Doughnut in a cage holds the key to bacteria's survival

A new study has revealed that bacteria have a unique doughnut-shaped protein that sits in a cage inside their cells to help them store potentially dangerous iron. Experts believe this discovery could lead to innovations in medical imaging and could even be used to track cancer cells, or look for damage caused by heart disease.

Almost all organisms have proteins in their cells called ferritins that are shaped like hollow balls. Since iron forms rust in the presence of oxygen, ferritin acts like a cage and safely stores oxidised iron until it's needed, preventing it from causing damage to DNA and other parts of the cell.

Now, for the first time, scientists have shown that bacteria have ferritin shaped like a ring-doughnut, not a ball.

The research team, a collaboration between Newcastle and Edinburgh universities, also showed that this ring ferritin cannot store iron like spherical ferritin. Instead, it sits within the shell of a larger protein cage that resembles the shell of a virus. This shell is much bigger than spherical ferritin cages so bacteria are able to store much more oxidised iron in contrast to other organisms.

Dr Jon Marles-Wright, Senior Lecturer in the School of Biology, Newcastle University, explains: "Our studies revealed that bacteria have an extra ferritin that is completely different to ferritins in other organisms.

"Normally ferritins are like a doughnut, but filled with rust instead of jam or custard. Our ferritin is shaped like a ring-doughnut and doesn't have the same sort of hollow cavity, so there's no space for the 'jam'. It can oxidise iron like other ferritins, but to store the iron, the doughnut ferritin is encapsulated inside an outer shell that is much bigger, allowing bacteria to store much more iron."

Spherical ferritins have already been used in MRI to track cells as the iron core gives them a high contrast, but their small size means that they are hard to see. Because the doughnut protein shell is two to three times bigger than standard spherical ferritins, the research team say that this could make it a useful tool in nanotechnology and for medical imaging since it could give a much stronger signal.

Dr David Clarke, Chancellor's Fellow in the School of Chemistry, University of Edinburgh explains: "Iron is an essential mineral required for life. However, in solution, the metal is potentially very toxic. Therefore it is important that all organisms have efficient mechanisms to store iron and release it in a controlled manner. Our findings are exciting because we are beginning to understand a completely new iron storage system used by bacteria."

Dr Marles-Wright adds "We don't know the details of what happens to the iron once it is stored, but there's clearly an advantage for bacteria in using these two proteins together in this way."