Structure of the substrate binding domain of the thermosome, an archaeal group II chaperonin

The crystal structure of the substrate binding domain of the thermosome, the archaeal group II chaperonin, has been determined at 2.3 Angstrom resolution. The core resembles the apical domain of GroEL but lacks the hydrophobic residues implied in binding of substrates to group I chaperonins. Rather, a large hydrophobic surface patch is found in a novel helix-turn-helix motif, which is characteristic of all group II chaperonins including the eukaryotic TRiC/CCT complex. Models of the holochaperonin, which are consistent with cryo electron microscopy data, suggest a dual role of this helical protrusion in substrate binding and controlling access to the central cavity independent of a GroES-like cochaperonin.