Opening image: Crystal structure of the asymmetrical cpn60-cpn10-ADP complex from E. coli. The seven ADP molecules are shown in magenta.

The bullet complex consist of three rings stacked on top of each other. In the case of cpn60, the two rings stack back to back. This double ring structure is stabilized by interactions between the equatorial domains of each cpn60 subunit. Each ring encloses a large central cavity. The two cavities alternate between two conformations. Seven ADP molecules are bound to the 'open' ring.

The cpn10 cap binds only to the apical rim of the cpn60 ring that is in the open conformation. The allosteric transition from the "closed" to "open" conformation is triggered by ATP binding. The entire asymmetrical chaperonin complex resembles a bullet.

The cpn10 lid closes the central cavity known as Anfinsen's cage. The cavity has a polar surface and thus provides a safe haven for a partially folded protein.

If we switch to spacefilled models, we'll get a better feel for the 3D structure of this wonderfully complex and highly dynamic protein-folding machine. Click to animate the assembly of four vertical panels (one cpn10 and two cpn60 subunits per panel) of the bullet complex.

At this stage only three panels are missing. Take a moment to play with the model to get a better feeling for the shape of the two central cavities.

The complete bullet complex with the panels colored. Color the rings. Note how the ADP molecules (magenta) barely show. Binding of ATP to the nucleotide binding sites of the adjacent ring triggers the allosteric "flip-flop" transition, releasing the ADP.

The top and bottom views along the seven-fold axis of symmetry are fascinating.