Hidden evolution: How the mammoth kept its cool

Last weekend I went to see an exhibition on mammoths in the Natural History Museum in London (unfortunately now closed). The descriptions and posters did a very good job of explaining our current knowledge of mammoths and how that knowledge had been gained. In particular, the curators were very keen to highlight the evolutionary relationships between different mammoth species and modern elephants.

A lot of the differences between the woolly mammoths and modern elephants are quickly apparent. The woolly mammoths are larger, hairier, and of course there are the tusks.

The tusks can be a big giveaway for a mammoth

The tusks can be a big giveaway for a mammoth

But one little sign caught my eye. It said that from the genetic material gathered from mammoth remains (we’ve recorded about 70% of their genome, so maybe not too far off until we can clone mammoths back into existence!), we can tell that woolly mammoths had a mutated form of haemoglobin that worked better in cold temperatures.

This haemoglobin discovery was published in 2010, so it isn’t anything new. But for me, this is evolution at its most interesting – not apes walking upright or fish breathing on land, but the tiny internal changes that adapt the body’s complex machinery.

Haemoglobin has a difficult balancing act to achieve – it needs to grab hold of oxygen from the lungs and it needs to release it around the body. If it holds the oxygen too tightly, the muscles can’t receive enough. If the haemoglobin holds on too weakly, it won’t be able to carry enough oxygen to where it is needed.

Elephant haemoglobin, like the human form, more readily releases oxygen at higher temperatures. That comes down to the energetics of breaking its hold, but it also means that harder-working (and therefore warm) muscles can take a greater share. This can be a serious issue if body-parts get too cold – they can suffocate to death. This is a danger for trekking around Siberia, even if you are large and hairy.

Researchers from the University of Manitoba, Canada, compared the haemoglobin genes of woolly mammoths to those of an Asian elephant from a nearby zoo. They noticed that there were four mutations that led to changes in the amino acid chains of the haemoglobin proteins.

Without any mammoth blood to hand, it can be very difficult to work out the effect of these changes. Fortunately, although we can’t resurrect a full mammoth now, we can insert the haemoglobin DNA into bacteria and resurrect one small part of them.

With the bacterium-produced mammoth haemoglobin, the researchers could run the tests needed to confirm that it was indeed adapted to the cold. Rather than the strong link to temperature seen in elephants, the mammoths’ haemoglobin can release oxygen at a similar level over a wide range of temperatures.

The structure of haemoglobin, with the blue regions showing where the mammoth mutations are

The structure of haemoglobin, with the blue regions showing where the mammoth mutations are (Credit: K Campbell et al, 2010)

The blue regions in the picture above show where the changes are in the structure of haemoglobin. These changes don’t appear to make any direct contact with the heme group (the part that carries the oxygen, shown in orange sticks), but they alter the shape of the protein subtly. This change in shape is enough to give mammoths a better chance of surviving freezing cold environments.

Mutations in haemoglobin are also seen in other animals adapted to cold environments, such as the arctic fox. The mutations seen in these animals are all different but lead to similar effects – an example of convergent evolution.

Just last year, the same researchers who made this discovery started work on an exciting new find: well-preserved mammoth blood. This could give them the chance to test the same idea on haemoglobin straight from a mammoth. There is even a (very small) chance that intact cells could be found in the sample, which would make cloning a full mammoth a lot easier.

So before long we might find out a lot more about these legendary creatures.

2012 Nobel prize in chemistry: G protein-coupled receptors

I couldn’t let today pass without writing something about the Nobel prize. Before long I’ll write a full piece about G protein-coupled receptors (GPCRs) but that’s quite a big story. For now, I just want to stress their importance and their place in current research.

A cell requires isolation from the outside world; maintaining different concentrations of ions inside and outside the cell is a vital part of how it functions. GPCRs play a hugely important role in detecting signals sent from outside the cell and relaying them to the inside. They are known as trans-membrane proteins because they extend from outside the cell, through the membrane and into the inner region. This means that when they bind a ligand on the outside of the cell (e.g. a hormone), the resulting structural rearrangement can trigger a chain of signals through to the inner machinery of the cell.

Beta-2 adrenergic receptor

Beta-2 adrenoceptor, the first structure of a human GPCR. It was solved by Brian Kobilka’s lab in 2007. The red portion indicates the membrane-bound section and the green indicates the intra-cellular section [the extra-cellular region was not observed].

Trans-membrane proteins are poorly studied, largely because it is so difficult to keep them stable when studying them in the lab. The parts of the protein that are outside the membrane will have positive and negative charges pointed outwards, interacting with water molecules. However, the membrane is made from fatty chains that repel water and so the region of the protein that stays in the membrane will be structured in a way that avoids water by having its hydrophobic amino acids pointing outwards. Keeping both of these regions stable in the same solution is tricky, but manageable. Detergents can mimic the membrane and are widely used.

Detergents can make it difficult to study the structure of GPCRs. Most protein structures are solved using X-ray crystallography or nuclear magnetic resonance (NMR). However, crystallography requires all the protein molecules to be very uniform which is difficult with the added complexity of a detergent that may form different shapes and sizes. NMR begins to lose its sensitivity as the size of the protein increases. In this case, the detergent counts towards the protein’s size which makes it difficult to get data.

This has meant that despite the huge importance of GPCRs in biochemistry and medicine (over a third of all drugs act via GPCRs), our understanding of them is still relatively poor. However we would be a lot further behind if it wasn’t for Lefkowitz and Kobilka – they are very deserving Nobel laureates.

Listening in on Lysozyme

Researchers from the University of California have used a ‘molecular microphone’ to listen to a single protein molecule at work.

One of the world’s smallest transistors, constructed from a single carbon nanotube, was attached to a molecule of lysozyme, a bacterium-fighting protein.

Lysozyme has been the subject of thousands of publications ever since Alexander Fleming, who was suffering from a cold at the time, first tried cultivating a sample of nasal mucus and discovered its antibacterial properties.

Proteins, which are very long, string-like molecules, need to be folded in a specific way in order to function properly. Lysozyme was the first enzyme to have its structure solved, back in 1965. However, knowing the structure alone is not enough to explain how lysozyme works – the way it moves is important too.

Lysozyme folds to form two ‘jaws’, which allow it to hold sugar chains in place in order to cut them apart by breaking a chemical bond. The development of the nano-transistor allowed scientists to study how this protein behaves in unprecedented detail. The team were able to detect lysozyme cutting through chemical bonds at a rate of 15 per second, before it got stuck and started opening and closing its jaws 300 times per second trying to find more bonds to cut (think Pac-Man!).

 

As the transistor is attached to a single molecule, the researchers can see how each protein behaves, rather than just looking at the average of millions of molecules. “Our circuits are molecule-sized microphones,” says Philip Collins, associate professor of physics and astronomy, who led the project with Professor Gregory Weiss from the department of biochemistry. “It’s just like a stethoscope listening to your heart, except we’re listening to a single molecule of protein.”

This research is the first to use carbon nanotubes for studying the activity of a single molecule. This would previously have been achieved by attaching a fluorescent dye to the protein but these often wear out within seconds. By using the nano-transistor, the team were able to ‘listen’ to the protein for ten minutes, which was crucial for them to observe the protein switching between its different forms.

This ‘microphone’ can be applied to countless other proteins, giving scientists a chance to listen to the machinery of our body at work.