First images of motor proteins in action


Rebecca Pool

Tuesday, September 15, 2015 - 12:15
Image: Dynein; understanding movement is critical for future treatment of disease.
A team of UK and Japan-based researchers have unveiled the first images of motor proteins in action, captured using cryo-electron microscopy.
While these proteins are vital to complex life, forming the transport infrastructure that allows different parts of cells to specialise in particular functions, movement has never been directly observed, until now.
Using a FEI Technai electron microscope, researchers at the University of Leeds, Chuo University and colleagues imaged the largest type of motor protein - dynein - during the act of stepping along its molecular track. 
Video: motor proteins caught “swinging" on microtubles.
“Dynein has two identical motors tied together and it moves along a molecular track called a microtubule," highlights Dr Stan Burgess, from the School of Molecular and Cellular Biology at Leeds. "It drives itself along the track by alternately grabbing hold of a binding site, executing a power stroke, then letting go, like a person swinging on monkey bars."
According to the researcher, until now dynein movement has only been tracked by tagging fluorescent molecules to the proteins and observing the fluorescence using very powerful light microscopes.
"It was a bit like tracking vehicles from space with GPS. It told us where they were, their speed and for how long they ran, stopped and so on, but we couldn’t see the molecules in action themselves," he says. "These are the first images of these vital processes.”
Dynein dimers display remarkable flexibility at a hinge close to the microtubule binding domain (the stalkhead) producing a wide range of head positions. [H. Imai et al., Nature Communications]
An understanding of motor proteins is important to medical research; many viruses '1hijack' motor proteins to reach the cell nucleus for replication.
What's more, cell division is driven by motor proteins and so insight into its mechanics could be relevant to cancer research.
To image the proteins, the team combined purified microtubules with purified dynein motors and added the chemical fuel adenosine triphosphate (ATP) to power the motor.
“We set the dyneins running along their tracks and then we froze them in ‘mid-stride’ by cooling them at about a million degrees a second, fast enough to prevent the water from forming ice crystals as it solidified," explains Burgess's colleague, Professor Hiroshi Imai from the Department of Biological Sciences at Chuo University, Japan. "Then using a cryo-electron microscope we took many thousands of images of the motors caught during the act of stepping."
"By combining many images of individual motors, we were able to sharpen up our picture of the dynein and build up a dynamic idea of how it moved," he adds. 
The researchers reckon their most striking discovery was the existence of a hinge between the dynein's, thin stalk and its ‘grappling hook’. This allows a lot of variation in the angle of attachment of the motor to its track.
Crucially, these findings provide a structural framework for understanding dyne in’s directionality and unusual stepping behaviour, as well as its many cellular functions.
The University has since announced a £17 million investment in state-of-the-art facilities that will allow even closer observation of life within cells.
New equipment includes two 300 kV electron microscopes , 950 MHz nuclear magnetic resonance spectrometer alongside existing 120kV and 200kV EMs, and 500, 600 and 750 MHz NMR machines.
Research is published in Nature Communications.
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