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

The crystal structure of the alpha alpha-apical domain of the thermosome, a n archaeal group II chaperonin from Thermoplasma acidophilum, has been dete rmined at 2.8 Angstrom resolution. The structure shows an invariant globula r core from which a 25 Angstrom long protrusion emanates, composed of an el ongated a-helix (H10) and a long extended stretch consisting of residues Gl uB245-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 adopt s an extended beta-like conformation rather than the alpha-helix seen in th e alpha-apical domain. Consequently, it appears that the protrusions of the apical domains from group II chaperonins might assume variety of context-d ependent conformations during an open, substrate-accepting state of the cha peronin. Sequence variations in the protrusion regions that are found in th e eukaryotic TRiC/CCT subunits may provide different structural propensitie s and hence serve different roles in substrate recognition. (C) 2000 Academ ic Press.