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Phase separation of multivalent protein and RNA molecules underlies the biogenesis of biomolecular condensates such as membraneless organelles. In vivo, these condensates encompass hundreds of distinct types of molecules that typically organize into multilayered structures supporting the differential partitioning of molecules into distinct regions with distinct material properties. The interplay between driven (active) versus spontaneous (passive) processes that are required for enabling the formation of condensates with coexisting layers of distinct material properties remains unclear. Here, we deploy systematic experiments and simulations based on coarse-grained models to show that the collective interactions among the simplest, biologically relevant proteins and archetypal RNA molecules are sufficient for driving the spontaneous emergence of multilayered condensates with distinct material properties. These studies yield a set of rules regarding homotypic and heterotypic interactions that are likely to be relevant for understanding the interplay between active and passive processes that control the formation of functional biomolecular condensates.

Phase separation of multivalent protein and RNA molecules underlies the biogenesis of biomolecular condensates such as membraneless organelles. In vivo, these condensates encompass hundreds of distinct types of molecules that typically organize into multilayered structures supporting the differential partitioning of molecules into distinct regions with distinct material properties. The interplay between driven (active) versus spontaneous (passive) processes that are required for enabling the formation of condensates with coexisting layers of distinct material properties remains unclear. Here, we deploy systematic experiments and simulations based on coarse-grained models to show that the collective interactions among the simplest, biologically relevant proteins and archetypal RNA molecules are sufficient for driving the spontaneous emergence of multilayered condensates with distinct material properties. These studies yield a set of rules regarding homotypic and heterotypic interactions that are likely to be relevant for understanding the interplay between active and passive processes that control the formation of functional biomolecular condensates.

The toxic effects of C9orf72-derived arginine-rich dipeptide repeats (R-DPRs) on cellular stress granules in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia remain unclear at the molecular level. Stress granules are formed through the switch of Ras GTPase-activating protein-binding protein 1 (G3BP1) by RNA from a closed inactive state to an open activated state, driving the formation of the organelle by liquid-liquid phase separation (LLPS). We show that R-DPRs bind G3BP1 a thousand times stronger than RNA and initiate LLPS much more effectively. Their pathogenic effect is underscored by the slow transition of R-DPR-G3BP1 droplets to aggregated, ThS-positive states that can recruit ALS-linked proteins hnRNPA1, hnRNPA2, and TDP-43. Deletion constructs and molecular simulations show that R-DPR binding and LLPS are mediated via the negatively charged intrinsically disordered region 1 (IDR1) of the protein, allosterically regulated by its positively charged IDR3. Bioinformatic analyses point to the strong mechanistic parallels of these effects with the interaction of R-DPRs with nucleolar nucleophosmin 1 (NPM1) and underscore that R-DPRs interact with many other similar nucleolar and stress-granule proteins, extending the underlying mechanism of R-DPR toxicity in cells. Our results also highlight characteristic differences between the two R-DPRs, poly-GR and poly-PR, and suggest that the primary pathological target of poly-GR is not NPM1 in nucleoli, but G3BP1 in stress granules in affected cells. [ABSTRACT FROM AUTHOR]