All viruses share the requirement for host ribosomes for the synthesis of viral proteins. Viral mRNAs must compete with host messages for access to the host translation machinery, and therefore viruses have evolved multiple mechanisms to ensure that viral mRNAs are preferentially translated during infection. Many viruses inhibit host protein synthesis, presumably to provide greater resources for the translation of viral mRNAs. Human cytomegalovirus (HCMV) is unusual in that infection does not inhibit host protein synthesis. How do HCMV mRNAs effectively compete with host mRNAs for access to ribosomes? The recruitment of ribosomes to mRNAs is facilitated by translation initiation factors. Control of translation initiation factor expression and activity is a key regulatory step in mRNA translation. Previous studies have shown that HCMV induces the expression and activity of the eIF4F translation initiation complex, which is required for the translation of capped mRNAs. As most mRNAs are capped, the eIF4F complex is required for the translation of most host mRNAs. However our preliminary studies suggest that, surprisingly, inhibiting host signaling pathways that control eIF4F activity has minimal impact on HCMV mRNA translation. This suggests that the mechanism of translation initiation on viral mRNAs is fundamentally different from that used by host mRNAs. Our data suggests that the HCMV TRS1 protein (pTRS1) acts as a viral translation initiation factor that replaces the function of the eIF4F complex on viral mRNAs. These data lead us to our overall hypothesis that viral proteins direct the loading of ribosomes onto HCMV mRNAs through a process that does not require the eIF4F complex. This hypothesis shifts the paradigm for the translation of HCMV mRNAs, from one in which host proteins are the critical regulatory factors, to one in which HCMV proteins are the primary determinants of viral protein synthesis. We test our hypothesis in two specific aims.
In Aim1 we use a combination of genetic, chemical, and genomics approaches to define the role of the eIF4F complex and it subunits in HCMV mRNA translation throughout infection.
Aim 2 tests our hypothesis that pTRS1 is a viral translation initiation factor that substitutes for the eIF4F complex during the translation of viral mRNAs. We will determine how i) pTRS1 associates with the translation machinery, ii) the full complement of mRNAs bound by pTRS1 during infection and iii) the role of pTRS1 interaction with the host translation machinery in HCMV mRNA translation and replication. Statement of public health relevance: Our novel hypothesis suggests the existence of HCMV proteins that specifically direct the synthesis of viral proteins. Viral translation initiation factors represent a novel class of targets for new antiviral drugs that woul specifically viral, but not host, protein synthesis. These drugs would allow for the continued synthesis of antiviral host proteins, and would have reduced toxicity due to their specificity for infected cells.
Our novel hypothesis suggests the existence of HCMV proteins that specifically direct the synthesis of viral proteins. Viral translation initiation factors represent a novel class of targets for new antiviral drugs that woul specifically viral, but not host, protein synthesis. These drugs would allow for the continued synthesis of antiviral host proteins, and would have reduced toxicity due to their specificity for infected cells. PUBLIC HEALTH RELEVANCE: HCMV is a significant human pathogen that causes severe disease in children and immunocompromised adults. By defining the mechanism(s) by which host and viral translation factors contribute to HCMV protein expression, we will uncover new targets for novel antiviral therapeutics specifically targeting viral protein expression. These studies will also investigate a novel use for existing compounds targeting host translation factors as antiviral drugs.
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