Tardigrade small heat shock proteins can limit desiccation-induced protein aggregation

Abstract

AbstractSmall heat shock proteins (sHSPs) are chaperones known for their response to heat stress. While sHSPs have well-characterized roles in heat tolerance, potential roles for sHSPs in desiccation tolerance have not been as thoroughly explored. We identified nine sHSPs from the genome of the tardigrade Hypsibius exemplaris, each containing a conserved alpha-crystallin domain flanked by disordered regions. Many of these sHSPs are highly expressed and in some cases are upregulated during desiccation. We found that tardigrade sHSPs were sufficient to improve desiccation tolerance when expressed in E. coli. Purification and subsequent analysis of two sHSPs, HSP21 and HSP24.6, revealed that these proteins can form large complexes in vitro, similar to oligomeric assemblies documented for other sHSPs. These proteins limited heat-induced aggregation of the model enzyme citrate synthase. Heterologous expression of HSP24.6 improved bacterial heat shock survival, and the protein significantly reduced heat-induced aggregation of soluble bacterial protein. Thus, HSP24.6 likely chaperones against protein aggregation to promote survival of heat stress. Furthermore, HSP21 and HSP24.6 also limited desiccation-induced aggregation and loss of function of citrate synthase. This suggests a mechanism by which tardigrade sHSPs promote desiccation tolerance: by limiting desiccation-induced protein aggregation, thereby maintaining proteostasis and supporting survival. These results suggest that sHSPs, classical chaperones, provide a mechanism of general stress resistance that can also be deployed to support survival during anhydrobiosis.