There is more actin in an animal or plant cell than any other protein. This is because actin is an important component of the cell’s cytoskeleton. Individual actin proteins string together to form actin filaments, sometimes called microfilaments. Much like your skeleton does for you, actin filaments in the cytoskeleton help the cell keep its shape.
Actin filaments also serve as tracks for motor proteins called myosins to walk on. (Remember myosin? If not, read about myosin here!) Actin and myosin often work together to move things around inside of the cell and to move the cell itself. In muscles, actin filaments are organized with myosin filaments in a very specific way so that they can team up to contract the muscle.
One actin protein has a “barbed” end and a “pointed” end. It also has a specific place for collecting energy, which it needs to form a filament.
In the actin filament, the pointed end of one actin is nestled into the barbed end of the next. Two strands of actin proteins, one after the other, twist together.
At first, it is very difficult for one actin protein to join with another to start a filament. (You can compare this process to starting a club.) But once enough actins have gathered together, it becomes much easier for the filament to recruit more actins and grow.
Unlike your skeleton, the actin filaments in the cytoskeleton are constantly forming and breaking down to helps cells change shapes and move, in addition to several more specific functions. An actin protein carrying some energy is more likely to join a filament, but once that energy has been spent, it becomes more likely to leave the filament. There are also several other proteins that interact with actin to encourage it or prevent it from joining a filament. All of this directs the ways that the actin filaments position themselves in the cell to do different jobs.
For an animation of these processes, see this video from the Mechanobiology Institute in Singapore: https://www.youtube.com/watch?v=VVgXDW_8O4U
Dominguez, R., & Holmes, K. C. (2011). Actin structure and function. Annual review of biophysics, 40, 169–186. https://doi.org/10.1146/annurev-biophys-042910-155359
Egelman E. H. (1985). The structure of F-actin. Journal of muscle research and cell motility, 6(2), 129–151. https://doi.org/10.1007/BF00713056