Dynamics and composition of small heat shock protein condensates and aggregates

Small heat shock proteins (sHSPs) are essential ATP-independent chaperones that protect the cellular proteome during stress. These proteins assemble into polydisperse oligomeric structures, the composition of which dramatically affects their chaperone activity. The biomolecular consequences of variations in sHSP ratios, especially inside living cells, remain elusive. Here, we study the consequences of altering the relative expression levels of HspB2 and HspB3. These chaperones are partners in a hetero-oligomeric complex, and genetic mutations that abolish their mutual interaction are associated with myopathic disorders. HspB2 displays three distinct phenotypes when co-expressed with HspB3 at varying ratios. Expression of HspB2 alone lead to formation of liquid nuclear condensates, while shifting the stoichiometry towards HspB3 resulted in the formation of large solid-like aggregates. Only cells co-expressing HspB2 with a limited amount of HspB3 showed a homogeneous nuclear distribution of HspB2. Strikingly, both condensates and aggregates were reversible, as shifting the HspB2:HspB3 balance in situ resulted in dissolution of these structures. To uncover the molecular composition of HspB2 condensates and aggregates, we used APEX-mediated proximity labelling. Most proteins interact transiently with the condensates and were neither enriched nor depleted. In contrast, we found that HspB2:HspB3 aggregates sequestered several disordered proteins among which autophagy factors, suggesting that the cell is actively attempting to clear these aggregates. This study presents a striking example of how changes in the relative expression levels of interacting proteins affects their phase behavior. Our approach can be a useful tool to study the role of protein stoichiometry in other biomolecular condensates. Graphical abstract Highlights - Small heat shock protein hetero-oligomerization affects their chaperone function - The HspB2:HspB3 expression ratio determines phase separation and aggregation - HspB2 condensates and HspB2:HspB3 aggregates are fully reversible - Proximity labelling unveils autophagy factor recruitment to HspB2:HspB3 aggregates - Stoichiometry-dependant regulation of phase behaviour may be widespread in biology