Effect of "new and old" actin on cell motility

A new study in the journal Nature provides a high-resolution structure showing how two key biochemical states of actin work with bending forces to determine how actin interacts
with other proteins.
At the leading edge of the crawling cell, a complex network of rod-shaped actin filaments extends toward the cell membrane at different angles, lengthening
the proteins one after the other.
Upon impact, staggered cell rods pass over the cell membrane, bending
as the combined force of countless fibers pushes the cell forward.
The flexibility of these fibers, and how efficiently they absorb the necessary regulatory proteins, depends on the properties of
the individual actins that make them up.

Rockefeller Gregory Alushin said, "When you add power in the mix, you see substantial changes
.
We provide clear evidence that these biochemical changes in actin can only be read
by the mechanical properties of the fiber.
”

Review of protein control

Actin filaments are long polymers of actin that are connected
end-to-end.
Actin within fibers can be present
in one of two important biochemical states.
Newly added actin to polymers contains phosphoric acid molecules, while aging actin does not contain phosphate molecules; Otherwise, the two states are more or less the same
.
But actin-binding proteins can distinguish them, and they bind or ignore filaments
depending on the state of actin.

How actin-binding proteins distinguish between these states has been a mystery
.
Some have proposed that phosphate somehow changes the shape of actin, making actin-binding protein stand out
from people in the body.
In fact, many enzymes can switch between different shapes when other molecules are attached to the enzyme, a process known as allosteric regulation
.
It makes sense to assume that actin is also true
.
But without knowing what the two biochemical states of actin look like, it's just a guess
.
Gregory Alushin wondered if there were more stories
.
"How to control proteins is an old problem," he said
.
"People haven't explored new ideas in a while
.
"

A leap in approach

Matthew Reynolds, a graduate student in Alushin's lab, began studying high-resolution structures
for each state.
When examining these structures, the bound phosphate and water molecules were clearly broken down, and the team found that the two actin states were still indistinguishable
effectively.
Whether actin binds to phosphate or not, its structure has almost the same filament lattice and protein backbone
.
If it participates in standard allosteric regulation, then when actin binds to phosphate, it changes significantly – the main difference that regulatory proteins use to distinguish one type of
actin.
But the observed differences appear to be too small for actin-binding proteins to distinguish between them
.

To find another explanation, the team developed a machine learning method to find a relatively small number of curved fibers in cryo-electron microscopy images in order to analyze their structure
.
They then determined the structure of the bent filament in two biochemical states, where the scale of the bending is consistent
with that found in the cell as the filament skims over the cell membrane during locomotion.
"Developing a way to capture this part of the image is critical," Alushin said
.
"This is an example of
methodological progress that requires scientific progress.
"

When bent, actin containing phosphate looks very different from actin without phosphate, so actin-binding protein can easily distinguish between the two states
.

"Changes in the biochemical state of the fiber affect how the fiber deforms when subjected to force," Reynolds said
.

A new pattern is beginning to emerge: when the polymer is bent, actin in the fiber can bend in a variety of ways, but this flexibility is limited
when phosphate bends its structure.
Imagine a hose with small doughnuts
side by side.
Some donut holes are open, some holes have golf balls, but otherwise they are the same
.
When the tube is bent, the doughnuts are all flattened and change shape, but those with golf balls will be different from
other donut deformations.

Similarly, the two states of actin are essentially indistinguishable until the filament is bending, but once pressure is applied, actin with phosphate is squeezed differently
than actin without phosphate.
"What's important is the deformability of the protein," Alushin said
.
"If there's a hole in the middle, it can bend
in one direction.
If you fill the hole with phosphate, it can't be squeezed
in the same way.
”

The results explain how actin-binding proteins distinguish actin's biochemical states and reveal a protein regulatory model that includes synergies between biochemical states and forces
.
In future studies, Alushin hopes to investigate whether other proteins have similar co-regulation
.
"Our study of actin is the first understanding of this phenomenon, but one limitation at the moment is that we don't have the structure of
other force-responsive proteins.
Studying these proteins is worthwhile because it is technically possible
.
”