Actin is the most abundant protein inside the cells of higher organisms, such as animals. It serves as the building-block for long, thin structures called filaments, which provide key structural support as part of the cell “cytoskeleton,” the system that gives cells their shape and polarity. Rapid changes in actin filaments underlie key cellular events such as movement along surfaces, cell-to-cell contact, and cell division, and actin filaments also are major elements in muscle fibers.
However, the mechanism that determines these differences has never been entirely clear to scientists.
Now, researchers from the Perelman School of Medicine have revealed key atomic structures of the ends of the actin filament through the use of a technique called cryo-electron microscopy (cryo-EM). The study, published in Science, provides fundamental insights that may help fill in details behind disorders affecting some muscle, bone, heart, neurological, and immune disorders that are the result of actin defects or deficiencies.
“The results of our study provide a mechanistic understanding of a process we have known about for more than 40 years, referred to as filament treadmilling, and impacts how we view the cellular roles of actin in health and disease,” says the study senior author Roberto Dominguez, the William Maul Measey Presidential Professor of Physiology at Penn.
In their study, the researchers, including two Penn students—Peter Carman, a recent graduate student in Dominguez’s lab, and Kyle Barrie, a graduate student currently in the lab, who served as co-first authors—analyzed actin filaments using cryo-EM. With this high-resolution imaging technique, a researcher obtains many thousands of snapshots of a target molecule, aligns them computationally, and then averages them to reduce random image “noise”—yielding a 3D reconstruction of the molecule that may be sharp enough to visualize individual atoms.
With artificial intelligence assistance, the researchers were able to focus on the ends of the filaments instead of their middle, as had previously been the norm in similar research. By doing so, they identified hundreds of thousands of filament end views, allowing them to obtain near-atomic scale reconstructions. These revealed a “flat” actin shape, or conformation, at the uncapped barbed end, versus a “twisted” conformation at the uncapped pointed end.
Read more at Penn Medicine News.