The role of the murine cytomegalovirus protein m139 in viral tropism

Abstract

Cytomegaloviruses (CMVs) have a highly restricted host range but exhibit a remarkably broad cell tropism in their natural host. The molecular basis of this broad tropism lies in the large viral genome, which contains a large number of genes encoding factors capable of overcoming cellular barriers and conferring the ability to replicate even in the face of specific cellular defense mechanisms. Macrophages and endothelial cells play important roles in the pathogenesis of CMV infection, as they promote viral dissemination and persistence in the host. Additionally, macrophages are important for the regulation of innate and inflammatory immune responses, thus linking the anti-viral immune defense to the immune-mediated pathogenesis. The protein m139 encoded by murine cytomegalovirus (MCMV) was previously identified as an important determinant of viral replication in murine macrophages, but the underlying mechanism has remained unknown. The aim of my doctoral research study was to characterize the function of m139 on the molecular level and to understand its role as a host range factor. I could show that m139 is an early protein, which localizes to the cytoplasm, and is also recruited to viral replication compartments within the host cell nucleus. By combining stable isotope labelling by amino acids (SILAC) with affinity purification and mass spectrometry, I identified two host proteins, the DEAD box RNA helicase DDX3 and the E3 ubiquitin ligase UBR5, as interaction partners of m139. Both factors were found to be recruited to viral replication compartments. Inactivation of m139 in the MCMV genome resulted in a replication defect in macrophages and endothelial cells. The latter was rescued in DDX3 and UBR5 knockouts cells, suggesting that m139 modulates the DDX3 and UBR5 dependent pathways to facilitate efficient MCMV replication in these cells. In macrophages, m139 was also found to be important for viral replication and a negative regulator of DDX3-dependent type I interferon induction. The biological importance of m139 in MCMV replication and dissemination in mice was confirmed in vivo. I could further show that inactivation of m139 facilitates MCMV replication in human epithelial cells. Thus, m139 has opposite functions: while it enhances MCMV replication in murine macrophages and endothelial cells, it is detrimental for viral replication in human cells.