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

Abstract

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 ( 528EKEKEKEGEK5 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.