53. Which one of the following inactivates the serine/threonine protein kinase, mTOR, related to cell growth in mammalian system?
    (1) Rifamycin (2) Rapamycin
    (3) Erythromycin (4) Chloramphenicol

The agent that inactivates the serine/threonine protein kinase mTOR, which is related to cell growth in the mammalian system, is:

(2) Rapamycin

Explanation

• Rapamycin, originally discovered as an antifungal agent, specifically inhibits mTOR (mechanistic target of rapamycin), a key regulator of cell growth, proliferation, and metabolism in mammals.​

• It forms a complex with the intracellular protein FKBP12 that binds to the FRB domain of mTOR, inhibiting its kinase activity, particularly in mTOR Complex 1 (mTORC1). This inhibits downstream signaling pathways including S6 kinase and 4E-BP1 phosphorylation, leading to decreased protein synthesis and cell cycle arrest at G1/S phase.​

• Rapamycin and its derivatives (rapalogs) are used clinically as immunosuppressants and anticancer agents due to their mTOR inhibitory activity.​

• Other antibiotics like Rifamycin, Erythromycin, or Chloramphenicol do not inhibit mTOR; their mechanisms target bacterial processes primarily.