Chaperone quality control in liquid-phase separated organelles

Molecular chaperones, central to the cellular proteostasis network, play an essential role in preventing the formation and proliferation of harmful aggregates associated with neurodegenerative diseases. Notably, for many intrinsically disordered proteins (IDPs), which are prone to form such damaging deposits, the formation of nano-clusters and phase separation into organelles prior to aggregation have been observed. The impact of molecular chaperones on such assemblies, remains unclear. In our study, we concentrated on the family of small heat shock proteins (sHsps), which are typically dynamic and form large oligomeric structures. While sHsps are mainly structured/folded proteins, they can undergo transient multivalent interactions, like many IDPs. Thus, sHsps might be a suitable regulator for vital and ubiquitous formation of membrane-less organelles in eukaryotic cells rich in IDPs and to inhibit aberrant aggregation. Here we show, using microfluidic diffusional sizing, that the formation of nano-clusters of FUS, associated with neurodegenerative diseases can be inhibited by the presence of sHsps. Furthermore, we identify that, depending on their assembly state, sHsps are capable of targeting specifically the interface between the dense droplet phase and the dilute phase not only of FUS but also of TDP-43, likely because the interface is the primary starting point for fibril formation or protein aggregation in general. Our findings emphasise the impact of molecular chaperones on maintaining the homeostasis of IDPs in the dilute and condensed phase. This could help to understand how chaperone dysregulation can influence aberrant protein association.