Cell Signalling 2 Flashcards
Some GPCRs exert effect via G-protein called Gq - how is that different from Gs and Gi?
The previous ones act on Adenylyl cyclase, while Gq activates the anzyme Phospholipase C
-> cleaves a lipid molecule from the plasma membrane = inositol phospholipid (a phospholipid with sugar inositol attached to its head)
What kind of second messenger molecules does phospholipase C generate? What do they do?
- Inositol 1,4,5 - triphosphate (IP3) = water soluable sugar phosphate
-> once release into cytosol -> binds to Ca2+ channels of ER -> Ca2+ stored there rushes out -> higher concentration -> signaling for other proteins
2.Diacylglyserol (DAG) = lipid produced by Phospholipase C that remains embedded in the plasma membrane
-> there,it recruits and activates protein kinase to translocate them from cytosol to plasma membrane = Protein Kinase C (PKC) which needs Ca2+ to become active
-> once Ca2+ binds -> acts similar as PKA just phosphorylating different proteins
In general, Ca2+ plays a large role in many intracellular signalling pathways - how come it is pushed into the cytosol so rapidly?
The concentration of Ca2+ in the cytosol is very low (in unstimulated cell) compared to extracellular space or ER
- the differences maintained by specialized pumps that force Ca2+ out of the cytosol
=> steep electrochemical gradient
-> once Ca2+ channels transiently open -> rushes into cytosol
- pumps can terminate the signal
The effect of Ca2+ is largely indirect - what kind of protein mediator could it use?
Calmodulin - most commonly, present in cytosol
- once Ca2+ binds to it -> conformational change -> now, can interact with other proteinsalong the pathway
-> e.g. Ca2+/Calmodulin-dependent protein kinases (Cam-Kinases) -> phosphorylation of target proteins/molecules
E.g. neurons exert this to induce learning or memory e.g. adding AMPA receptors to the plasma membrane
How does light get transduced in photoreceptors?
You can find it in structure biology :)
What is meant by adaptation in terms of signalling pathways? What mechanism is responsible?
= modulation of response to extracellular signal molecules depending on their amount
- If there is little of it (e.g. dark conditions -> small amount of photons) -> cells engage in more amplification
- If there is too much (e.g. bright conditions) -> cell can decrease their amplification to not get overwhelmed
- This is achieved by positive or negative feedback loops
- e.g. in photoreceptors - intense light response decreases cytosolic Ca2+ concentration -> inhibits enzymes responsible for amplification
How are enzyme-coupled and G-protein coupled receptors similar ahd how different?
- Similar = both are transmembrane receptors with ligand-binding domain facing the extracellular site
- Different = ligand-coupled have a cytosolic domain that either acts like an enzyme or forms a larger complex with an enzyme
-they also tend to be just single spanning alpha helix (monomer) -> so simple conformational change won’t cut it (often they have to cooperate and form a dimer)
Do enzyme-coupled receptors show effect immediately or over some longer period?
Both:
- Longer - require many intracellular steps diffused over cytosol that ultimatelly lead to change in gene expression => cell proliferation, differentiation, survival
- Rapid - usually proteins attached to the membrane => reconfiguration of cytoskeleton, change in the cell’s shape
What is the largest family of enzyme-coupled receptors -> explain the steps of its activation.
Receptor Tyrosine Kinases
- Ligand bonds to two separate monomers -> dimerization (conformational change) -> autophosphorylation of tyrosine tails -> adaptor proteins or other protein kinases with a specific interaction domain can bind to the tyrosine => while binded proteins can simultaneously trigger several routes of signalling
- leading to complex responses such as proliferation, differentiation
NOTE: although the receptors differ from one another they often involve similar proteins e.g. phospholipase C, Ras
How do we terminate response of receptor tyrosine kinases?
- Tyrosine phosphorylations are reversed by tyrosine phosphatases - removes the phosphate group from both RTKs and other intracellular signalling proteins activated
- In some cases the RTKs can also be removed from the membrane by endocytes and destroyed in lysosomes
What is the signalling protein most readily activated by the RTKs? How is it similar and different from G-proteins mentioned earlier?
Ras protein - monomeric GTPase
- resembles the single alpha subunit, bound to the membrane by a lipid tail
- cycles in the same way as other G-proteins i.e. active conformational state with GTP bound and inactive with GDP
- Virtually all RTKs bind Ras
How does Ras protein function ?
- RTKs phosphorylate -> bind an adaptor protein with Ras-GEF -> Ras-GEF forces Ras to kick out GDP and take up GTP from the cytosol -> cascase of signalling of serine/threonine kinases
- E.g. MAP-kinase signaling module (mitogen-activated protein kinase)
- Ras -> MAP kinase kinase kinase -> MAP kinase kinase -> MAP kinase -> effector proteins e.g. transcription regulators -> gene expression e.g. cell proliferation, survival, differentiation
- E.g. MAP-kinase signaling module (mitogen-activated protein kinase)
- To terminate it uses Ras-GAP which promotes hydrolysis of GTP to GDP again
Ras was first discovered thanks to mutation - why?
Mutation in Ras often leads to inactivation of the Ras GTPase leading to constant activation -> constant signalling -> e.g. cell growth
- present in 30% of cancers
- NOTE: the rest of cancer types often involve proteins present in the Ras pathway
Name a crucial signalling pathway belonging to insulin-like growth factor (IGF) family.
- This pathway promotes cell growth and survival
- Activates phosphoinositide 3-kinase (PI 3-kinase) -> phosphorylates inositol phospholipids in the plasma membrane -> these phospholipids can now serve as docking sites for intracellular signalling proteins -> they can activate each other at the plasma membrane
Name the most important relocated signalling proteins. What can it do?
Serine/threonine protein kinase Akt - also called Protein kinase B (PKB)
Cell survival
- phosphorylates/inactivates the cytosolic protein Bad -> encourages cells survival
- At its active version, bad promotes apoptosis
Cell growth
- the pathway with PKB indirectly stimulates serine/threonine kinase Tor -> enhances protein synthesis and inhibits protein degradation
NOTE: anticancer medication “rapamycin” inactivates Tor
Look at an overview of signalling pathways:
True/False: Nucleus receives a message only via a complex route of signalling. Explain.
No, there are few proteins that can have an impact on gene expression in a direct way
- E.g. protein Notch
- essential for neural developmnent in drosophila
- the receptor itself acts as a transcriptor regulater:
- Activated by binding of Delta (=transmembrane signalling protein at the surface of a neighboring cell) -> Notch gets cleaved -> cytosolic tail of the receptor is now free to diffuse into the nucleus -> activate Notch-responsive genes
Why and how are plant and animal cells different in terms of cell-cell communication?
Although they share common ancestor, this one existed in time where each cell function by itself -> so once multicellular organisms were starting to form they had to figure out the cell-cell communication separately -> different mechanisms
- Different receptor serine/threonine kinases
- Don’t use RTKs and GPCRc but rather steroid-hormon-type nuclear receptors and cyclic AMP
One of the best studied signalling systems in plants is mediated by ethylene - how does it work?
- Involved in seed germination, fruit ripening
- In the ABSENCE of ethylene -> protein kinase activate -> inhibit ethylene-responsive genes in the nucleus- Ethylene binds to receptor -> inhibiting protein kinases -> genes are free to transcribe related reaction
NOTE: In actuality the complexity of these signaling pathways is much greater then shown here.
There is a lot of different types of kinases, different receptors -> many interact with one another (e.g. protein kinase phosphorylates molecules from pathways other than just their own)
- There are even interaction proteins helping to regulate the complex communication
