Modular structure of RNA 3’ processing condensates involving the Arabidopsis RNA binding protein FCA

The Arabidopsis RNA binding protein FCA is found in liquid-like nuclear condensates and promotes proximal polyadenylation of specific nascent transcripts. To understand the in vivo interactions required for these condensates we used single-particle tracking experiments on FCA expressed at endogenous levels in live cells. These revealed FCA forms a core tetramer that multimerizes into higher-order particles (∼16 to 56 FCA molecules), corresponding to condensates observed using confocal microscopy. The coiled-coil protein FLL2, genetically required for FCA function and condensate formation, showed co-localization primarily to the larger condensates. FCA regulates FLC , however, its in vivo co-localization with an FLC lacO /LacI transgene was found to be infrequent. A missense mutation in the FCA RRM domain showed RNA binding to FCA is necessary for function. This reduced RNA-binding activity, attenuated FCA condensate formation, 3’ RNA processing and FLC repression, however it did not influence the core tetramer. Our work points to a modular structure for FCA condensates centred around a core of four FCA molecules, which multimerize to larger functionally important condensates via associated RNA and FLL2 interaction, with only transient residency at their site of action.