Taking close-ups of cell doors

Myriad proteins are found on the surface of our cells. Some of them send signals between the inside and the outside; others act as doors and conduits. At present, we have a true picture (the molecular structure) of only a few of these proteins. Pontus Gourdon at Lund University wants to determine structures of more of them, so he can understand exactly how they work.

Pontus Gourdon

PhD, Membrane Protein Structural Biology

Wallenberg Academy Fellow 2015

Institution:
Lund University

Research field:
3D-imaging of membrane proteins

Our bodies manufacture some 30,000 different proteins that are necessary for innumerable bodily processes. One-third of them are membrane proteins embedded in the cell membrane. More than half of all drugs act by binding to membrane proteins.

Some proteins serve as tiny channels enabling various substances to pass through the cell membrane. They include TRP (transient receptor potential) ion channels. It is these channels that Gourdon will be studying as a Wallenberg Academy Fellow at Lund University.

“TRP ion channels are found in places such as the mouth and on the skin. They enable us to feel pressure, cold, heat and taste certain substances, such as chili and mint. The brain perceives mint as cold and chili as hot because the same channels are activated as when we experience heat and cold, and the same processes start in the cell,” Gourdon explains.

Now he wants to understand – “in nerdish detail” – the way these channels, as well as channels for copper ions, work. The way to do this is to take really high quality HD pictures of the proteins, elucidating their molecular structures.

“It gives an enormous sense of freedom to be able to focus solely on research for five years. Many grants last for three years, and you then have to lower the bar so as to achieve results in time to apply for further funding. Being admitted as a Wallenberg Academy Fellow will enable me to adopt a long-term approach, and to raise the bar.”

Imaging sensitive proteins – a tricky task

Taking 3D photographs of membrane proteins is not easy. Although humans have 10,000 of these proteins, only about twenty of them have so far been imaged. One reason for this is that the proteins are sensitive, and easily destroyed during the imaging process.

To obtain a 3D picture of a protein it is first necessary to synthesize it in a completely pure form. A fairly simple genetically modified organism, such as yeast, is used to produce the protein, which is then purified with the help of chemicals. Purification long represented an obstacle in research. The proteins were indeed purified, but they also often coagulated – like a boiled egg. This gave them an entirely different shape from the one they have in the cell membrane.

About twenty years ago researchers succeeded in developing a detergent that was mild enough to purify the proteins. Since then some 700 proteins have been imaged in 3D – although only about twenty of them are human proteins.

Luck needed for good crystals

The commonest imaging technique is called X-ray crystallography, and this is the main method used by Gourdon.

“We make crystals from the protein, which means that it “sets” in its natural form. We make the crystals by mixing the protein with chemicals in a way that experience has taught us usually produces crystals. But we need a modicum of luck. Sometimes results are good, sometimes not; it’s a little tricky to predict how it will go,” Gourdon comments.

The crystallized protein is then exposed to a strong X-ray beam, called synchrotron light. Most of the beam goes straight through, but some of it is diffracted by the crystal. With the help of advanced computation software, the researchers can use the information to create an image of the protein, obtaining a picture of its three-dimensional structure.

Gourdon points at his screen, which shows one of the latest images:

“Fewer than ten people in the world have seen this. That knowledge – being able to see something for the first time – gives enormous impetus to my research.”

But once he has seen it, he also wants to understand how the proteins work. Just what is it that happens when mint binds to the ion channel, and opens a tiny “gate”? Knowledge about how the proteins work could benefit other research, perhaps leading to drugs that bind even better to the channels.

New technology on the way – patience still needed

Gourdon does not recall when he first began to consider a career in research, but he was interested in biochemistry even while completing his first degree, in bio engineering.

“When I was doing my PhD I had a gifted and incredibly inspirational supervisor. In retrospect, it feels as though I joined this research team almost by accident – it could just have easily been another one. But I had great good fortune. It led me to where I am.”

Now Gourdon and his team are busy acquainting themselves with a complementing technology: cryo-electron microscopy. This involves use of an electron microscope to take pictures of proteins in a frozen solution. The advantage is that the proteins do not need to be crystallized, a process that, as mentioned, often fails in X-ray crystallography. The drawback is that images are not as detailed. But it looks as though this will not be a problem for much longer.

“That’s why we are jumping on this bandwagon, which is very much on the move,” Gourdon comments.

Perhaps in the future he will be able to use a technique that yields even more reliable results. But for the time being, resolve in the face of adversity is part of the job description.

“We experience many setbacks. Often no crystals at all form, or they’re not of good enough quality. But I’m stubborn. I think that, as a researcher, you have to be – stubborn and curious.”

Text Lisa Kirsebom
Translation Maxwell Arding
Photo Magnus Bergström