IP3 and Calcium Release Flashcards
Where is the majority of the IP3 receptor?
In cytosol
What is the structure of the IP3 receptor?
- Tetrameric homodimer
- Small hole in centre where Ca2+ moves
What is the Hill coefficient of IP3 receptor and why?
- 3 because 3 molecules of IP3 need to bind for the channel to go into an intermediate state
What is required for the IP3 receptor to go from an intermediate to an open state?
Requires free intracellular Ca2+ to bind
What happens to the IP3 receptor when [Ca2+] intracellular increases?
It inhibits the activity of the IP3 receptor and stops the flux of Ca2+ out of the ER
What is the relationship between probability of an open IP3 receptor and concentration of intracellular calcium?
Why do Ca2+ oscillations occur due to IP3?
- Because the agonist IP3 is still present a point is reached where the receptor is resensitised so can now generate the next ‘spike’
- This is why amplpitude is the same every time
What is required to sustain Ca2+ fluxes and why?
- Ca2+ must enter the cell because each time Calcium goes back to a resting level some is lost to out of the cell
How does calcium influx occur in sustained calcium signalling?
- Dissociation of Ca2+ occurs away from lumenal side of STIM1
- STIM1 forms digomeric assemblies which assemble in cortical ER and couple to Orai1
- Then opens the Orai1 channel and allows Ca2+ to come into cytoplasm
Where is STIM1 located?
ER-resident integral protein
What is the function of STIM1?
Senses the ER luminal free Ca2+ concentration
What is the function of Orai1?
It is a Ca2+ influx channel
What structure does STIM1 usually have?
Dimer locked in dimer conformation
What causes [Ca2+] to drop in ER lumen?
Stimulation with IP3
How does IP3 affect STIM1?
- IP3 causes [Ca2+] in ER lumen to drop as fluxing to cytosol causes dissociation of Ca2+ away from lumenal side of STIM1 which alters STIM1 structure
What happens to STIM1 and Orai1 when ER refills?
STIM1 goes back to dimeric state which stops coupling to Orai1 and channel shuts
What is the structure of Orai1?
- Hexamer of 6 Orai subunits arranged around a central axis
- Gated central core which allows Ca2+ to flux in
How does STIM1 cause Orai1 to open and Ca2+ enter the cell?
- STIM1 reconfigures cytoplasmic structure of Orai1 channel and pulls apart from helices making up pore of channel
- Conformational coupling
Where are the Ca2+ binding sites located on calmodulin?
On amino and carboy termini
What happens to calmodulin when Ca2+ is bound?
- Alters conformation
- Allows calmodulin to bind specifically to a variety of target proteins, locking them into certain conformations and allowing them to be calcium responsive
What is the structure of Calmodulin-dependent PKII?
- 6 kinase subunits
How is calmodulin dependent PKII activated?
- amino-carboxy terminal end of calmodulin can bind to the regulatory segment
- this catches the kinase domain when it ‘pops out’ and activates it
- Kinase phosphorylates itself and neighbouring subunit to activate the enzyme
How is the active state of calmodulin dependent PKII further stabilised?
By calmodulin locking kinases
How is calmodulin dependent PKII deactivated?
- When [Ca2+] decreases calmodulin dissassociates and becomes partially active
- Phosphatases remove phosphates
What happens to activity of calmodulin dependent PKII at high calcium concentrations?
- Phosphatases don’t have enough time to deactivate enzyme before Ca2+-calmodulin spike
- Build up of activity
where is the IP3 receptor located?
between the ER lumen and the cytosol
Where is STIM1 located?
In the ER
Where is Orai1 located?
In the plasma membrane
How can Ca2+ leave the cell?
- PMCA
- Sodium Calcium exchanger
How does STIM1 activate Orai1?
Binding pulls the helices apart and opens the channel to allow Ca2+ to enter
