New insights in the ubiquitylation-related mechanisms in African swine fever virus infection

Resumen

In this work, we have investigated ubiquitylation-related mechanisms in African swine fever virus (ASFV) infection. The ubiquitin-proteasome system (UPS) is a tightly regulated organelle that control many cellular processes, including viral infections in the cell. Several viruses manipulate the ubiquitin-proteasome system (UPS) to initiate a productive infection. We determined that inhibition of proteasome blocked a post-internalization step, affecting ASFV replication in Vero cells. Under proteasome inhibition, ASF viral genome replication, late gene expression and viral production were severely reduced. Also, ASFV enhanced proteasome activity at late times and induced the accumulation of Lys63-polyubiquitinated proteins surrounding viral factories. Core-associated and/or viral proteins involved in DNA replication may be targets for the ubiquitin-proteasome system that could possibly assist virus uncoating at final core breakdown and viral DNA release. At later steps, polyubiquitinated proteins at viral factories could exert regulatory roles in cell signaling. Determined viral proteins are able to control the host cell ubiquitin machinery and some viruses even encode their own ubiquitinating or deubiquitinating enzymes. African swine fever virus (ASFV) encodes a gene homologous to the E2 ubiquitin conjugating (UBC) enzyme. We verified that the viral ubiquitin-conjugating enzyme (UBCv1) is an early protein that expressed throughout ASFV infection and accumulates at late times. UBCv1 is also present in the viral particle suggesting that the ubiquitin-proteasome pathway could play an important role at early stages of ASFV infection. Indeed, we corroborated the conjugating activity of this viral E2 enzyme, depending on its catalytic domain, that was able to bind several types of polyubiquitin chains. We also characterized potential UBCv1 host targets by mass spectrometry. This proteomic analysis revealed that the early viral protein interacted with the initiation translation factor eIF4E. This was consistent with previous results that pointed a relation with the 40S ribosome subunit RPS23. These interactions indicated a possible function of UBCv1 in the viral regulation of host translation. Further analysis also revealed the interaction with the G protein Arf3, related with membrane traffic and organelle structure, and the interplay with the E3 ligase Cullin4B. ASFV-mediates innate immune response inhibition through a number of genes that have been previously studied in detail. Here, we contribute a new ASFV gene involved in the regulation of the innate immune response. UBCv1 impaired NF-κB and AP-1 transcription factors activation while had no effect neither in interferon β production nor in interferon regulatory factor (IRF) activation. We detected that UBCv1 induced a decrease in IκBα phosphorylation and the inhibition of p65 translocation into the nucleus. We propose that UBCv1 blocked both signalling pathway at the level of IKK kinases. Finally, our studies and results were completed with the transcriptome analysis obtained by Next Generation sequencing of ASFV infected macrophages