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 (⁵²⁸EKEKEKEGEK5³⁷) 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 Ca²⁺, especially in the mutant Cpn lacking C-terminal Lys residues. This suggests that Glu-Glu interactions between C termini might be mediated by Ca²⁺. 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.