Targeting viral liquid–liquid phase separation

Liquid–liquid phase separation (LLPS) has recently emerged as a novel mechanism of compartmentalization of biomolecules that form membraneless organelles within cells. Studies have shown that several viral proteins and nucleic acids undergo LLPS to form condensates that support viral replication. Now, two recent studies provide evidence that LLPS during viral replication is a target for antiviral therapy. Risso-Ballester et al. discovered that a steroidal alkaloid cyclopamine and its chemical analogue (A3E) inhibit respiratory syncytial virus (RSV) replication by disorganizing and hardening viral inclusion body (IB) condensates, where viral RNA synthesis occurs. In untreated infected cells, viral IBs had a dynamic nature and the RSV phosphoprotein had a liquid-like behaviour, consistent with the formation of viral IBs through LLPS. In compound-treated cells, IB disorganization occurred within minutes, suggesting that these molecules directly inhibit the liquid properties of RSV IBs. The authors also demonstrated that A3E suppresses viral replication in RSV-infected mice. In a separate study, Wang et al. found that targeting LLPS of the SARS-CoV-2 nucleocapsid protein (NP) with a peptide promotes innate antiviral immunity in vitro and in vivo. Previous work showed that NP phase separates into condensates that contain viral RNA and the viral RNA polymerase. In this study, Wang et al. found that the NP dimerization domain (DD) is required for NP to undergo LLPS with RNA, which prevents Lys63-linked poly-ubiquitination and aggregation of mitochondrial antiviral-signalling protein (MAVS), thus suppressing innate immunity in infection. Using a screening approach, the authors identified a peptide that targets the DD. Remarkably, treatment with the peptide led to the disruption of NP LLPS and increased innate antiviral responses in vitro and in mice. Together, these two studies highlight the potential of targeting LLPS to treat viral infections by disrupting viral replication or boosting host immunity.