CASC3 Biomolecular Condensates Restrict Turnip Crinkle Virus by Limiting Host Factor Availability

Area of Science:

• Plant virology
• Molecular biology
• RNA virus-host interactions

Background:

• The exon-junction complex (EJC) regulates gene expression and has known antiviral roles against some RNA viruses.
• The full spectrum of EJC-targeted viruses and the mechanisms involved remain largely uncharacterized.

Purpose of the Study:

• To investigate the antiviral activity of Arabidopsis thaliana EJC components against Turnip crinkle virus (TCV).
• To elucidate the mechanisms underlying CASC3-mediated antiviral activity.

Main Methods:

• Screening of plant EJC components for antiviral activity against TCV in Nicotiana benthamiana.
• Overexpression and knock-down studies of CASC3.
• Analysis of CASC3 condensate formation and co-localization.
• Mass spectrometry and bimolecular fluorescence complementation to identify CASC3 interacting proteins.
• Assessing the impact of host factor (Hsp70-1, GAPDH) overexpression on CASC3 antiviral activity.

Main Results:

• Overexpression of CASC3 significantly inhibited TCV accumulation, while its knock-down enhanced viral load.
• CASC3 forms distinct cytoplasmic condensates, and its SELOR domain is crucial for antiviral activity.
• CASC3 sequesters host factors Hsp70-1 and GAPDH, which are vital for tombusvirus replication.
• CASC3 also restricted the accumulation of the unrelated Tobacco mosaic virus (TMV), indicating broad antiviral potential.
• CASC3's antiviral activity is not solely dependent on condensate formation, as demonstrated by ARF19.

Conclusions:

• CASC3 exhibits broad-spectrum antiviral activity against positive-strand RNA viruses by limiting the availability of essential host factors.
• CASC3-mediated viral inhibition is linked to its ability to form unique cytoplasmic condensates and sequester host proteins.
• These findings expand our understanding of EJC's role in plant antiviral defense mechanisms.