Deciphering the role of CCR4-NOT in human cytomegalovirus infection

Human cytomegalovirus (HCMV) is a common herpes virus which is not usually problematic for those in the population with a normally functioning immune system. However, congenital infections can lead to deafness and other neurological problems and place HCMV as the leading infectious cause of birth defects in the UK. HCMV is also a major cause of complications following organ transplantation, where infections of immunosuppressed transplant recipients typically result from reactivation of latent HCMV.

In other DNA virus infections RNA decay is manipulated to evade immune responses and to ensure efficient and precisely timed viral gene expression. The importance of cellular RNA decay pathways and how they might be manipulated during HCMV infection however has not been fully explored.

Significantly, I have recently discovered that a cellular RNA decay enzyme, the CCR4-NOT deadenylase complex is required for efficient HCMV replication. CCR4-NOT acts by degrading the poly(A)-tails on messenger RNAs (mRNAs) which is generally the first and rate-limiting step of RNA decay for most mRNAs. Poly(A)-tails are considered important determinants of the stability and efficiency of translation of mRNAs into protein.

We also found that viral mRNA poly(A)-tails are longer than host mRNA poly(A)-tails and are less sensitive to CCR4-NOT, providing clues to understand viral regulation by this complex. Underlining the importance of the complex to the virus, HCMV produces a viral protein that interacts with CCR4-NOT and we found HCMV also upregulates two CCR4-NOT components.

How these changes impact complex function during infection is presently unknown. All our studies thus far have been in cells that support lytic infection of HCMV, but whether CCR4-NOT could also be involved in the maintenance of a latent infection that occurs in select cell types such as haematopoietic progenitor cells, is also unknown.

In this proposal we will investigate how CCR4-NOT regulates HCMV by establishing how mRNA stability and translation are affected when CCR4-NOT is disrupted during infection (Aim 1). We will also investigate how infection and the viral CCR4-NOT interacting protein physically and functionally affect CCR4-NOT (Aim 2). Finally, we will investigate the importance of CCR4-NOT and poly(A)-tail length regulation during HCMV latency, since this lifecycle phase is particularly important to HCMV disease in transplant recipients and how viral gene expression is regulated during latency is not well understood (Aim 3).

Together, these research objectives will help me meet my overall goal of deciphering the role of CCR4-NOT in HCMV infection.

Understanding this will illuminate a fundamental facet of HCMV's interaction with its host cell, with the potential to reveal new ways to inhibit HCMV and which could also have significant implications for other related medically important viruses. Furthermore, by leveraging HCMV as a cell biology discovery tool, this project will reveal novel aspects to the regulation of the CCR4-NOT complex, an essential node in eukaryotic post-transcriptional gene expression control.