Structure of Mycobacterium tuberculosis chaperonin-10 at 3.5 A resolution.

Chaperonin-60 (cpn60) and chaperonin-10 (cpn10) are essential proteins involved in ATP-dependent folding of several intracellular proteins in the bacterial cell. Folding of the nascent substrate polypeptide takes place in the large central cavity formed by each ring of the tetradecameric cpn60. This large cavity is closed upon capping by the heptameric cpn10. Cpn10s interact with cpn60s primarily through a 17-residue mobile loop and regulate the release and binding of the substrate polypeptide from the cpn60 surface. Here, the structure of M. tuberculosis cpn10 is reported at 3.5 A resolution. The overall structure of the cpn10 monomer is formed of a four-stranded beta-barrel and two long stretches of highly flexible segments: the dome loop and the mobile loop. The seven subunits in the heptamer show very little conformational difference and exhibit nearly perfect sevenfold geometry. The binding sites for metal ions in the dome loop of cpn10 have been identified, suggesting the role of metal ions in the stabilization of the protein. Comparisons with the available cpn10 structures indicate several interesting features.