The Mycobacterium tuberculosis chaperonin 10 monomer exhibits structural plasticity

The conditions which favor dissociation of oligomeric Mycobacterium tuberculosis chaperonin 10 and the solution structure of the monomer were studied by analytical ultracentrifugation, size exclusion chromatography, fluorescence, and circular dichroism spectroscopies. At neutral pH and in the absence of divalent cations, the protein is fully monomeric below approximately a 4.7 (mu)M concentration. Under these conditions the monomer forms completely unfolded and partially folded conformers which are in equilibrium with each other. One conformer accumulates over the others which is stable within a very narrow range of temperatures. It contains a (beta)-sheetstructured C-terminal half and a mostly disordered N-terminal half. Other components of the equilibrium include partially helical structures which do not completely unfold at high temperature or under strong acidic conditions. Complete unfolding of the monomer occurs in the presence of denaturants or below 14°C. Cold-denaturation is detected at an unusually high temperature and this may be due to the concentration of hydrophobic residues, which is larger in chaperonins than in other globular proteins. Finally, the monomer self-associates in the pH range 5.8-2.9, where it forms small oligomers. A structure-activity relationship was investigated with the sequences known to be involved in the various biological activities of the monomer.