The evolutionarily conserved checkpoint protein kinase, TOR (target of rapamycin), regulates cell growth and proliferation by controlling protein synthesis. Recent studies highlight the complex upstream and downstream pathways of mTOR. The mammalian target of rapamycin (mTOR) was identified and cloned, showing high sequence identity to yeast TOR proteins. mTOR contains multiple structural domains, including HEAT repeats, a kinase domain, and FAT domains, which are crucial for its function. mTOR activity is regulated by nutrients, growth factors, and energy metabolism. Nutrient availability influences mTOR through protein complexes containing Raptor, mLST8, and other proteins. Growth factors, such as insulin, activate mTOR via the PI3K/Akt pathway, which also involves TSC1 and TSC2, upstream negative regulators of mTOR. Akt phosphorylates TSC2, leading to mTOR inhibition. Rheb, a small GTPase, is regulated by TSC2 and activates mTOR. Energy metabolism, particularly through AMPK, also influences mTOR activity by modulating TSC2. The interplay between nutrient and growth factor signaling pathways converges on mTOR, with TSC1/TSC2 acting as a convergence point. Downstream targets of mTOR include components of the translation machinery, such as eIF4E and eIF4G, which regulate ribosome recruitment to mRNA. 4E-BP1 and S6K1 are key downstream effectors of mTOR, with their phosphorylation states reflecting mTOR activity. S6K1 regulates mRNA translation by promoting the translation of 5'TOP mRNAs, while 4E-BP1 inhibits this process by preventing eIF4E from binding to eIF4G. S6K1 also phosphorylates eIF4B, enhancing ribosome recruitment to mRNA. Dephosphorylation of 4E-BP1 and S6K1 by phosphatases like PP2A and TAP42 regulates mTOR activity. eIF4G, a scaffolding protein, plays a critical role in ribosome initiation complex assembly. The regulation of mTOR is tightly linked to cellular energy status and nutrient availability, with AMPK and TSC2 playing key roles in this process. Overall, mTOR is a central regulator of cell growth, proliferation, and metabolic processes, with implications for cancer and synaptic plasticity.The evolutionarily conserved checkpoint protein kinase, TOR (target of rapamycin), regulates cell growth and proliferation by controlling protein synthesis. Recent studies highlight the complex upstream and downstream pathways of mTOR. The mammalian target of rapamycin (mTOR) was identified and cloned, showing high sequence identity to yeast TOR proteins. mTOR contains multiple structural domains, including HEAT repeats, a kinase domain, and FAT domains, which are crucial for its function. mTOR activity is regulated by nutrients, growth factors, and energy metabolism. Nutrient availability influences mTOR through protein complexes containing Raptor, mLST8, and other proteins. Growth factors, such as insulin, activate mTOR via the PI3K/Akt pathway, which also involves TSC1 and TSC2, upstream negative regulators of mTOR. Akt phosphorylates TSC2, leading to mTOR inhibition. Rheb, a small GTPase, is regulated by TSC2 and activates mTOR. Energy metabolism, particularly through AMPK, also influences mTOR activity by modulating TSC2. The interplay between nutrient and growth factor signaling pathways converges on mTOR, with TSC1/TSC2 acting as a convergence point. Downstream targets of mTOR include components of the translation machinery, such as eIF4E and eIF4G, which regulate ribosome recruitment to mRNA. 4E-BP1 and S6K1 are key downstream effectors of mTOR, with their phosphorylation states reflecting mTOR activity. S6K1 regulates mRNA translation by promoting the translation of 5'TOP mRNAs, while 4E-BP1 inhibits this process by preventing eIF4E from binding to eIF4G. S6K1 also phosphorylates eIF4B, enhancing ribosome recruitment to mRNA. Dephosphorylation of 4E-BP1 and S6K1 by phosphatases like PP2A and TAP42 regulates mTOR activity. eIF4G, a scaffolding protein, plays a critical role in ribosome initiation complex assembly. The regulation of mTOR is tightly linked to cellular energy status and nutrient availability, with AMPK and TSC2 playing key roles in this process. Overall, mTOR is a central regulator of cell growth, proliferation, and metabolic processes, with implications for cancer and synaptic plasticity.