Phase separation of both a plant virus movement protein and cellular factors support virus-host interactions

Phase separation concentrates biomolecules, which should benefit RNA viruses that must sequester viral and host factors during an infection. Here, the p26 movement protein from Pea enation mosaic virus 2 (PEMV2) was found to phase separate and partition in nucleoli and G3BP stress granules (SGs) in vivo . Electrostatic interactions drive p26 phase separation as mutation of basic (R/K-G) or acidic (D/E-G) residues either blocked or reduced phase separation, respectively. During infection, p26 must partition inside the nucleolus and interact with fibrillarin (Fib2) as a pre-requisite for systemic trafficking of viral RNAs. Partitioning of p26 in pre-formed Fib2 droplets was dependent on p26 phase separation suggesting that phase separation of viral movement proteins supports nucleolar partitioning and virus movement. Furthermore, viral ribonucleoprotein complexes containing p26, Fib2, and PEMV2 RNA were formed via phase separation in vitro and could provide the basis for self-assembly in planta . Interestingly, both R/K-G and D/E-G p26 mutants failed to support systemic trafficking of a Tobacco mosaic virus (TMV) vector in Nicotiana benthamiana suggesting that p26 phase separation, proper nucleolar partitioning, and systemic movement are intertwined. p26 also partitioned in SGs and G3BP over-expression restricted PEMV2 accumulation >20-fold. Expression of phase separation-deficient G3BP only restricted PEMV2 5-fold, demonstrating that G3BP phase separation is critical for maximum antiviral activity. AUTHOR SUMMARY Phase separation of several cellular proteins is associated with forming pathological aggregates and exacerbating neurodegenerative disease progression. In contrast, roles for viral protein phase separation in RNA virus lifecycles are less understood. Here, we demonstrate that the p26 movement protein from Pea enation mosaic virus 2 phase separates and partitions with phase-separated cellular proteins fibrillarin and G3BP. The related orthologue from Groundnut rosette virus has been extensively studied and is known to interact with fibrillarin in the nucleolus as a pre-requisite for virus movement. We determined that basic residues and electrostatic interactions were critical for p26 phase separation. Furthermore, mutation of charged residues prevented the rescue of a movement-deficient Tobacco mosaic virus vector in Nicotiana benthamiana . Stress granules form through phase separation and we found that p26 could partition inside stress granules following heat shock. Phase separation of the stress granule nucleator G3BP was required for maximum antiviral activity and constitutes a host response that is dependent on cellular protein phase separation. Collectively, we demonstrate that phase separation of a plant virus protein facilitates virus-host interactions that are required for virus movement and phase separation of cellular proteins can simultaneously restrict virus replication.