A multidisciplinary approach defining the functions and regulation of the antiviral protein IFIX

Abstract

A critical aspect of cellular defense is the intracellular recognition of foreign pathogens. Among these important defense factors are DNA sensors, proteins able to bind to foreign DNA and subsequently trigger immune responses. Recently, our lab identified a new viral DNA sensor, the interferon inducible protein IFIX. However, its mechanisms of action remained unknown. Therefore, for my PhD studies, I used an integrative approach that couple mass spectrometry-based proteomics with virology, molecular biology, and microscopy to characterize this important antiviral factor. I find that IFIX is capable of localizing to viral DNA both in the nucleus and cytoplasm of cells, in response to herpes simplex virus 1 (HSV-1) infection or vaccinia virus transfection, respectively. As IFIX is predominantly nuclear, I characterize the IFIX localization-dependent sensing by defining nuclear localization signals, and demonstrate its modulation by acetylation. Since the location of viral DNA is largely dependent on the type of virus and the host cell, these studies highlight the importance of mechanistic studies in understanding the regulation of early innate immune responses. To discover how IFIX exerts its antiviral functions, I focused on the human virus, HSV-1. By characterizing IFIX interactions with human and viral proteins during the progression of infection, I discovered potential antiviral functions. I establish that IFIX associates with proteins involved in transcriptional regulation. I demonstrate a dependency on IFIX in reducing HSV-1 viral progeny as well as viral gene transcription. Interestingly, I found that during infection with HSV-1, IFIX is targeted for proteasomal degradation. This indicates yet another host defense protein targeted by HSV-1 to combat host cell immune responses. Altogether, my studies define IFIX as a critical anti-viral factor against DNA viruses and highlight several mechanisms involved in its defense functions.