Actin is the Lego of the cell. The small proteins can be assembled into many different forms for a wide variety of uses: serving as a scaffold to keep the cell’s shape, a railroad for shipping packages, or a powerful motor to propel the cell or tear it in half. But actin itself is a blank slate, an interchangeable material that needs guidance to do anything more than stick together in chains called filaments. To truly understand the Lego of the cell, you have to understand the factors that prompt it to form into its many useful conformations.
“The actin itself is boring. It’s just a building block,” said David Kovar, assistant professor of cell & molecular biology and molecular biophysics at the University of Chicago. “These filaments have to be assembled at the right time and place, they have to be organized with the right architecture, and the dynamics have to be correct – some structures are assembled and stable for minutes to hours, whereas others are assembled and disassembled on the order of seconds.”
Kovar’s lab studies actin-binding proteins, the cellular tools that shape formless actin into functional filaments. This area of research has exploded as scientists discovered multiple actin-binding proteins, each with their own unique properties. One element, the actin-related protein 2/3 complex (Arp2/3 for short), creates branches in the normally linear filaments. Another, called formin, attaches to the end of the filament and steps on the gas, causing it to grow at an accelerated rate.
Scientists have learned a lot about actin engineering by using a method called TIRF microscopy, which allows them to watch as actin filaments form, grow, and take shape. [A short video of Arp2/3-induced branching is available below.]
“This enables us to actually watch these things in real time, and it has revolutionized the field,” Kovar said.
In a new paper, published last week in the journal Nature Structural & Molecular Biology, Kovar’s laboratory and collaborators at the University of Pennsylvania eavesdropped on the activity of a newly-discovered class of actin-binding proteins, named for a shared feature called the WH2 domain. By studying one such WH2 domain protein, isolated from a water-dwelling bacteria that causes gastrointestinal problems in humans, the lab found themselves watching a new, chaotic kind of actin-forming behavior – akin to how a toddler might choose to play with Legos.
Bacteria are fond of “hijacking” the actin system to their own purposes. The species that cause the disease listeria use actin to push themselves from one end of a cell to the other, so that they can burst free and infect neighboring cells. In the case of this bacteria, Vibrio parahaemolyticus, the actin-binding protein is called VopL. When this protein is deleted, the bacteria loses its infectious bite, emphasizing the biological importance of its actin-binding activity. But when Kovar’s laboratory combined VopL with fluorescent green actin in a dish, he found a very surprising effect (viewable in the video below). Instead of the elegant branching or elongation seen with other factors, VopL acts like an actin machine gun, spraying filaments in all directions like the Atari game Asteroids.
“I think something that makes a filament and then shoots it out into space is very bizarre,” Kovar said. “The simplest thing I can think of is that’s a good way to kill a cell. If you over-express an actin-binding factor in a cell it kills it because it essentially assembles actin everywhere. It uses it up. That’s why it’s so important that a cell preserves a balance between unassembled and assembled actin.”
Curiously, the WH2 domain of VopL is shared by a number of normal cellular factors whose purpose, one would think, is less war-like. One example is a protein in animal cells called Spire, which appears to cooperate with formin to rapidly grow long, straight filaments. By studying the characteristics of VopL – what end of the filament it binds, how long it stays bound to the end – scientists can gain insight into the more constructive role of Spire and similar factors. While an actin “machine gun,” may not appear to be a useful function, other aspects shared by VopL and Spire likely contribute to the broad range of tools used to build important structures out of the cell’s Legos.
“That’s where the story is yet to be told,” Kovar said. “These factors are vastly different from each other and it turns out that the complexity is large. So understanding how each of those different mechanisms would be important for their particular role is not yet clear.”
Namgoong, S., Boczkowska, M., Glista, M., Winkelman, J., Rebowski, G., Kovar, D., & Dominguez, R. (2011). Mechanism of actin filament nucleation by Vibrio VopL and implications for tandem W domain nucleation Nature Structural & Molecular Biology, 18 (9), 1060-1067 DOI: 10.1038/nsmb.2109