RNA promotes synapsin phase separation providing a platform for local translation.

Condensates at synapses organize synaptic vesicle (SV) clusters and are essential for efficient neurotransmitter release. While it is established that RNA granules traffic along the axons, the function of RNA at the presynapse remains unclear. Here, we uncover a direct structural role of coding RNAs in organizing presynaptic condensates by focusing on SV clusters, condensates between synapsin-1 and lipid vesicles, a defining feature of nerve terminals. Using in vitro reconstitution systems, we show that RNA drives synapsin-1 coacervation, with bias toward structured RNAs being more effective at promoting phase transitions. The importance of RNA was confirmed in living synapses, where acute disruption of native RNA induces a dispersion of SVs and synapsin. Conversely, ectopically expressed SV-like condensates have the ability to recruit the translational machinery. The microscopy-based in vitro translation assay demonstrates increased translation efficiency within synapsin-1/RNA condensates. Together, our work indicates a novel structural role of RNAs in modulating SV condensates.