RNA-dependent proteome solubility maintenance in Escherichia coli lysates analysed by quantitative mass spectrometry: Proteomic characterization in terms of isoelectric point, structural disorder, functional hub, and chaperone network.

Protein aggregation, a consequence of misfolding and impaired proteostasis, can lead to cellular malfunctions such as various proteinopathies. The mechanisms protecting proteins from aggregation in complex cellular environments have long been investigated, often from a protein-centric viewpoint. However, our study provides insights into a crucial, yet overlooked actor: RNA. We found that depleting RNAs from Escherichia coli lysates induces global protein aggregation. Our quantitative mass spectrometry analysis identified over 900 statistically significant proteins from the Escherichia coli proteome whose solubility depends on RNAs. Proteome-wide characterization showed that the RNA dependency is particularly enriched among acidic proteins, intrinsically disordered proteins, and structural hub proteins. Moreover, we observed distinct differences in RNA-binding mode and Gene Ontology categories between RNA-dependent acidic and basic proteins. Notably, the solubility of key molecular chaperones [Trigger factor, DnaJ, and GroES] is largely dependent on RNAs, suggesting a yet-to-be-explored hierarchical relationship between RNA-based chaperone (termed as chaperna) and protein-based chaperones, both of which constitute the whole chaperone network. These findings provide new insights into the RNA-centric role in maintaining healthy proteome solubility in vivo, where proteins associate with a variety of RNAs, either stably or transiently.

RNA-binding as chaperones of DNA binding proteins from starved cells.

DNA-binding proteins from starved cells (Dps) in Escherichia coli protects DNA from multiple stresses during the stationary phase by forming a stable Dps-DNA complex. In contrast, Dps cannot bind to DNA during the exponential phase and it has not been clear why Dps conditionally binds to DNA depending on the growth phase. In this study, we show that DNA-free Dps in the exponential phase can also bind to RNA and the preemptive binding of RNA precludes DNA from interacting with Dps. The critical role of RNA in modulating the stability and functional competence of Dps and their morphology, leads us to propose a two-state model of Dps in executing stress responses. In the exponential phase, Dps is present predominantly as ribonucleoprotein complex. Under starvation, RNAs are degraded by up-regulated RNases, activating Dps to bind with chromosomal DNAs protecting them from diverse stresses. A dual role of RNA as an inhibitor of DNA binding and chaperone to keep dynamic functional status of Dps would be crucial for operating an immediate protection of chromosomal DNAs on starvation. The holdase-type chaperoning role of RNA in Dps-mediated stress responses would shed light on the role of RNAs as chaperone (Chaperna).