THE MOLECULAR CHAPERONIN TF55 FROM THE THERMOPHILIC ARCHAEON SULFOLOBUS-SOLFATARICUS - A BIOCHEMICAL AND STRUCTURAL CHARACTERIZATION

The purification and characterization of a new type of thermostable ch aperonin from the archaebacterium Sulfolobus solfataricus is described . The chaperonin forms a heterooligomeric complex of two different, bu t closely related, subunits, which we have assigned TF55-alpha and TF5 5-beta. Their N-terminal sequences and amine acid residue compositions are reported. Two-dimensional projections of the chaperonin have been reconstructed from electron microscopy images, showing a 9-fold symme trical complex, about 17.5 nm in height and 16 nm in diameter, with a central cavity of 4.5 nm. The complex is resistant to denaturing agent s at room temperature and only pH values lower than 2 lead to dissocia tion. The separated subunits do not reassemble spontaneously but requi re Mg2+ and ATP for complex formation. Both subunits are necessary for formation of the TF55 oligomer. Significant structural changes have b een observed after phosphorylation, thus providing evidence for a stru ctural mobility during the chaperonin-assisted folding process of a pr otein. The phosphorylation reaction is modulated by potassium and magn esium ions. Magnesium seems to have an inhibitory effect, whereas pota ssium enhances this reaction.