New research by the University of Groningen and the FOM Institute for Atomic and Molecular Physics (AMOLF) offers new insights into the folding and unfolding of protein molecules, which partly enable the cells in our bodies to live. We have known for a long time that proteins fold, but not exactly how it happens. RUG biologist Arnold Driessen and his PhD student Philipp Bechtluft have changed that. Together with their colleagues from Amsterdam, this has resulted in an article in the scientific journal Science (30 Nov. 2007).
The cells in our bodies delegate virtually all their work to protein molecules, of which there are thousands of different ones. Each protein type has a special task, for example food intake. They can only perform that task if they are wrapped around like a ball of wool. Some proteins perform their task outside the cell. That means they have to pass through the outside wall of the cell. However, if they have already folded within the cell, they are too large and do not fit through the transportation channels they use to get to their 'work floor'. Sometimes the rounded proteins unwrap themselves during their journey into long, thin strings which do fit through the channels. Once the protein reaches its destination, it folds itself into a ball again and then sets to work.
The chaperone
This protein folding does not happen spontaneously. The proteins need help – from special chaperone proteins. Driessen and Bechtluft were mainly interested in chaperones that keep unfolded proteins nice and straight. They wanted to know whether an unfolded protein string with the help of a chaperone is indeed completely unfolded or whether there were still tiny folds in it.
Folding and unfolding themselves
In order to investigate this, Driessen and Bechtluft first had to unfold folded proteins themselves so that they could observe the entire process of folding and unfolding in detail. ‘We called on our Amsterdam colleagues at AMOLF for help with the unfolding mechanism – they have microscopically small tweezers in which we could place a single protein’, says Driessen. Thus he and Bechtluft could pull gently on that protein and then let it bounce back into position. At the same time they were calculating how much force it was taking.
Jerks
The calculations revealed that a protein under pressure from the tweezers unfolded in a series of jerks. The jerks are an indication that the protein unfolds in several steps. It looked like the biologists were pulling on a tangled ball of wool. That doesn’t go in one fell swoop, either, but with stops and starts. ‘You can draw a pattern of the sudden unfoldings’, says Driessen, pointing to a graph. And every time that the scientists pulled at the protein, the same pattern emerged. ‘This means that the protein – after we have completely straightened it and let it spring back – falls back into its original folds.' In other words, the scientists could follow the unfolding of a protein molecule without any problems.
The chaperone takes over
But what happens if the chaperone also joins in? The question is whether it ensures that the protein remains completely unfolded. So Driessen and Bechtluft repeated the pulling experiment, but this time with a chaperone protein in the equation. And lo and behold, when the biologists expanded the protein for a second time with a chaperone present, the jerky pattern did not appear. It was as if the scientists were pulling on a floppy cord instead of a tangled ball of wool. According to Driessen, this is hard evidence that the protein no longer folds up, but remains unfolded thanks to the chaperone. As a result, the protein can pass easily through the cell's transport channels.
Alzheimer’s
The researchers have also experimented with proteins that are stuck together. They, too, were much easier to stretch out after adding a chaperone protein. According to Driessen, this could be very important for research into diseases like Alzheimer’s and BSE. ‘With Alzheimer’s, proteins that have folded wrongly stick to each other. They eventually overrun the brain cells. Using this technique, we have discovered that chaperone proteins definitely prevent this sticking together. I have to say that this was an unexpected result, but it certainly opens up new research perspectives.’
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Press release AMOLF
