Functional diversity in the Hsp60 of Sulfolobus acidocaldarius: mosaic of Group I and Group II chaperonin

Abstract

External stress can disrupt protein homeostasis in organisms, necessitating the involvement of heat shock proteins (Hsps) to restore equilibrium and ensure survival. Unlike other organisms, the thermoacidophilic crenarchaeon Sulfolobus acidocaldarius lacks Hsp100, Hsp90, and Hsp70, possessing only two small heat shock proteins (Hsp14 and Hsp20) and one group II chaperonin, Hsp60. This raises questions about how protein substrates are protected and transferred to Hsp60 for refolding without other chaperones. Our study focused on ATP-dependent Hsp60 in S. acidocaldarius, revealing its formation of oligomeric structures in the presence of ATP. While ATP hydrolysis is not essential for oligomer formation and lid closure, it is crucial for Hsp60s chaperone activity, effectively folding stress-denatured substrate proteins by stabilizing their folded conformations. The mechanism involves hydrophobic recognition of unfolded substrates, encapsulating and releasing them in a more folded state. Negatively charged inner surface of the ring seems to be responsible for driving the folding of the substrate. Importantly, Hsp14 was found to transfer substrate proteins to Hsp60{beta}, orchestrating their refolding into an active state. Beyond protein folding, Hsp60{beta} protects the membrane under stress, contributing to maintaining membrane rigidity. Hsp60 exhibits nested cooperativity in ATPase activity, adapting to ATP concentration changes and interestingly Hsp60{beta} and Hsp60{beta} complex shows a mosaic behaviour during ATP hydrolysis belonging to both group I and group II chaperonin respectively. In conclusion, our study provides insights into the intricate mechanisms employed by Hsp60 in S. acidocaldarius to maintain protein homeostasis. It offers a comprehensive understanding of Hsp60s role in the heat shock response pathway, shedding light on fundamental cellular processes in extremophilic archaea.