The myosin protein is well known for walking, but now it seems it also sleeps.
Myosin is a motor protein that helps muscles contract. Motor proteins have the fun quirk of looking as though they walk along the cell’s cytoskeleton to change the shape of the cell and make our muscles move.
Now, a team led by Charlotte Scarff from the University of Leeds, UK, and colleagues from East Carolina University, US, have shown that myosin has another quirk when not in use; it bunkers down for a “nap”. The team combined 96,000 electron microscope images to show, with remarkable detail, exactly how the myosin folds up when it is shutdown.
The paper, published in Nature, shows a 3D visualisation of how the shape of myosin changes to conserve energy when inactive.
The myosin protein looks like a two-headed tadpole with a very long tail. When active, the two heads attach to actin and perform the walk, dragging its tail behind it. But when it is no longer in use, the myosin folds itself up into a compact bundle by wrapping its tail around its heads.
Once folded, the tail is locked in place, making it small and compact instead of long and unwieldly. This helps other molecules move the myosin around the cell for use later.
When the muscles need to move again, the tail is unlocked and springs back into position, ready for action.
“The analogy here is that the folded myosin is like a Brompton bicycle, kept in a folded state when not needed, and able to be quickly unfolded when it is, by releasing a simple catch,” says Michelle Peckham, University of Leeds.
“The compact folded myosin is also more easily transported through a crowd to where it’s needed.”
They found that the locking mechanism was controlled by phosphorylation – a process where a phosphorus-based molecule is added to change the shape or action of a protein. When the phosphorus was removed, the lever was locked to keep the tail in place. When it was added again, the catch was released, and the tail quickly unravelled.
This is quite relevant for muscle disorders because they found that many known mutations were at, or close to, the phosphorylation sites needed to lock the protein. This could prevent the shutdown process from happening and cause muscle disease.
Thankfully, the incredible detail they described provides a good model for researching these types of mutations in other myosin-related diseases.
Dr Deborah Devis is a science journalist at The Royal Institution of Australia.
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