New Nature research answers fundamental questions about life that scientists have been trying to answer for decades

"We're answering fundamental questions about life that scientists have been trying to answer for decades," Raunser said
.
In eukaryotic cells, actin is abundant and easily binds (polymerizes) into filaments
.
These filaments form a network that forms the cytoskeleton of the cell and controls various cellular processes
through movement.
For example, immune cells use actin filaments to move and trap bacteria and viruses
.
Researchers already know that fiber dynamics are regulated by ATP hydrolysis — ATP's reaction with water causes a phosphate group to split and produce energy
.
However, the exact molecular details behind this process have not been answered
.

Too flexible or too big?—not suitable for cryo-electron microscopy

Because actin filaments are too flexible or too large for X-ray crystallization and NMR, cryogenic electromagnetism is the only viable technique
for obtaining detailed images.
In 2015, Raunser's team used cryo-electron microscopy (cryo-EM) to create a novel three-dimensional atomic model of the fiber with a resolution of 0.
37 nanometers
.
In 2018, his team described three different states that actin acquires in filaments: binding to ATP, binding to ADP in the presence of split phosphate, and binding
to ADP after phosphate release.

How water molecules move

In their current study, Raunser and his colleagues created a new resolution record: They obtained all three actin states at a resolution of about 0.
2 nanometers, making previously invisible details visible
.
The three-dimensional map shows not only all the amino acid side chains of the protein, but also the location of
hundreds of water molecules.
By comparing these new structures to isolated actin structures, they were able to infer how water molecules move.

During polymerization, the water molecules are repositioned in the ATP pocket in such a way that only one water molecule remains in front of the ATP, ready to attack a phosphate and initiate hydrolysis
.
The accuracy obtained by this method could help further research in the field: "Our high-resolution model could push scientists to design small molecules for light microscopy studies of tissues and, ultimately, for therapeutic applications," Raunser said
.

Door opener! ?

The authors also shed light on the ultimate fate
of phosphates.
Prior to this, scientists thought there was a back door in the ATP pocket that remained open after ATP hydrolysis to facilitate phosphate withdrawal
.
However, the new cryogenic - electromagnetic structure shows no traces
of opening the back door.
Therefore, the release mechanism remains a mystery
.
"We believe there is a door, but it may be open temporarily," commented Raunser, who now wants to use mathematical simulations and time-resolved cryo-electron microscopy methods to demonstrate how phosphate
exits.
Clearly, these exciting discoveries open the door for scientists to dig deeper into the more details
behind the actin filaments that promote cell motility.