Three types of protein filaments make up the cytoskeleton: actin filaments (microfilaments), intermediate filaments, and microtubules. These thread-like structures fill the interior of every eukaryotic cell, working together to maintain cell shape, enable movement, and transport cargo from one part of the cell to another. Each filament type is built from different protein subunits, has a distinct diameter, and handles different jobs.
Actin Filaments (Microfilaments)
Actin filaments are the thinnest of the three, measuring about 7 nm in diameter. They’re built from a small, globe-shaped protein called actin. Individual actin molecules link together end to end, like beads on a string, to form flexible fibers that can stretch several micrometers long. These fibers are especially dense just beneath the cell’s outer membrane, where they form a mesh that acts like scaffolding. That mesh gives the cell its shape, lets it crawl across surfaces, and allows it to pinch in half during cell division.
Actin filaments have a built-in polarity, meaning one end (the “barbed end”) grows faster while the other end (the “pointed end”) tends to lose subunits. At steady state, new actin monomers are added at the barbed end while old ones fall off the pointed end, creating a slow conveyor-belt effect called treadmilling. Cells speed this process up dramatically with helper proteins, giving them the ability to rapidly reshape their actin networks in response to signals. This constant remodeling is what lets a white blood cell chase down bacteria or a wound-edge cell crawl forward to close a gap.
Intermediate Filaments
Intermediate filaments sit in the middle size-wise, roughly 10 nm in diameter. Unlike actin filaments and microtubules, which are each built from a single type of protein, intermediate filaments are assembled from a large and diverse family of proteins. Which proteins a cell uses depends on what kind of cell it is.
Keratins (about 30 different varieties) are found in skin, hair, and the cells lining organs. Vimentin shows up in connective tissue cells, smooth muscle, and white blood cells. Desmin is specific to muscle cells, where it helps anchor the contractile machinery. Neurons rely on neurofilament proteins to reinforce their long axons. And nearly every cell in the body contains nuclear lamins, intermediate filament proteins that line the inside of the nucleus and help organize DNA.
The main job of intermediate filaments is mechanical strength. They act like ropes, absorbing tension and preventing cells from tearing apart under stress. They don’t serve as tracks for motor proteins the way actin filaments and microtubules do, and they don’t show the same rapid assembly and disassembly behavior. They’re the most stable, most durable part of the cytoskeleton.
Microtubules
Microtubules are the largest cytoskeletal filaments, forming hollow tubes about 25 nm across on the outside and 15 nm on the inside. They’re assembled from pairs of proteins called alpha-tubulin and beta-tubulin. These pairs stack into long chains (protofilaments), and 13 protofilaments wrap side by side to form the tube wall.
Microtubules radiate outward from a structure near the nucleus called the centrosome, creating a network of tracks that span the cell. Two families of motor proteins walk along these tracks: kinesins generally carry cargo toward the cell’s outer edge (the “plus end” of the microtubule), while dyneins haul cargo back toward the center (the “minus end”). This system is the cell’s internal shipping network, responsible for moving organelles, vesicles, and signaling molecules to where they’re needed.
During cell division, microtubules take on a critical role. They reorganize into a football-shaped structure called the mitotic spindle, which grabs chromosomes and pulls one copy to each side of the dividing cell. Without properly functioning microtubules, chromosomes can’t separate correctly.
How Motor Proteins Use the Cytoskeleton
Each filament type partners with its own set of molecular motors. Myosin motors walk along actin filaments, powering muscle contraction and helping cells change shape. Kinesin and dynein motors walk along microtubules, ferrying cargo in opposite directions. No motor proteins are known to travel along intermediate filaments, which is consistent with their role as structural cables rather than transport highways.
Septins: A Possible Fourth Component
Some researchers now consider septins a fourth component of the cytoskeleton. Septins are proteins that bind a small energy molecule called GTP and assemble into rings and short filaments. They act as scaffolds that recruit other proteins to specific locations and as diffusion barriers that keep molecules corralled in the right part of the cell. Septins play important roles in cell division, helping define where a cell pinches apart. While the classic three-filament model still dominates textbooks, septins are increasingly recognized as a genuine cytoskeletal element conserved across a wide range of organisms.
What Happens When Cytoskeletal Proteins Go Wrong
Because the cytoskeleton is so fundamental, defects in its proteins cause serious disease. Neurons are particularly vulnerable because of their extreme length and dependence on intracellular transport. Mutations in neurofilament genes (intermediate filament proteins) are linked to Charcot-Marie-Tooth disease, a condition that damages peripheral nerves and causes progressive weakness in the hands and feet. Mutations in intermediate filament genes are also associated with some forms of amyotrophic lateral sclerosis (ALS).
Abnormalities in tau, a protein that normally stabilizes microtubules in neurons, are the hallmark of a group of conditions called tauopathies. These include Alzheimer’s disease, progressive supranuclear palsy, and certain forms of frontotemporal dementia. In these disorders, tau detaches from microtubules and clumps into tangled aggregates inside nerve cells, disrupting transport and eventually killing the neurons. Tau gene mutations directly cause some inherited forms of these diseases, while certain genetic variations increase risk for the sporadic versions.
Comparing the Three Filament Types
- Actin filaments: 7 nm diameter, built from actin, flexible, concentrated near the cell membrane, responsible for cell shape and movement, paired with myosin motors.
- Intermediate filaments: ~10 nm diameter, built from a diverse family of proteins (keratins, vimentin, desmin, neurofilaments, lamins), provide mechanical strength, no associated motor proteins.
- Microtubules: 25 nm diameter, built from tubulin dimers, form hollow tubes, serve as tracks for kinesin and dynein motors, organize the mitotic spindle during cell division.
All three filament systems are connected to each other and to the cell membrane through a web of accessory proteins. This interconnected architecture is what allows a cell to be both structurally rigid and dynamically adaptable, holding its shape one moment and completely reorganizing the next.