Sunday, February 11, 2007

Heat Shock and Molecular Chaperones

 
Protein folding takes place inside cells at their normal temperature. This temperature is 37°C in the case of mammals. If cells encounter higher temperatures, their proteins become unfolded since the minimal energy conformation at one temperature isn't the same minimum at a another temperature.

The difference in temperature isn't large. Many mammalian proteins become unstable at 42°C. This is a temperature encountered with high fever but it's also common in skin cells that have been exposed to the sun or the water in a hot tub. Temperature differences are even more common in species that do not regulate their temperature (e.g., plants, fungi, bacteria, invertebrates).

All living cells have defense mechanisms that protect against exposure to high temperature. They produce special proteins called heat shock proteins in order to recover damaged proteins that have unfolded at the high temperature. Many of the heat shock proteins are molecular chaperones. These are proteins that guide proper refolding of damaged proteins.

Shortly after the discovery of heat shock proteins we learned that these proteins are always present even in cells that haven't been stressed. In other words, molecular chaperones play a role in normal protein folding as well as helping to recover from damage. This role is illustrated in the energy diagram (top left).

One of the pathways to the energy minimum follows the line labeled "B." This traverses a local energy well and proteins can get trapped in this pit. What happens is that they adopt a less-than-optimal conformation but they can't easily unfold and refold to fall down into the deeper well because that would require an increase in energy. It will happen eventually, but it could take a long time. Chaperones help direct folding along the proper pathway and his speeds up the folding process.

Recall from the earlier discussion of How Proteins Fold that folding is an entropy-driven process where the end result is to bury hydrophobic residues in the central core of the protein. Another thing that can happen during folding is that exposed hydrophobic surfaces of one protein can interact with similar surfaces in another protein leading to aggregation. Chaperones can bind to these hydrophobic surfaces and prevent aggregation. This is another way of speeding up folding.

There are many different chaperones. My personal favorite is HSP70 (Heat Shock Protein of relative molecular mass 70,000). This is a protein that's found in every type of living species. It is the most highly conserved protein in all of biology so it's an excellent protein for looking at deep phylogeny. The role of HSP70 as a molecular chaperone is to bind to proteins as they are being synthesized in order to prevent improper folding. It also prevents aggregation.

Another famous chaperone used to be called HSP60 since it was a heat shock protein of Mr= 60,000. It was also known as GroE since it allowed for the growth of bacteriophage λ gene E mutants. Now it's known as chaperonin. Note that the term "chaperonin" refers to a specific molecular chaperone.

Chaperonin is a barrel-shaped molecule consisting of two rings of seven subunits
surrounding a central cavity (one subunit is colored green in the image) . The top of the cavity is sealed by cap of seven smaller subunits. The chaperone works by capturing small unfolded proteins in the cavity where the hydrophobic environment encourages rapid folding to the correct conformation. Aggregation is also prevented by keeping the folding protein away from other proteins.

The inside of the cavity is referred to as an Anfinsen cage after Christian Anfinsen, a Nobel Laureate who worked on protein folding (see next Wednesday's Nobel Laureate). The release of unfolded protein is coupled to the hydrolysis of ATP. This is a common feature of most molecular chaperones.


10 comments :

  1. Larry,

    Owing to the tremendous quality of this post and others, you have won the Thinking Blog award:

    http://gregladen.com/wordpress/?p=290

    Respond only if you desire and don't if you like snakes a LOT.

    ReplyDelete
  2. A very interesting post and blog in general. Found it through Megite, because the title of the post invited me...

    Is there a similar relation between protein folding and temperature within the region of the human eye?

    Specifically, I am interested in the relationship between temperature and "cell death" inside the human eye - as it happens while performing a LASER retinopathy. It would be helpful if you can provide me with some specific research literature

    You can write to me at ommachi AT gmail DOT com

    thanks in advance

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  3. This comment has been removed by the author.

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  4. " The chaperone works by capturing small unfolded proteins in the cavity where the hydrophobic environment encourages rapid folding to the correct conformation."

    So chaperones are like 'university' molecules, providing a stimulating environment while letting a molecule develop according to its individuality.

    Now, considering what was said about binding on hydrophobic surfaces I expected that the chaperone subunits present hydrophilic surfaces at all times. I can't square a general "hydrophobic environment" with entropically forcing hydrophobic driven folding either. Where do I go wrong?

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  5. arunn asks,

    Is there a similar relation between protein folding and temperature within the region of the human eye?

    High temperature will affect all cells and all proteins. I assume that those in the eye will be affected by heat and other stress.

    Incidently, some of the small heat shock proteins have evolved to become lens crystallins in some species. This isn't directly related to your question but it's a cool little factoid.

    Specifically, I am interested in the relationship between temperature and "cell death" inside the human eye - as it happens while performing a LASER retinopathy. It would be helpful if you can provide me with some specific research literature.

    Sorry, I'm not familiar with any literature on this topic. You should search PubMed.

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  6. Larry,

    I was wondering if you had encountered the literature relating chaperonins to disease. One M.D. I know mentioned in passing that there is a theory that when damaged cells release HSP70-related proteins into the bloodstream that this may be a potent trigger for inflammation responses. On a related note, it appears that the application of sensitive Mass Spectrometry techniques for protein identification (often referred to as proteomics), when focused on diseased tissues, will frequently turn up chaperonin proteins. I have seen some authors speculate that this may be relevant for symptoms and progression (of whatever particular ailment they are studying), while others feel that if the disease is mucking up protein structure, then obviously you're going to trigger the cell's response.

    Since my own blog is thus far vacant, I wonder if you'd be interested in a poll for the first identified molecular chaperone? My candidate would obviously (?) be coliphage T4 protein gp63 which as we all know encodes an RNA ligase enzyme. However Wood and colleagues used classical biochemical purification to show that this protein has dual roles and is also required for proper assembly of the phage's tail fibres (although gp63 itself is not found in the mature phage tails).

    Marc

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  7. Marc. The first protein to be called a molecular chaperone is nucleoplasmin, a nuclear protein required in the assembly of nucleosomes from histones and DNA -nothing to do with protein folding! The first protein to be discovered that was later identified as a molecular chaperone is PDI (protein disulfide isomerase) discovered by Anfinsen in the 1960s. See TiBS 31,395. 2006. John Ellis

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  8. Torbjorn. The quote is incorrect. The internal environment of the Anfinsen cage changes from hydrophobic to hydrophilic once the GroES cap is attached. So the encapsulated protein completes its folding in a hydrophilic (and highly crowded) environment. See Nature 388, 741, 1997. John Ellis

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  9. How many proteins are known to require a molecular chaperone for proper folding? I heard that it is small (5%). Are heat shock proteins involved in temperature-dependent sex determination in amphibians?

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  10. thanks for your article, very understandable, clean and direct

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