No abstract

Heat shock proteins maintain protein homeostasis and facilitate the survival of an organism under stress. Archaeal heat shock machinery usually consists of only sHsps, Hsp70, and Hsp60. Moreover, Hsp70 is absent in thermophilic and hyperthermophilic archaea. In the absence of Hsp70, how aggregating protein substrates are transferred to Hsp60 for refolding remains elusive. Here, we investigated the crosstalk in the heat shock response pathway of thermoacidophilic crenarchaeon Sulfolobus acidocaldarius. In the present study, we biophysically and biochemically characterized one of the small heat shock proteins, Hsp14, of S. acidocaldarius. Moreover, we investigated its ability to interact with Hsp20 and Hsp60 to facilitate the substrate proteins' folding under stress conditions. Like Hsp20, we demonstrated that the dimer is the active form of Hsp14, and it forms an oligomeric storage form at a higher temperature. More importantly, the dynamics of the Hsp14 oligomer are maintained by rapid subunit exchange between the dimeric states, and the rate of subunit exchange increases with increasing temperature. We also tested the ability of Hsp14 to form hetero-oligomers via subunit exchange with Hsp20. We observed hetero-oligomer formation only at higher temperatures (50 °C-70 °C). Furthermore, experiments were performed to investigate the interaction between small heat shock proteins and Hsp60. We demonstrated an enthalpy-driven direct physical interaction between Hsp14 and Hsp60. Our results revealed that Hsp14 could transfer sHsp-captured substrate proteins to Hsp60, which then refolds them back to their active form.

No abstract

The signal recognition particle (SRP) and its receptor constitute universally conserved and essential cellular machinery that controls the proper membrane localization of nascent polypeptides with the transmembrane domain. In the past decade, there has been an immense advancement in our understanding of this targeting machine in all three domains of life. A significant portion of such progress came from the structural analysis of archaeal SRP components. Despite the availability of structural insights from different archaeal SRP components, little is known about protein translocation in this domain of life compared to either bacteria or eukaryotes. One of the primary reasons being limited availability of the genetic and cell biological tools in archaea. In the present review, an attempt has been made to explore the structural information available for archaeal SRP components to gain insights into the protein translocation mechanism of this group of organisms. Besides, many exciting avenues of archaeal research possible using the recently developed genetic and cell biological tools for some species have been identified.

Small heat shock proteins (sHsp) are a ubiquitous group of ATP-independent chaperones found in all three domains of life. Although sHsps in bacteria and eukaryotes have been studied extensively, little information was available on their archaeal homologs until recently. Interestingly, archaeal heat shock machinery is strikingly simplified, offering a minimal repertoire of heat shock proteins to mitigate heat stress. sHsps play a crucial role in preventing protein aggregation and holding unfolded protein substrates in a folding-competent form. Besides protein aggregation protection, archaeal sHsps have been shown recently to stabilize membranes and contribute to transferring captured substrate proteins to chaperonin for refolding. Furthermore, recent studies on archaeal sHsps have shown that environment-induced oligomeric plasticity plays a crucial role in maintaining their functional form. Despite being prokaryotes, the archaeal heat shock protein repository shares several features with its highly sophisticated eukaryotic counterpart. The minimal nature of the archaeal heat shock protein repository offers ample scope to explore the function and regulation of heat shock protein(s) to shed light on their evolution. Moreover, similar structural dynamics of archaeal and human sHsps have made the former an excellent system to study different chaperonopathies since archaeal sHsps are more stable under in vitro experiments.