Which statement is true of mTOR?

• A. It is a transcription factor that regulates ribosome biogenesis
• B. It is a phosphatase that inhibits growth
• C. It has no role in muscle adaptation
• D. It is a protein kinase that is the major regulator of protein synthesis and muscle size

Answer: (D)

mTOR acts as a serine/threonine protein kinase that sits at a central hub integrating nutrients, energy status, and growth signals to control cell growth. In muscle, its activity is mainly through the mTORC1 complex, which promotes protein synthesis by phosphorylating targets like S6K1 and 4E-BP1, thereby lifting inhibition on translation initiation and supporting larger, more rapid protein production. This mechanism is a key driver of muscle protein accretion and hypertrophy in response to resistance training and sufficient amino acids, especially leucine. It is not a transcription factor, nor a phosphatase, and it does play a crucial role in muscle adaptation. So describing mTOR as a protein kinase that is the major regulator of protein synthesis and muscle size captures its essential function in coordinating signals to boost muscle growth.