Scientists have studied molecular interactions in the chaperonin protein GroEL. GroEL is alternately bound and unbound by a co-chaperonin, GroES. Against expectations, the 'football' complex where two GroES units cap the cylindrical GroEL at either end was roughly as prevalent as the single-bound 'bullet' complex.

This implies that negative allosteric interactions preventing double-binding of GroEL sometimes fail, and the double-bound complex plays an active role in protein folding. The study was published in Philosophical Transactions.

Molecular Chaperones

Proteins must fold in a specific way to function. This is often assisted by molecular chaperones, small proteins whose job is to help others fold to the right shape. Now, Japanese researchers have discovered that for one molecular chaperone at least, there's more to the process than was suspected.

Target Protein

The rough outline is understood: GroEL captures an unfolded target protein (the substrate) within a cavity, where it can fold correctly without aggregating. However, the mechanistic details are hard to unravel with traditional ensemble methods. In the new study, high-speed atomic force microscopy (HS-AFM) was used to visualize events more directly.

Co-Chaperonin

GroEL is a cylinder-shaped molecule, made of two rings stacked back to back. A key partner in its function is GroES, a ring-shaped "co-chaperonin" that binds to each end of GroEL like a domed lid. Only when GroEL is capped by GroES can it trap the substrate protein. Then, when folding is complete, GroES dissociates from GroEL, and the folded substrate is released.

The rings are identical, and both can be capped by GroES. When only one end is capped, the resulting complex is termed a "bullet," by virtue of its pointed appearance.In one conventional model, the cycle of capping, protein folding, and uncapping alternates between each ring.

Allostery

Capping at one ring of GroEL (which has cis stereochemistry) prevents simultaneous capping at the other (trans) end. Such intramolecular communication is known as allostery.

Protein Folding

In the predominant Type I, when the active ring of GroEL completes its task and the other end takes up the baton, the two rings also exchange cis and trans conformations. However, around 25% of the time (in Type II), the conformations are not exchanged, disrupting the circular, alternating rhythm of Type I. Nonetheless, protein folding still occurs. 

The football structure is so abundant, it must play a more active role than they thought. This complex mechanism is important, because chaperonins are a natural class of molecular machines. The subtleties of GroEL may help us to understand the role of allostery in molecular machines more generally.