Phase Separation and Aggregation of a Globular Folded Protein Small Ubiquitin-like Modifier 1 (SUMO1)

Liquid-liquid phase separation (LLPS) plays a crucial role in cellular organization, primarily driven by intrinsically disordered proteins (IDPs) leading to the formation of biomolecular condensates. A folded protein SUMO that post-translationally modifies cellular proteins has recently emerged as a regulator of LLPS. Given its compact structure and limited flexibility, the precise role of SUMO in condensate formation remains to be investigated. Here, we show the rapid phase separation of SUMO1 into micrometer-sized liquid-like condensates in inert crowders under physiological conditions. Subsequent time-dependent conformational changes and aggregation are probed by label-free methods (tryptophan fluorescence and Raman spectroscopy). Remarkably, experiments on a SUMO1 variant lacking the N-terminal disordered region further corroborate the role of its structured part in phase transitions. Our findings highlight the potential of folded proteins to engage in LLPS and emphasize further investigation into the influence of the SUMO tag on IDPs associated with membrane-less assemblies in cells.

Automated summary

This study reveals the important role of folded protein SUMO in liquid-liquid phase separation (LLPS) in cells. The researchers show that SUMO1 can rapidly undergo phase separation into liquid-like condensates under physiological conditions. Moreover, the structured part of SUMO1 is found to play a crucial role in the phase transition process.