mTOR

Question 1

What is rapamycin?

Answer 1

Produced by soil bacterium Streptomyces Hygroscopius
A macrolide (drug with a large lactone ring).
Causes cells to arrest in G1.

Question 2

What is the genetic evidence of mTOR signaling?

Answer 2

Tsc1 mutants - double negative are visibly larger.

TOR mutants - double negative visibly smaller

Double mutants - same size as TOR mutants.
(TOR must act downstream).

Question 3

What is the biochemical evidence?

Answer 3

Insulin stimulation - increases S6K1, S6 and 4EBP1.

Rapamycin stimulation - insulin increased phosphorylation disappears.

Torin stimulation - insulin increase in phosphorylation disappears (proves mTOR acts downstream of insulin).

Question 4

What are the actions of S6K and how are they mediated by mTORC1?

Answer 4

phosphorylate eIF2B - enhancing eIF4A activity to unwind structured 5’ UTRs.

Promotes phosphorylation dependent degradation of ODCD4 - usually blocks association of eIF4A with translation ore-initiation complex.

Activates SKAR - at exon junction complex to enhance translation of newly generated mRNAs.

Itself phosphorylated by mTORC1.

Question 5

How does mTORC2 aid cell survival?

Answer 5

Akt mediated inhibition of FOXO1 and FOXO3 (phosphorylated by Akt and prevented from translocating to nucleus and activiating gene expression programmes promoting apoptosis.)

Question 6

How does mTORC2 affect metabolism?

Answer 6

activates akt inhibiting GSK3B which deprives activation of glycogen synthase.

Question 7

What is the rationale for linking mTOR and lifespan?

Answer 7

Growth promoting programmes generate metabolic by products which accelerate aging.

Suppress growth –> activation of salvage programmes
Cell functionality preserved for longer time periods.