ?

Answer 6

Via pumps, carriers and channels.

Question 7

For a typical mammalian cell, give the intra- and extracellular concentrations (in mM) of:
Na+, K+, Mg2+, Ca2+, H+, Cl-.

Answer 7

Na+: intra = 5-15; extra = 145
K+: intra = 140; extra = 5
Mg2+: intra = 0.5; extra = 1-2
Ca2+: intra = 1E-4; extra = 1-2
H+: intra = 7e-5; extra = 4e-5
Cl-: intra = 5-15; extra = 110

Question 8

Are their ionic gradient w/in the cell itself ?

Where and for what ions ?

Answer 8

ER [Ca2+] = 0.5mM; cytosolic [Ca2+] = 100nM
Lysosomal pH = 4.5; cytosolic pH = 7.2
Mitochondrial pH = 7.8; cytosolic pH = 7.2

Question 9

What is the main factor affecting passive ion transport ?

Answer 9

The electrochemical gradient.

Question 10

What maintains the K+/Na+ gradient across the membrane ?

How does it work (broadly) ?

Answer 10

The Na+/K+ATPase pump.
For every 2K+ going in, 3Na+ go out.
~6 conformational states, binds Na+ upon phosphorylation, binds K+ upon dephosphorylation.

Question 11

What generates Ca2+ gradients across the plasma membrane ?

What about across intracellular stores like the SER ?

Answer 11

Ca2+ pumps:

• PMCA (Plasma Membrane Ca2+ ATPase) = a P-type ATPase
• SERCA (Smooth Endoplasmic Reticulum Ca2+ ATPase) = P-type ATPase, pumps 2Ca2+ in for every ATP –>ADP

Question 12

What is 2dary active transport ?

What are the 2 types of these ?

Answer 12

When ion gradients are used (instead of ATP) to transport other ions against their electrochemical gradient.
This is coupled transport: symport or antiport.

Question 13

Give an example of 2dary active transport.

Answer 13

The Na+ gradient is used to drive Ca2+ efflux via the Na+/Ca2+ exchanger (antiporter).
For every 3 Na+ going in, 2 Ca2+ go out.

Question 14

What are the 3 main types of ion channels ?

Answer 14

• voltage gated
• ligand gated (intra- or extracellular ligand)
• mechanically gated

Question 15

How can we tell that v-gated ion channels are evolutionarily related ?

Answer 15

Because their share the same “building block”:
6 alpha-helical transmembrane domains S1 to S6, w/
- S1-S4 = volatge sensing domain
- S4 = voltage sensor (+ve AAs)
- S4-S5 linker to pull the channel open when S4 “sense” the voltage change
- S5-S6 forming the pore domain

Question 16

How are the K+ channel and the Na+ channel related ?

Answer 16

The K+ channel protein is just 6 transmembrane domains. Four of these come together to form the actually ion channel.
The Na+ channels is made of 4 times these 6 transmembrane domains, so although the subunits of the protein are rather similar, in the Na+ channel they are part of the same structure.

Question 17

Who revealed the 3D structure of the K+ channel ?

W/ which method ?

Answer 17

Roderick McKinnon (Nobel Laureate 2003, Chemistry) by using X-ray crystallography.

Question 18

How is K+ selectivity mediated ?

Answer 18

By the selectivity filter, in which the carboxyl groups of 4 glutamate side chains (in the “high field strength site”) stabilize the K+ ion in its dehydrated form.
X-ray crystallography shows that, if the smaller Na+ ion entered this selectivity filter, only 2 of the 4 carboxyl groups would stabilize it, thus making its entry thermodynamically unfavorable.

Question 19

How does the S4 segment “sense” voltage change ?

What effect does this have on the overall structure of the channel ?

Answer 19

Regularly spaced +vely charged residues, arginine (R) and lysine (K), “sense” potential changes. This mediates a conformational change in the ion channel by pulling on the S4-S5 linker and opening the pore domain.