MRC-University of Glasgow Centre for Virus Research

Scientists at the MRC-University of Glasgow's Centre for Virus Research (CVR), in collaboration with the European Molecular Biology Laboratory (EMBL) Heidelberg/University of Bayreuth, have made significant strides in understanding how our immune system combats RNA viruses, which include some of the world's most dangerous pathogens such as influenza and coronaviruses. Their new research focuses on the protein TRIM25, a key player in the body’s innate immune response, and uncovers how its ability to bind RNA is crucial for fighting off viral infections.

TRIM25, an E3 ligase enzyme, has long been recognised for its role in activating immune responses against RNA viruses. Previous studies showed that TRIM25 also binds to RNA, but the precise residues responsible for this interaction and the biological significance of RNA binding remained unclear. This knowledge gap limited our understanding of how TRIM25 functions in immune defense, and ultimately, in the broader antiviral response.

The CVR and EMBL team set out to solve this puzzle. They aimed to pinpoint the RNA-binding residues in TRIM25 and to create a version of the protein that retains its core enzymatic activity but is unable to bind RNA. This would allow the researchers to investigate the role of RNA binding in TRIM25’s antiviral function.

To achieve this, the team employed a comprehensive set of tools and methodologies, integrating structural biology, biochemistry, and cell biology. Led by the Hennig group at EMBL Heidelberg/University of Bayreuth, the team used advanced techniques like NMR, ITC, and SEC-MALS to study how TRIM25 binds to RNA. Then, at the CVR, the Castello group explored the protein’s antiviral role using iCLIP2 technology to map RNA-protein interactions and cellular assays to measure viral replication.

A highlight of the project was its collaborative nature, made possible by the prestigious EIPOD4 grant, which facilitated seamless synergy between the two groups. This grant enabled a researcher to travel between the two laboratories, combining expertise from both structural and cellular biology to unravel TRIM25's complex behaviour.

The research team made a critical discovery: specific amino acid residues in TRIM25 are responsible for its ability to bind RNA. Moreover, they engineered a mutant TRIM25 protein that cannot bind RNA but retains its E3 ligase activity. This breakthrough allowed them to demonstrate that RNA binding is not just a supplementary function—it is essential for TRIM25’s antiviral response.

The mutant protein failed to colocalise with viral replication organelles, and viral gene expression skyrocketed when TRIM25 could not bind RNA, mimicking the effects of completely losing TRIM25's enzymatic activity. By mapping TRIM25’s binding sites on viral RNA, the study provided a detailed view of how the protein targets viral genomes and exerts its antiviral effects.

Understanding how TRIM25 fights viral infections, particularly through its interaction with RNA, opens up new avenues for developing antiviral therapies. The research shows that disrupting RNA binding significantly weakens the immune response, suggesting that targeting this interaction could be a promising strategy in designing novel treatments.

As the world continues to face threats from RNA viruses, including the ongoing challenges posed by emerging viruses, this research provides a crucial piece of the puzzle in understanding our immune system's first line of defense. By deepening our knowledge of TRIM25's antiviral mechanisms, this study lays the foundation for future exploration into therapeutic interventions aimed at enhancing immune responses to viral infections.

To dive deeper into this exciting discovery, the full research article is available now.

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First published: 8 October 2024