Adaptation of a hyperthermophilic group II chaperonin to relatively moderate temperatures

Local Flexibility of a New Single-Ring Chaperonin Encoded by Bacteriophage AR9 Bacillus subtilis

Chaperonins, a family of molecular chaperones, assist protein folding in all domains of life. They are classified into two groups: bacterial variants and those present in endosymbiotic organelles of eukaryotes belong to group I, while group II includes chaperonins from the cytosol of archaea and eukaryotes. Recently, chaperonins of a prospective new group were discovered in giant bacteriophages; however, structures have been determined for only two of them. Here, using cryo-EM, we resolved a structure of a new chaperonin encoded by gene 228 of phage AR9 B. subtilis. This structure has similarities and differences with members of both groups, as well as with other known phage chaperonins, which further proves their diversity.

Characterization of group II chaperonins from an acidothermophilic archaeon Picrophilus torridus

Chaperonins are a type of molecular chaperone that assist in the folding of proteins. Group II chaperonins play an important role in the proteostasis in the cytosol of archaea and eukarya. In this study, we expressed, purified, and characterized group II chaperonins from an acidothermophilic archaeon Picrophilus torridus. Two genes exist for group II chaperonins, and both of the gene products assemble to form double‐ring complexes similar to other archaeal group II chaperonins. One of the Picrophilus chaperonins, PtoCPNα, was able to refold denatured GFP at 50 °C. As expected, PtoCPNα exhibited an ATP‐dependent conformational change that is observed by the change in fluorescence and diffracted X‐ray tracking (DXT). In contrast, PtoCPNα lost its protein folding ability at moderate temperatures, becoming unable to interact with unfolded proteins. At lower temperatures, the release rate of the captured GFP from PtoCPNα was accelerated, and the affinity of denatured protein to PtoCPNα was weakened at the lower temperatures. Unexpectedly, in the DXT experiment, the fine motions were enhanced at the lower temperatures. Taken together, the results suggest that the fine tilting motions of the apical domain might correlate with the affinity of group II chaperonins for denatured proteins.

A modulator domain controlling thermal stability in the Group II chaperonins of Archaea

Archaeal Group II chaperonins (Cpns) are strongly conserved, considering that their growth temperatures range from 23 to 122°C. The C-terminal 15-25 residues are hypervariable, and highly charged in thermophilic species. Our hypothesis is that the C-terminal is a key determinant of stabilization of the Cpn complex. The C-terminus of the Cpn from the hyperthermophile Pyrococcus furiosus was mutated to test this hypothesis. C-terminal deletions and replacement of charged residues resulted in destabilization. The stability of ATPase activity declined in proportion to the reduction in charged residues with Ala or Gly. An EK-rich motif ((528)EKEKEKEGEK5(37)) proved to be a key domain for stabilization at or near 100°C. Mutations "tuned" the Cpn for optimal protein folding at lower optimal temperatures, and Glu substitution was more potent than Lys replacement. Pf Cpn stability was enhanced by Ca(2+), especially in the mutant Cpn lacking C-terminal Lys residues. This suggests that Glu-Glu interactions between C termini might be mediated by Ca(2+). The C-terminal of a Cpn from the psychrophilic archaeon Methanococcoides burtonii was replaced by a domain from the hyperthermophile, resulting in increased thermostability and thermoactivity. We conclude that localized evolutionary variation in the C-terminus modulates the temperature range of archaeal Cpns.