Crimean-Congo hemorrhagic fever virus (CCHFV) is a ssRNA (-) nairovirus that produces fever, prostration, and severe hemorrhages in humans. Fatality rates associated with CCHFV range from 5- 80% based on phylogenetic variation of the virus, transmission route, and different treatment facilities. Originally identified in Russia and he Congo, CCHFV has rapidly spread across large sections of Europe, Asia, and Africa. Recently, U.S. citizen traffic has increased substantially to the regions endemic with CCHFV, specifically South Central Asia. As a result, there is a substantial risk for transmission of CCHFV and/or its tick vector to the United States. Intriguingly, CCHFV is not the only nairovirus that threatens the public. Nairobi Sheep Disease virus (NSDV) as well as nairoviruses Hazara, Dugbe and Erve can cause human disease of varying severity and economic distress. Currently, there is no vaccine or prophylactic available for treatment of CCHF or other any other nairovirus related diseases. Recent reports have identified a viral homologue of the ovarian tumor protease (vOTU) located within the nairovirus genome and implicated its possible involvement in down-regulation of the Interferon type 1 immune response through cleavage of post-translational modifying proteins ubiquitin (Ub) and Ub-like interferon-simulated gene 15 (ISG15). As a result, it has been suggested to be a virulence factor. This proposal will determine whether dual deubiquitinating and deISGylating activities are conserved functions of this subclass of proteases in vitro and in vivo. Additionally, these experiments will gain insight into their mechanism of recognition for Ub and ISG15 allowing greater predictability of currently unknown vOTUs. The proposal will also seek to evaluate a potential correlation between the in vitro activity/substrate specificity of these nairovirus vOTUs and overall virulence of the nairoviruses in question. The resulting information will also provide critical insight into the role of vOTUs ply in viruses that may ultimately have practicality in the development of prophylactics targeting vOTUs.