Dealing with stress: Human cytomegalovirus maintains translation in cells under adverse conditions

The Human Cytomegalovirus (HCMV) requires its host cell's translational machinery to produce its proteins. Mammalian target of rapamycin (mTOR) kinase-mediated signaling regulates translation initiation so that it occurs only when nutrient and energy levels are sufficient to support protein production. HCMV-induced increases in cellular processes and metabolism can deplete resources enough that the infected cell initiates stress signaling. Stress signaling inhibits mTOR mediated-signaling, and thus translation, which would impede HCMV protein production and growth. The experiments described herein evaluate the ability of HCMV to translate proteins and grow under conditions that simulate the inhibitory affects of stress on mTOR-mediated signaling. MTOR associates in two complexes distinguishable by their functions and sensitivities to the drug rapamycin. The rapamycin-sensitive Raptor complex regulates translation while the rapamycin-insensitive Rictor complex regulates Akt kinase activity. The Raptor complex promotes cap-dependent translation by phosphorylating and inactivating the eIF4E binding protein (4E-BP). This allows eIF4E and the capped mRNA to which it is bound to associate in the eIF4F complex which delivers mRNAs to ribosomes for their translation. The data show HCMV activates both mTOR complexes and promotes eIF4F formation under normal infection conditions. Furthermore, they show that HCMV can adapt both upstream regulators and downstream targets of the Raptor complex to ensure that the translational machinery remains active in the presence of stress or other conditions that normally inhibit its activity. Raptor and rictor shRNA-mediated depletion studies show that rictor is more significant than raptor for both HCMV growth and the inhibitory effects of rapamycin on HCMV growth. Interestingly, HCMV appears to change the substrate specificity of the Rictor complex such that it can phosphorylate the two Raptor complex targets, 4E-BP and p70S6K, in infected cells. HCMV also makes the Rictor complex rapamycin sensitive towards the hyperphosphorylation of both targets, and the Raptor complex rapamycin-insensitive towards 4E-BP phosphorylation. In toto, the data show that HCMV significantly adapts mTOR-mediated signaling such that translation and virus growth proceed under normal and cellular stress conditions.