Researchers have used visualization techniques to see how formins bind to actin filaments at the molecular level. This has helped them understand how formins facilitate the addition of new actin molecules to a growing filament. Additionally, the researchers have uncovered the reasons for the varying speeds at which different formins promote this process.
Actin is a protein that is found in abundance and is responsible for controlling the shape and movement of our cells. It does this by forming filaments, with one actin molecule joining at a time. The formin family of proteins plays a crucial role in this process, as they are located at the end of the filament.to understand the underlying process of actin filament growth. The researchers used a technique called cryo-electron microscopy to capture high-resolution images of formins interacting with actin filaments. They observed that formins recruit new actin subunits and remain associated with the end by “stepping” with the growing filament. Our cells contain up to 15 different formins that drive actin filament growth at varying speeds and for different purposes. However, the precise mechanism of action of formins and the reasons for their different inherent speeds have remained unclear. This groundbreaking research provides the first-ever visualization at the molecular level of how formins bind to the ends of actin filaments, shedding light on the process of actin filament growth. conducted at the Max Planck Institute of Molecular Physiology in Dortmund by researchers from the groups of Stefan Raunser and Peter Bieling. They used cryo-electron microscopy to capture detailed images of formins interacting with actin filaments. The images revealed that formins recruit new actin subunits and maintain their association with the end by “stepping” with the growing filament. This study offers insight into the mechanism of action of formins and the factors influencing their varying speeds in driving actin filament growth.Joining forces with biochemical strategies and electron cryo-microscopy, researchers at the MPI have uncovered the mechanisms through which formins facilitate the addition of new actin molecules to growing filaments. This breakthrough, published in the journal Science, sheds light on the different speeds at which formins promote this process and provides valuable insights into the implications of mutations in formins for various diseases. Specifically, the study reveals how ring-like formin proteins promote the growth of actin filaments at the molecular level in cells.Forces
“Our finding allows us to view decades of biochemical research on formins in a new way, addressing many longstanding questions in this area,” explains Peter Bieling. Previous X-ray crystallization structures showed that formins consist of two identical parts that wrap around the actin filament in a ring-like formation and move along with it as it grows. Previous speculative models suggested that formins interact with actin through all four of their binding domains, and that slow and fast-moving formins would take on different shapes on the filament. “However, these studies did not have high-resolution structures of foAccording to Wout Oosterheert, a postdoc in Stefan Raunser’s group at MPI Dortmund and co-first author of the publication, formins are bound to specific sites of activity, specifically the barbed end of actin filaments. These formins are dynamic proteins that quickly assemble filaments, making it challenging to obtain enough filament ends for detailed structure determination. The MPI scientists analyzed three distinct formins from fungi, mice, and humans, all of which elongate actin filaments at different speeds. Oosterheert describes one of the formins as very fast, likening it to the “Ferrari among formins.””Another type of formin acts more like a tractor,” explained Stefan Raunser. Through testing and optimizing a variety of conditions, the scientists were able to increase the number of formin-bound filaments. “We drew from our previous studies to improve both the biochemistry and cryo-EM sample preparation, which was crucial in obtaining these structures,” said Micaela Boiero Sanders, the other co-first author of the study.
The new structures, with resolutions around 3.5 Ångstrom, reveal that formins encircle actin in an asymmetrical ring formation: One half of the ring isThe team discovered that half of the formin protein is tightly attached to the actin filament, while the other half is more loosely connected and can grab onto a new subunit. Oosterheert and Boiero Sanders stated that analyzing the structures provided a breakthrough moment in understanding the mechanism. When a new actin subunit joins the filament, it disrupts the formin arrangement, causing the stable half-ring to move onto the new subunit and become loose, while the other half-ring becomes stable. This coordinated mechanism allows formins to remain connected to the growing actin filament end over long distances.
Researchers analyzed formins and found that only three binding domains were interacting with actin simultaneously.
By introducing mutations into formins, MPI scientists explained the differences in speed among actin-formin complexes: if the formin ring is bound more tightly to the actin filament end, it is more challenging for the ring to release and move onto a new incoming actin subunit. This results in slower filament growth. Peter Bieling stated, “We now understand how a formin, which acts like a tractor, can be made faster by giving it some Ferrari-like features.” The MPI team anticipates that their findings will benefit many scientists in the field.The actin cytoskeleton is studied by various researchers around the world. Raunser concludes that the new insights gained from their research can help in understanding the specific roles of the fifteen human formins at the cellular level. This understanding can also shed light on how mutations in formin genes lead to severe diseases.
