Autophagy Mechanism Flashcards
What cellular signals can induce autophagy?
Amino acid sensing, signals such as growth factors and reactive oxygen species, infection.
Which protein kinases are key autophagy regulators and how do they regulate?
mTOR:
Suppresses autophagy through inhibitory phosphorylation of the Unc-51-like kinases ULK1 and ULK2
AMPK:
Induces autophagy by directly and indirectly activating ULK1 in response to cellular distress (mostly nutrient deprivation).
ULK1/2:
Activate autophagy, part of the ULK2-complex which is needed for early autophagosome biogenesis
What are ULK1 and ULK2, what do they do, and what can inhibit them?
Unc-51-like kinases: autophagy activating kinases (they are two isoforms).
They work particularly in response to amino acid deprivation.
Can be phosphorylated and inhibited by mTOR and phosphorylated and activated by AMPK. ULK can also phosphorylate itself, as well as ATG13 and Beclin-1.
Why does rapamycin induce autophagy?
Because it inhibitors mTOR (Mammalian Target Of Rapamycin), and mTOR inhibits autophagy.
What is mTOR and what is it’s significance for autophagy?
Mammalian Target Of Rapamycin:
A kinase that is a core component of the complexes mTOR complex 1 and mTOR complex 2. It regulates cell growth, proliferation, motility, survival, protein synthesis, autophagy, and transcription.
When active, mTORC1 inhibits autophagy by phosphorylating both ULK1 and ATG13, which reduces the kinase activity of ULK1. Under starvation conditions, mTORC1 is inhibited (by AMPK) and dissociates from ULK1 allowing it to become active.
What is AMPK?
AMP-activated protein kinase
It is a kinase enzyme that plays a role in cellular energy homeostasis, largely to activate glucose and fatty acid uptake and oxidation when cell energy is low.
AMPK activates autophagy by directly and indirectly activating ULK1. AMPK is activated when intracellular AMP increases which occurs under starvation conditions. It then inactivates mTORC1, and thus indirectly activates ULK1. AMPK also directly phosphorylates ULK1 at multiple sites.
What other molecules are part of the ULK1-complex and what does this complex do?
Atg13, Atg101, and the FAK family kinase interacting protein of 200kD (FIP200 or RB1CC1).
This complex is needed in the early steps of autophagosome biogenesis.
What has research recently shown to be a likely target for phosphorylation by ULK1?
Beclin-1 at serine 14.
What is Beclin-1?
Not a huge amount known!
Beclin-1 is the mammalian orthologue of the yeast Atg6.
Schizophrenia is associated with low levels of Beclin-1 in the hippocampus which causes diminished autophagy.
Part of the autophagy-inducible Beclin-1 complex.
What molecules are part of the autophagy-inducible Beclin-1 complex?
Beclin-1, p150, Atg14L, and the class III phosphatidylinositol 3-phosphate kinase (PI(3)K) Vsp34.
What do the active ULK and Beclin-1 complexes do?
Re-localise to the site of autophagosome initiation (the phagophore) where they both contribute to the activation of downstream autophagy proteins.
What is the name of the site of autophagosome initiation?
The phagophore
What complex is VPS34 part of?
The autophagy inducible Beclin-1 complex
Once active, what does VPS34 do?
VSP34 is a
phosphatidylinositol 3-phosphate kinase (PI(3)K), so it does what the name says.
Phosphorylates the lipid phosphatidylinositol on the surface of the phagophore to generate phosphatidylinositol 3-phosphate (PtIns(3)P) on the surface of the phagophore.
What is the purpose of the PtdIns(3)P on the surface of the phagophore?
Acts as a docking point for proteins harbouring a PtdIns(3)P binding motif such as WIPI2.
