The Thermosome of Thermoplasma acidophilum and Its Relationship to the Eukaryotic Chaperonin TRiC

A high molecular‐mass protein complex from the archaebacterium Thermoplasma acidophilum, referred to here as the ‘thermosome’, is built from two subunits (Mr 58 and 60). The thermosome has been purified to homogeneity. The molecular mass of the native complex was determined to be 1061 ± 30 Da by scanning transmission electron microscopy. It shows a weak ATPase activity and is able to bind denatured polypeptides. Averages obtained from electron micrographs of negatively stained molecules in the end‐on and side‐on orientations, respectively, were compared with those of the t‐complex polypeptide 1 ring complex (TRiC), isolated from bovine testes. Both molecules consist of two stacked pseudo eightfold symmetric rings which build up a cylindrical particle with a large cavity in the center. Sequence alignments of peptides generated from both subunits of the thermosome and different subunits of TRiC reveal a high partial similarity to each other and to the archaebacterial chaperonin thermophilic factor 55 from Sulfolobus shibatae as well as to eukaryotic TCP1 proteins. These striking structural similarities confirm the proposition that all these molecules belong to a single protein family which is structurally and functionally related to the GroEL class of molecular chaperones.