Actin filaments are tiny protein fibers that act as the structural skeleton, movement engine, and internal highway of nearly every cell in your body. They hold cells in shape, power muscle contraction, drive cell movement, carry cargo from one part of the cell to another, and pinch cells in half during division. Few proteins do as much, which is why actin is one of the most abundant proteins in any animal cell.
How Actin Filaments Are Built
Actin exists in two forms. The raw material is a single round protein called G-actin (the “G” stands for globular). When conditions are right, these individual units snap together like beads on a twisted string to form a long fiber called F-actin (filamentous actin). The shift from one form to the other involves a physical flattening: each subunit rotates about 12 to 13 degrees as it joins the chain, making the filament version roughly 17 to 18 degrees flatter than the free-floating version.
Each actin unit carries a molecule of ATP, the cell’s energy currency. Once a unit locks into the growing filament, its ATP is broken down into ADP, which weakens its grip on its neighbors. This creates a built-in expiration date for every section of the filament. New ATP-carrying units add to one end (the “plus” or barbed end), while older ADP-carrying units fall off the opposite end (the “minus” or pointed end). The result is a process called treadmilling: the filament appears to stay the same length while constantly rebuilding itself, like a conveyor belt made of protein. In lab conditions, the plus end starts growing when actin concentration exceeds about 0.12 micromolar, while the minus end requires concentrations above 0.5 micromolar to grow. This difference in stickiness between the two ends is what makes treadmilling possible.
Giving Cells Their Shape
Just beneath the outer membrane of most animal cells sits a thin, dense mesh of actin filaments called the cortex. This network is stitched together by over a hundred different helper proteins and anchored directly to the membrane by specialized linker proteins. Think of it as a flexible scaffolding that gives the cell its form and stiffness.
Motor proteins embedded in this cortex pull on the actin filaments, generating a baseline tension across the cell surface, much like the tension in the skin of a drum. This cortical tension is a key factor in determining how round, flat, or irregular a cell looks. When a cell needs to change shape, whether it’s a white blood cell squeezing between tissues or an embryonic cell rearranging during development, it remodels this actin cortex.
Powering Muscle Contraction
The most familiar job of actin filaments is making muscles work. Inside each muscle fiber, actin filaments are arranged in parallel alongside thicker filaments made of the motor protein myosin. When your brain signals a muscle to contract, myosin heads latch onto the actin filaments, pull them a short distance in what’s called a power stroke, release, and grab again. Each cycle is fueled by one molecule of ATP. Billions of these tiny rowing motions happening simultaneously slide the actin and myosin filaments past each other, shortening the muscle fiber and producing force. This sliding filament mechanism is the basis for every voluntary movement you make, from blinking to sprinting.
Driving Cell Movement
Cells that need to crawl, such as immune cells hunting bacteria or embryonic cells migrating to form organs, rely on actin polymerization to push their leading edge forward. They do this primarily through two types of structures.
The first is the lamellipodium, a broad, flat sheet that extends from the front of a moving cell. Inside it, a branching protein complex creates a rapidly expanding web of actin filaments. As new filaments grow and push against the inside of the membrane, they generate enough force to overcome the membrane’s natural tension and shove the cell edge forward. The second structure is the filopodium, a thin, finger-like spike made of tightly bundled parallel actin filaments. Filopodia probe the environment ahead of the cell, sensing chemical signals and surface features. Both structures rely purely on the force of actin assembly rather than on motor proteins to extend outward.
Splitting Cells During Division
When a cell divides, it needs to physically pinch itself into two daughter cells. Actin filaments are central to this final step, called cytokinesis. A ring of actin filaments and myosin motors assembles around the cell’s equator, positioned between the two newly separated sets of chromosomes. Myosin grabs and pulls on the actin filaments in the ring, generating tension that constricts the ring like a drawstring on a bag. As the ring tightens, it squeezes the cell membrane inward until the cell is cleaved in two. In fission yeast, a well-studied model organism, each myosin cluster pulls on actin filaments with a force of about 4 piconewtons, and the filaments self-organize into a tight, tension-generating bundle through repeated myosin interactions.
Transporting Cargo Inside Cells
Actin filaments also serve as tracks for intracellular delivery. Specialized myosin motors (different from the muscle type) walk along actin filaments while carrying organelles, vesicles, and other molecular cargo. This system handles relatively short-range transport, particularly near the cell’s outer edges where actin networks are dense. The properties of each type of myosin transporter are tuned to the actin structure it travels on. Some move along the disordered actin meshwork of the cortex to distribute cargo locally, while others travel along organized actin bundles to cover longer distances within the cell.
Detecting Sound in the Inner Ear
One of actin’s more surprising roles is in hearing. The sensory hair cells of the inner ear have bundles of tiny projections called stereocilia on their surfaces, and each stereocilium is built around a core of tightly packed, hexagonally arranged actin filaments connected by protein cross-bridges. When sound waves cause these stereocilia to bend, the actin filaments inside don’t compress or stretch. Instead, they slide past one another, tilting the cross-bridges relative to the bundle’s long axis.
The resistance to bending depends on the number of cross-bridges, which in turn depends on how many actin filaments are present and how long they are. Hair cells in different regions of the cochlea have stereocilia of different, predictable widths and lengths. This means the force needed to bend them varies by location, effectively tuning each hair cell to respond to a specific range of sound frequencies. Fine-tuning of hearing, in other words, is a built-in structural property of actin bundles.
How Cells Control Actin Networks
Cells don’t just build actin filaments and leave them alone. A large cast of regulatory proteins constantly reshapes actin networks to match the cell’s needs at any given moment.
- Arp2/3 complex: Creates new branches off existing filaments, building the dense, web-like networks that push the cell membrane forward during movement. It binds to three consecutive actin subunits along a filament and nucleates a new branch at a 70-degree angle.
- Cofilin: Acts as the demolition crew. At low concentrations, cofilin severs actin filaments into shorter pieces. It also dismantles Arp2/3-created branch points by competing for binding sites on the filament and changing the filament’s structure in ways that weaken the branch connection. At high concentrations, cofilin can actually help start new filaments, making it one of the more versatile regulators.
- Profilin: Loads fresh ATP onto free actin monomers, recharging them so they’re ready to join the plus end of a growing filament. It essentially recycles the building blocks.
- Tropomyosin: Wraps along the length of actin filaments and can protect them from being severed by cofilin, stabilizing parts of the network that the cell wants to keep intact.
- Formins: Sit at the plus end of a filament and speed up its growth while staying attached, producing the long, straight filaments found in filopodia and the contractile ring during cell division.
The interplay between these proteins lets cells build, maintain, and tear down actin structures in seconds. A migrating cell, for instance, continuously polymerizes actin at its leading edge while cofilin disassembles older filaments at the rear, recycling monomers back to the front. This rapid, targeted turnover is what makes actin one of the most dynamic and versatile components of the cell.