Arthropod-borne viruses (arboviruses) are a constant source of emerging infectious diseases. While all viruses co-opt the biological processes of the host cell and elude its antiviral defenses, arboviruses do so in both vertebrate hosts and arthropod vectors. How arboviruses are able to hijack the cell in such vastly divergent organisms is an enduring enigma in viral biology. At the molecular scale, viral misappropriation of cellular processes and evasion of antiviral defenses are believed to be largely mediated by physical interactions between viral components – both viral proteins and nucleic acid – and host cell proteins. As their outcome determines the susceptibility or resistance of cells to infection, these interactions represent major determinants of critical features of viral pathobiology, including host range, zoonotic potential and virulence, as well as the capacity of a virus to be transmitted by a given arthropod vector (vector competence). Ultimately, these interactions represent key molecular drivers of viral emergence. While interactions between viral proteins and cellular proteins have been extensively studied, at least so far as the human host is concerned, viral RNA interactomes have recently come into their own. The RNA genome of certain mosquito-borne flaviviruses has been shown to interact with numerous cellular proteins –RNA-binding proteins (RBP) – which may support viral replication or exert antiviral activity, as viral dependency or restriction factors, respectively. Remarkably, a 3’ fragment of viral RNA – subgenomic flaviviral RNA (sfRNA) – accumulates to high levels in flavivirus-infected cells. sfRNA is thought to antagonize antiviral immunity in both mammalian hosts and arthropod vectors, partly by sequestering critical RBP. The BeatNIC project will address how arboviruses usurp the cell in phylogenetically distant species, and more particularly for an understudied flavivirus, the tick-borne encephalitis virus (TBEV). TBEV is a medically significant arbovirus in Europe, where it is principally transmitted by Ixodes ricinus ticks. To elucidate how TBEV usurps the cell in evolutionarily distant species, we will apply state-of-the-art RNA-centric proteomics to resolve the RNA interactome of TBEV – for both genomic and sfRNA – in human cellular models, and notably in primary human neural cells, as well as in I. ricinus cell lines. Based on the sets of discovered RBP, gene perturbation studies, performed by RNA interference, will be deployed to define their functional role in the viral life cycle, as restriction or dependency factors, and in neurovirulence. For selected I. ricinus RBP, their role in not only viral replication, but also vector competence, will be explored in vivo, in ticks themselves, again using a gene perturbation strategy. For RBP shown to condition the replication of TBEV, their impact on the replication of other arboviruses will be explored. We will, moreover, specifically address the role of sfRNA in TBEV replication – including in ticks – and in neurovirulence. Data acquired separately for H. sapiens and I. ricinus species will be integrated by bioinformatic analyses, so as to determine shared and distinct features of TBEV biology within its mammalian host and arthropod vector. These data will be integrated with comparable data sets for mosquito-borne flaviviruses. Comparison of viral RNA and viral protein interactomes in both the human host and the I. ricinus tick vector should elucidate the strategies that allow the same viral components to subjugate cells in vastly different intracellular milieux. While functionally conserved interactions will highlight the unnegotiable requisites for viral replication, human- or arthropod-specific interactions will shed light on virus-host co-evolution in phylogenetically remote species. We thus expect our study to illuminate the evolutionary processes that allow tick-borne viruses to alternate between phylogenetically distant species.