Cellular responses to retroviral capsid recognition

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PROJECT SUMMARY: TRIM5 is a multi-functional antiviral protein whose various actions in host defense are still being uncovered. Understanding the molecular mechanisms underlying these antiviral actions is an essential step towards the possible development of TRIM5-based host-directed antiviral therapies. TRIM5 is best known as an antiviral effector against diverse families of viruses including flaviviruses and retroviruses. TRIM5 also has roles in antiviral signaling that can trigger the expression of cytokines including type 1 interferon in response to retroviral pattern recognition. We previously reported a third major role for TRIM5: it acts as a positive regulator of autophagosome biogenesis and it physically interacts with proteins acting in multiple steps of the autophagy pathway. This raises the question of what the autophagy pathway and/or the autophagy machinery might be contributing to TRIM5’s antiviral activities. In this project, we will answer this question and work towards the long-term goal of understanding how TRIM5 coordinates its actions in defending against retroviral infection. Our preliminary data demonstrate that cells lacking autophagy-related proteins (ATGs) involved in upstream autophagy regulation, autophagosome membrane elongation, and autophagic cargo selection are unable to carry out TRIM5-directed inflammatory signaling. Whereas autophagy is typically considered a degradative process, in this setting the ATGs tested contributed to assembling active TRIM5 signaling complexes. This suggests that TRIM5 orchestrates novel, non-canonical functions of the ATGs with which it interacts. These findings support a hypothesis in which TRIM5’s actions in inflammatory signaling and in establishing an antiviral state are linked to its actions in autophagy. We will use cell biological, immunological, and proteomic approaches to test this hypothesis. We will uncover the role(s) of the autophagy pathway and individual autophagy-related proteins in TRIM5-dependent antiviral signaling (Aims 1 and 2). Our third Aim will uncover a novel TRIM5 signaling pathway connected to the inflammatory and autophagy- regulatory kinase TBK1, which we identified as a retrovirus-responsive TRIM5 interactor through proteomic analysis. Understanding TRIM5 signaling is significant, since TRIM5 signaling could explain why certain TRIM5 alleles confer protection against HIV infection in people despite human TRIM5’s relative inability to directly restrict HIV. As outcomes, we anticipate that our proposed studies will: i) reveal novel pathways for antiviral defense; ii) enable our understanding of how cells respond to detection of retroviral infection; and iii) provide mechanistic insight into how TRIM5, a protein that has shaped the evolution of primate retroviruses, acts in antiviral defense and innate immunity. We also expect that our findings will shed light on the broader TRIM family of proteins (TRIMs). This protein family consists of roughly 80 genes in humans. Like TRIM5, many TRIMs also have roles in antiviral defense, inflammation, and autophagy; thus understanding TRIM5 may provide a firm foundation for the study of other TRIMs.