14. Molecular Chaperones in Thermophilic Eubacteria and Archaea

Thermophilic organisms tolerate or adapt to high temperatures by making their proteins thermostable or thermophilic. Even though, thermophilic bacteria have their own optimal growth temperatures, and heat-shock responses are still induced at the temperatures higher than optimal temperatures. The molecular chaperone systems of thermophilic eubacteria are very similar to those of mesophilic eubacteria. On the contrary, the molecular chaperone system of hyperthermophilic archaea is much simpler than those of other organisms. Within the hyperthermophilic archaea, only the following six kinds of chaperones have so far been identified: group II chaperonins, prefoldin, small heat-shock proteins, peptidyl-prolyl cis-trans isomerases, AAA proteins, and NAC. These archaea lack the Hsp70 chaperone system as well as Hsp90 and Hsp100, though these are thought to be indispensable chaperones in all other organisms. Since group II chaperonins are highly induced at elevated temperatures and related to the stress response of hyperthermophilic archaea, this manuscript focuses on the group II chaperonin and its cofactor, prefoldin, in addition to sHsps that are ubiquitous in thermophilic eubacteria and archaea. The limited number of molecular chaperones in hyperthermophilic archaea might be due to the relatively high stability of their proteins. The molecular chaperones in hyperthermophilic archaea might contribute to the protection of only a limited number of relatively unstable proteins.