Cauliflower mosaic virus protein P6 forms a microenvironment for RNA granule proteins and interferes with stress granule responses

Biomolecular condensation is a multipurpose cellular process that viruses use ubiquitously in their multiplication. CaMV replication complexes are condensates that differ from most viruses in being non-membranous assemblies and consist of RNA and protein, mainly viral protein P6. Despite description of these viral factories already half a century ago with many observations that followed since, functional details of the condensation process, their properties and relevance has remained enigmatic. Our main findings include a large dynamic mobility range of host proteins within viral factories, while the viral matrix protein P6 is immobile in accordance with representing the central node of these condensates. As novel components of VFs we identify stress granule (SG) nucleating factors G3BP7 and the UBP1 family. Similarly, as SG components localize in VFs during infection, ectopic P6 localizes to SGs and reduces their assembly after stress. Intriguingly, it appears that soluble rather than condensed P6 suppresses SGs and mediates also other essential P6 functions, suggesting that the increased condensation over the infection time-course may accompany a progressive shift in selected P6 functions. Together, this study highlights VFs as dynamic condensates and P6 as a complex modulator of SG responses.