TRANSLATIONAL EFFICIENCY OF HERPESVIRUS MESSENGER RIBONUCLEIC ACIDS

Abstract

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the causative agent of Kaposi’s sarcoma and primary effusion lymphoma. Herpesvirus genomes are decoded by host RNA polymerase enzymes, generating messenger ribonucleotides (mRNA) that are post-transcriptionally modified and exported to the cytoplasm through the combined work of host and viral factors. These viral mRNA bear 5ʹ-m7GTP caps and poly(A) tails that should permit assembly of canonical host eIF4F cap-binding complexes to initiate protein synthesis. However, the precise mechanisms of translation initiation remain to be investigated for KSHV and other herpesviruses. Mechanistic target of rapamycin complex 1 (mTORC1) normally promotes eIF4F assembly in response to sufficient growth signals and nutrient abundance, while simultaneously suppressing autophagy, an important mechanism of cellular catabolism. Here I demonstrate that mTORC1 activity supports viral reactivation from latency, but this requirement can be relieved by silencing the essential autophagy gene ATG14. I show that mTORC1 is activated during KSHV lytic replication, but unresponsive to normal inhibitory signals. mTORC1 activity is required for eIF4F assembly during viral latency and lytic replication, but dispensable for suppression of autophagy. eIF4F contributes to bulk protein synthesis during lytic replication, but it is dispensable for the efficient synthesis of viral proteins and subsequent release of infectious progeny virions. KSHV virions also accumulate normally when known non-eIF4F translation initiation factors are depleted. To identify proteins required to support viral protein synthesis, I isolated and characterized actively-translating messenger ribonucleoprotein (mRNP) complexes by ultracentrifugation and sucrose-gradient fractionation followed by quantitative mass spectrometry. This analysis revealed the presence of viral proteins in mRNP complexes that had not previously been shown to play roles in viral protein synthesis. This work demonstrates that KSHV can disrupt cellular mTORC1 regulation to ensure efficient viral protein synthesis and evasion of autophagy during lytic replication.