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heat shock 60kDa protein 1 (chaperonin)
Identifiers
Symbol	HSPD1 SPG13
HUGO	5261
Entrez	3329
OMIM	118190
RefSeq	NM_002156
UniProt	P10809
Other data
Locus	Chr. 2 q33.1

GroEL is a protein chaperone required for the proper folding of many proteins in prokaryotes, chloroplasts, and mitochondria. To function properly, it requires the cochaperone protein GroES. (GroEL and GroES are also sometimes referred to as chaperonin and cochaperonin, or chaperonin 60 and chaperonin 10 for their molecular weights.) In eukaryotes the proteins Hsp60 and Hsp10 are structurally and functionally nearly identical to GroEL and GroES, respectively.

A class of chaperones, characterized by GroEL/GroES act in this way. The GroEL/GroES complex traps a string of exposed and unfolded amino acids in its bottle-like enclosure. At this initial stage, the interior of the chaperone complex is highly hydrophobic. This surface is suitable for binding unfolded proteins. Once the protein (or one domain of a larger protein) is properly folded within this capsule, the interior changes to a hydrophilic environment. This releases the folded protein to the aqueous environment outside of the chaperone; the cycle can then repeat. The cycling between hydrophilic and hydrophobic interiors requires a conformational change in the GroEL/GroES structure, and is driven by ATP hydrolysis.

Contents 1 Structure 2 Why Chaperonins? 2.1 What does GroEL do? 2.2 Once More from the Top, with Substrate 2.3 GroEL has no Specific Mechanism 3 See also 4 References 5 External links

Structurally, GroEL is a dual-ringed tetradecamer, with both the cis and trans rings consisting of seven subunits each. The inside of GroEL is hydrophobic, and is likely where protein folding takes place.

GroEL (side)

GroEL (top)

GroES/GroEL complex (side)

GroES/GroEL complex (top)

Chaperonins were originally characterized as a subpopulation of "heat shock proteins", proteins whose synthesis was promoted by heat stress. This is the reason that GroEL and its homologues, for example, are often referred to as "Hsp60" proteins. GroEL is a 60kDa (actually 57kDa) heat shock protein.

It was realized quite early on that chaperonins help to turn polypeptides, fresh from the ribosome, into properly folded proteins. No one knew how the trick was done. In fact, the details are still obscure. However, it was clear that randomly ordered polypeptides passed through the double-ring of the chaperonin molecule and emerged as globular proteins, more or less correctly folded. The chaperonin was thus like a stage magician who runs a scarf through a magic ring, from which it emerges a different color. This trick is normally accomplished by using a scarf that is actually a bag, with the inside made of a cloth of a different color. While feeding the "scarf" through the ring, the magician simply turns the bag inside-out. The analogy is not perfect, but the chaperonin protein manages the trick in somewhat the same way.

But why bother? Some proteins are absolutely dependent on chaperonins to attain their correct conformation. These proteins are sometimes very important, but they are few in number. Why would cells evolve general purpose chaperonins, rather than a few, special purpose types specialized to handle the bulk protein products of the cell? This generality of chaperonins became even more inexplicable when it was found that many proteins which are not chaperonin-dependent fold themselves just fine, and at about the same speed, if they are simply left to themselves in a test tube. So what is the point?

The answer seems to relate to the fact that a cell is not a test tube. In the test tube, proteins are relatively pure and dilute. In the cell, proteins are at high concentration. Before they can fold properly, they've gotten tangled up with other things and the whole business becomes a gooey mess. In many cases, the function of chaperonins may be simply to keep newly formed polypeptides from blundering into other things and clogging up the machinery of the cell.

In addition to folding new proteins, chaperonins also re-fold old proteins which have somehow gotten twisted out of shape. This "personal trainer" function seems to be why chaperonins are induced by heat shock. Heat stress can denature proteins, and the cell mobilizes chaperonins like a small army of physical therapists to twist everything back into its proper conformation.

The blue-colored image above shows some low-resolution electron density maps of the holoenzyme. As the top view shows, the top half is a ring made of seven identical monomers. The bottom half is a mirror image. The third picture shows a vertical cross-section. Note the large amount of empty space in the middle of the rings.

The key to the activity of GroEL is in the structure of the monomer, shown in the next image from Ranson et al. (1998). The Hsp60 monomer has three distinct sections separated by two hinge regions. The apical section contains a large number of hydrophobic binding sites for "native" (unfolded) protein substrates. Note that most sensible globular proteins won't bind to the apical domain because they have their hydrophobic parts tucked away inside, away from the aqueous medium since this is the thermodynamically optimum conformation. So, these "substrate sites" will only bind to proteins which are not optimally folded. The apical domain also has binding sites for the Hsp10 monomers of its helper protein, GroES, which we'll get to in a minute.

The equatorial domain has a slot near the hinge point for ATP, as well as two attachment points for the other half of the GroEL molecule. The rest of the equatorial section is moderately hydrophilic.

What happens when we plug in ATP and GroES? The answer is shown in the next set of figures. For purposes of the following discussion, we will refer to the "activated" half of the GroEL molecule as the cis domain and the other half as the trans domain.

The addition of ATP and GroES has a drastic effect on the conformation of the cis domain. This effect is caused by flexion and rotation at the two hinge points on the Hsp60 monomers. The intermediate domain folds down and inward about 25° on the lower hinge. This effect, multiplied through the cooperative flexing of all monomers, increases the equatorial diameter of the GroEL cage. But the effect on the upper hinge is far more drastic. The apical domain rotates a full 60° degrees up and out on the upper hinge, and also rotates 90° around the hinge axis. This motion opens the cage very widely at the top of the cis domain, but completely removes the substrate binding sites from the inside of the cage.

Despite all this beautiful work with empty cages, it is not clear what happens when actual proteins are provided as substrate. Plainly, the non-polar, hydrophobic interactions with the inactive state of GroEL select suboptimally folded peptides. These confused and potentially antisocial proteins are suckered into the GroEL cage by non-polar binding sites. Then the trap is sprung, and they find themselves caught, alone in a relatively polar environment. A second or two later, the ATP is hydrolyzed, and the peptide is pushed out through the trans domain, emerging into the cellular environment once more, but now transformed into a model molecular citizen. How has this rehabilitation taken place?

On this topic a great deal of elegant thermodynamic and kinetic work has been done in the last 5-7 years. Most of this effort, we submit, has been wasted because of a general failure to take account of the phylogenetic framework in which this all occurs. Some thermodynamic generalities are worth emphasis. The constricted nature of the molecular cage strongly favors compact molecular conformations of the substrate protein. Free in solution, long-range, non-polar interactions can only occur at a high cost in entropy [2]. In the crowded quarters of the GroEL cage, the relative loss of entropy is much smaller. Thirumalai et al. (2003); see also Takagi et al. (2003). The method of capture also tends to concentrate the non-polar binding sites separately from the polar sites. When the GroEL non-polar surfaces are removed, the chance that any given non-polar group will encounter a nonpolar intramolecular site are much greater than in bulk solution. One might well speculate that the trick is very much like the magician's ring. The hydrophobic sites which were on the outside are gathered together at the top of the cis domain and bind each other. The geometry of GroEL requires that the polar structures lead, and they envelope the non-polar core as it emerges from the trans side. In effect, the GroEL complex, like the magician's ring, may simply work by turning the substrate inside out.

But these are just general themes. The real point is that there is no need to invoke any specific mechanism. In fact, one of the key observations made by Ranson et al. in their review is that there seems to be considerable variability in exactly when ATP and GroES bind, exactly how they effect the conformational changes, how the substrate complex forms, and how it is released. There is no reason to invoke a completely general mechanism by which GroEL works in exactly the same way for all substrates.

In phylogenetic context it is easy to see why this is so. GroEL and its homologues have not changed much since the basal split between Archaea and Eubacteria. In short, proteins have been learning how to optimize their reactions with GroEL, or very similar molecules, for about three billion years. Given this enormous span of time, and the virtual immutability of GroES, as opposed to the huge evolutionary laibility of its substrates, it is much more reasonable to assume that cellular proteins have evolved to make the best possible use of GroEL -- not that GroEL has some magic quality which assures optimal results for all proteins.

GroEL is a very complicated molecule and, doubtless, there are any number of ways in which a polypeptide can interact with it. Merely by way of example, it is known that certain larger polypeptides, which can't fit inside the cis chamber, extend into the trans portion and interact in some (unknown) way with the trans heptamer. Since GroEL has changed hardly at all over the last several billion years, it seems far more likely that the rest of the genome has adapted to GroEL, and uses it in various specialized ways. GroEL itself cannot specialize. GroEL, like many anatomical and biochemical features at the intersection of many developmental pathways, cannot change significantly without lethal effects. Thus the search for a specific GroEL mechanism will inevitably fail. Aside from qualitative thermodynamic considerations, such as those mentioned above, there probably is no general mechanism.

• Chaperonin
• Heat shock protein

• MeSH GroEL+Protein