Coronavirus nucleocapsid proteoforms inhibit antiviral biomolecular condensates

Viruses are small but mighty. Despite having tiny genomes, they make big changes in host cells, hijacking host cell machinery and stealing resources to copy the viral genome, produce viral proteins, and assemble these components into new viruses, all while evading highly sophisticated cellular surveillance systems. Viruses can multitask in this way by making proteins that have more than one function. A good example is the multifunctional nucleocapsid (N) protein of coronaviruses (CoVs). N protects the viral genome while supporting genome copying and packaging genomes into new viruses. N also suppresses cellular antiviral immune responses by inhibiting the function of cellular granules called processing bodies and stress granules. How can N have so many different roles, and how are these roles of the N protein regulated in space and time? Our lab's recent work has revealed the answer - N is not one protein, but rather a collection of proteins. We have discovered that for SARS-CoV-2, the cause of COVID-19, and common-cold CoVs, OC43 and 229E, N exists as an array of smaller truncated versions, as well as the full-length N protein. This means that some of the roles previously attributed to full-length N may be performed by truncated N proteins. We discovered that one truncated form of SARS-CoV-2 N decreases the cellular response to viral infection by causing processing body disassembly, blocking stress granules, and binding to double-stranded RNA in a manner distinct from its full-length parent N molecule. Our project will elucidate the role of CoV truncated N products during infection and define how these products inhibit cellular responses to viruses that are governed by antiviral granules. Together, this will reveal how truncated N production allows CoVs to make the most of their limited genetic capacity, enabling N to be 'everything, everywhere, all at once'.