26Bluetongue virus (BTV) is an arbovirus transmitted by blood-feeding midges to a wide 27 range of wild and domestic ruminants. In this report, we showed that BTV, through its virulence 28 non-structural protein NS3 (BTV-NS3), is able to activate the MAPK/ERK pathway. In 29 response to growth factors, the MAPK/ERK pathway activates cell survival, differentiation, 30 proliferation and protein translation but can also lead to the production of several inflammatory 31 cytokines. By combining immunoprecipitation of BTV-NS3 and mass spectrometry analysis 32 from both BTV-infected and NS3-transfected cells, we identified the serine/threonine-protein 33 kinase B-Raf (BRAF), a crucial player of the MAPK/ERK pathway, as a new cellular interactor 34 of BTV-NS3. BRAF silencing led to a significant decrease of the MAPK/ERK activation by 35 BTV supporting a model where BTV-NS3 interacts with BRAF to activate this signaling 36 cascade. Furthermore, the intrinsic ability of BTV-NS3 to bind BRAF and activate the 37 MAPK/ERK pathway is conserved throughout multiple serotypes/strains but appears to be 38 specific to BTV compared to other members of Orbivirus genus. Inhibition of MAPK/ERK 39 pathway with U0126 reduced viral titers, suggesting that BTV manipulates this pathway for its 40 own replication. Therefore, the activation of the MAPK/ERK pathway by BTV-NS3 could 41 benefit to BTV replication by promoting its own viral protein synthesis but could also explain 42 the deleterious inflammation associated with tissue damages as already observed in severe cases 43 of BT disease. Altogether, our data provide molecular mechanisms to explain the role of BTV-44 NS3 as a virulence factor and determinant of pathogenesis. 45 author/funder. All rights reserved. No reuse allowed without permission. Importance 46 47Bluetongue Virus (BTV) is responsible of the non-contagious arthropod-borne disease 48 Bluetongue (BT) transmitted to ruminants by blood-feeding midges. Despite the fact that BTV 49 has been extensively studied, we still have little understanding of the molecular determinants 50 of BTV virulence. In this report, we found that the virulence protein NS3 interacts with BRAF, 51a key component of the MAPK/ERK pathway. In response to growth factors, this pathway 52 promotes cell survival, increases protein translation but also contributes to the production of 53 inflammatory cytokines. We showed that BTV-NS3 enhances the MAPK/ERK pathway and 54 this activation is BRAF-dependent. Our results demonstrate, at the molecular level, how a 55 single virulence factor has evolved to target a cellular function to ensure its viral replication. 56On the other hand, our findings could also explain the deleterious inflammation associated with 57 tissue damages as already observed in severe cases of BT disease. 58 author/funder. All rights reserved. No reuse allowed without permission.

Bluetongue virus (BTV) is an arbovirus transmitted by blood-feeding midges to a wide range of wild and domestic ruminants. In this report, we showed that BTV, through its nonstructural protein NS3 (BTV-NS3), is able to activate the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway, as assessed by phosphorylation levels of ERK1/2 and the translation initiation factor eukaryotic translation initiation factor 4E (eIF4E). By combining immunoprecipitation of BTV-NS3 and mass spectrometry analysis from both BTV-infected and NS3-transfected cells, we identified the serine/threonine-protein kinase B-Raf (BRAF), a crucial player in the MAPK/ERK pathway, as a new cellular interactor of BTV-NS3. BRAF silencing led to a significant decrease in the MAPK/ERK activation by BTV, supporting a model wherein BTV-NS3 interacts with BRAF to activate this signaling cascade. This positive regulation acts independently of the role of BTV-NS3 in counteracting the induction of the alpha/beta interferon response. Furthermore, the intrinsic ability of BTV-NS3 to bind BRAF and activate the MAPK/ERK pathway is conserved throughout multiple serotypes/strains but appears to be specific to BTV compared to other members of Orbivirus genus. Inhibition of MAPK/ERK pathway with U0126 reduced viral titers, suggesting that BTV manipulates this pathway for its own replication. Altogether, our data provide molecular mechanisms that unravel a new essential function of NS3 during BTV infection. IMPORTANCE Bluetongue virus (BTV) is responsible of the arthropod-borne disease bluetongue (BT) transmitted to ruminants by blood-feeding midges. In this report, we found that BTV, through its nonstructural protein NS3 (BTV-NS3), interacts with BRAF, a key component of the MAPK/ERK pathway. In response to growth factors, this pathway promotes cell survival and increases protein translation. We showed that BTV-NS3 enhances the MAPK/ERK pathway, and this activation is BRAF dependent. Treatment of MAPK/ERK pathway with the pharmacologic inhibitor U0126 impairs viral replication, suggesting that BTV manipulates this pathway for its own benefit. Our results illustrate, at the molecular level, how a single virulence factor has evolved to target a cellular function to increase its viral replication.