Human cytomegalovirus infection maintains mTOR activity and its perinuclear localization during amino acid deprivation

The mammalian target of rapamycin (mTOR) kinase is present in 2 functionally distinct complexes, mTOR complex 1 (mTORC1) and complex 2 (mTORC2). Active mTORC1 mediates phosphorylation of eIF4E-binding protein (4E-BP) and p70 S6 kinase (S6K), which is important for maintaining translation. During human cytomegalovirus (HCMV) infection, cellular stress responses are activated that normally inhibit mTORC1; however, previous data show that HCMV infection circumvents stress responses and maintains mTOR kinase activity. Amino acid deprivation is a stress response that normally inhibits mTORC1 activity. Amino acids can signal to mTORC1 through the Rag proteins, which promote the colocalization of mTORC1 with its activator Rheb-GTP in a perinuclear region, thereby inducing 4E-BP and S6K phosphorylation. As expected, our results show that amino acid depletion in mock-infected cells caused loss of mTORC1 activity and loss of the perinuclear localization; however, there was no loss of activity or perinuclear localization in HCMV-infected cells where the perinuclear localization of Rheb-GTP and mTOR coincided with the perinuclear assembly compartment (AC). This suggested that HCMV infection bypasses normal Rag-dependent amino acid signaling. This was demonstrated by short hairpin RNA (shRNA) depletion of Rag proteins, which had little effect on mTORC1 activity in infected cells but inhibited activity in mock-infected cells. Our data show that HCMV maintains mTORC1 activity in an amino acid- and Rag-independent manner through the colocalization of mTOR and Rheb-GTP, which occurs in association with the formation of the AC, thus bypassing inhibition that may result from lowered amino acid levels.