C. PATZINA¹, F. GOLEBIOWSKI¹, B.G. HALE<sup>1

1</sup>Institute of Medical Virology, University of Zurich, Zurich, Switzerland

Type-I interferon (IFN) signaling is an essential part of the body`s innate defense against viruses. Upon detection of foreign pathogen-associated patterns, every cell is capable of mounting this rapid response to put itself and its neighbors into an antiviral state. Since exacerbated IFN signaling is detrimental to the host, while insufficient activation can result in severe infection, the pathway is tightly regulated by mechanisms including posttranslational protein modifications like SUMOylation. SUMO has just recently been tied to innate immunity, and can have a profound impact on the antiviral action of IFN, as well as negatively regulating IFN signaling. Since many aspects of IFN regulatory functions of protein SUMOylation are still unknown, we aim to elucidate the IFN regulating activity of SUMO by characterizing the specific cellular targets of (de-)SUMOylation upon induction of IFN signaling, as well as by defining the SUMO enzymes involved in this response. Therefore, we established a screening system using reporter cell lines that allowed us to identify SUMO enzymes, i.e. putative and known SUMO E3 ligases and SUMO-specific proteases (SENPs), that mediate SUMOylation of specific target proteins involved in the IFN induction and signaling pathways. In addition, we defined the subset of target proteins that change in their SUMOylation status upon induction of IFN signaling by performing a SILAC-based comparative mass spectrometry screen of SUMO targets in the presence or absence of a strong IFN-inducing agent. Functional validation of these factors will give insights into their impact on the IFN pathway, as well as on their influence on virus replication. These findings will ultimately broaden our knowledge on how innate immunity is fine-tuned by SUMO to prevent excessive as well as insufficient responses to pathogens, and might eventually reveal novel treatment options for innate immune disorders.