The archaeal molecular chaperone machine: Peculiarities and paradoxes

A major finding within the field of archaea and molecular chaperones has be en the demonstration that, while some species have the stress (heat-shock) gene hsp70(dnaK), others do not. This gene encodes Hsp70(DnaK), an essentia l molecular chaperone in bacteria and eukaryotes. Due to the physiological importance and the high degree of conservation of this protein, its absence in archaeal organisms has raised intriguing questions pertaining to the ev olution of the chaperone machine as a whole and that of its components in p articular, namely, Hsp70(DnaK), Hsp40(DnaJ), and GrpE. Another archaeal par adox is that the proteins coded by these genes are very similar to bacteria l homologs, as if the genes had been received via lateral transfer from bac teria, whereas the upstream flanking regions have no bacterial markers, but instead have typical archaeal promoters, which are like those of eukaryote s. Furthermore, the chaperonin system in all archaea studied to the present , including those that possess a bacterial-like chaperone machine, is simil ar to that of the eukaryotic-cell cytosol. Thus, two chaperoning systems th at are designed to interact with a compatible partner, e.g., the bacterial chaperone machine physiologically interacts with the bacterial but not with the eucaryal chaperonins, coexist in archaeal cells in spite of their appa rent functional incompatibility. It is difficult to understand how these hy brid characteristics of the archaeal chaperoning system became established and work, if one bears in mind the classical ideas learned from studying ba cteria and eukaryotes. No doubt, archaea are intriguing organisms that offe r an opportunity to find novel molecules and mechanisms that will, most lik ely, enhance our understanding of the stress response and the protein foldi ng and refolding processes in the three phylogenetic domains.