Crystal structure of the β-apical domain of the thermosome reveals structural plasticity in the protrusion region

The crystal structure of the β-apical domain of the thermosome, an archaeal group II chaperonin from Thermoplasma acidophilum, has been determined at 2.8 Å resolution. The structure shows an invariant globular core from which a 25 Å long protrusion emanates, composed of an elongated α-helix (H10) and a long extended stretch consisting of residues GluB245-ThrB253. A comparison with previous apical domain structures reveals a large segmental displacement of the protruding part of helix H10 via the hinge GluB276-ValB278. The region comprising residues GluB245-ThrB253 adopts an extended β-like conformation rather than the α-helix seen in the α-apical domain. Consequently, it appears that the protrusions of the apical domains from group II chaperonins might assume a variety of context-dependent conformations during an open, substrate-accepting state of the chaperonin. Sequence variations in the protrusion regions that are found in the eukaryotic TRiC/CCT subunits may provide different structural propensities and hence serve different roles in substrate recognition.