As you read this, the cells of your immune system are busy eliminating invaders. Their weapons are awe-inspiring – but they go awry in autoimmune diseases, attacking your own cells. Studying how these weapons work could lead to treatments for autoimmune disorders.
One weapon resembles a cookie-cutter. It is rapidly assembled from component proteins, and can punch a lethal hole into a cell’s membrane. The cell either bursts or dies from the chemicals that immune cells deliver through the breach.
Once they identify a target cell, hole-punching proteins self-assemble into a ring on the surface of that cell. The ring is built from about 20 identical protein units. The target cell may be a bacterium or a resident cell that has been infected by a virus. For example human perforin proteins, stored inside immune cells called lymphocytes, are deployed on to virally infected cells. Once 18 perforins link into a ring, the cutting machine kicks into action to punch through the cell membrane. We are researching how it does this by studying the oyster mushroom.
Hole-punching proteins are ancient weapons conserved across many species. Oyster mushrooms need to defend themselves against bacteria. They also trap and digest millimetre-long roundworms and employ a host of hole-punching proteins including pleurotolysin. Assembled from 11 to 13 proteins, it’s a tidy, stable ring, which can be easily manipulated.
By changing its structure, such as pinning parts of it together, I can see how the ring packs its punch. The work requires specialised electron microscopes. My collaborator Helen Saibil is an expert at snapping 2-D images of pleurotolysin rings and merging them to reconstruct their 3-D shape. Doing this at different stages of a punch lets us see how the protein moves – it is somewhat like making a flipbook.
We found the inner part of the ring unravels like a sock turned inside out and flips down with a punch that smashes through a cell membrane. We also identified the control centre – a region called TMH2. By finding which parts of hole-punching proteins drive the action, we may be able to find ways to control them, leading to treatments for human autoimmune disorders such as multiple sclerosis.
PAPER: Conformational changes during pore formation by the perforin-related protein pleurotolysin, PLoS Biology, 2015, vol 13, p1-15.
Michelle Dunstone is a biochemist based at Monash University in Melbourne, Australia.
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