Organization of the Proteostasis Network of Membraneless Organelles

Abstract Membraneless organelles (MLOs) are dynamic macromolecular condensates that act as crucibles to modulate biological process within the cell. Since MLOs form in the absence of lipid membranes, it is important to understand how their effective regulation is achieved by the protein homeostasis (proteostasis) system. To address this question, we report a comprehensive mapping of the proteostasis network (PN) of MLOs, comprising over 220,000 protein-protein interactions. The analysis of this PN reveals how the regulatory proteins (PN proteins) occupy central roles in the overall protein-protein interaction network of MLOs. We then investigate which branches of the PN are most important in the regulation of MLOs, and find that the anabolic component, which makes up ∼30% of the PN, is more closely involved with MLOs than the catabolic component, which makes up the remaining ∼70% of the PN. We also find that translation-related PN proteins and molecular chaperones play central roles in the regulation of MLOs. We then analyse the similarity between the PNs of different MLOs, revealing that stress granules and P-bodies share large portions of their PNs. Finally, to understand how specificity is achieved despite these similarities, we further analyse the PN organization, showing that HSP70 chaperones are generic in regulating MLOs, while client-specific HSP70 co-chaperones confer specificity to the chaperone action. Taken together, these results identify the composition of the PN of MLOs, rationalize its organization, and reveal a central role of molecular chaperones in the regulation of protein conformations within MLOs.