Actin and myosin are the two proteins responsible for generating nearly every deliberate movement your body makes, from the beating of your heart to the blinking of your eyes. Actin forms long, thin filaments that act as tracks, while myosin is a motor protein that grabs onto those tracks, pulls, and lets go in a repeating cycle powered by the cell’s energy currency, ATP. Their interaction inside muscle cells is what produces force and shortens muscles, but the partnership extends well beyond muscles: actin and myosin help divide cells, move cargo around inside them, and allow cells to crawl from one place to another. The basic mechanics are elegant and surprisingly well understood, though researchers continue to uncover layers of complexity that matter for everything from heart disease to drug design.
The Sliding Filament Model
The modern understanding of how muscles contract dates to 1954, when two unrelated scientists named Huxley, working independently with their respective collaborators, proposed what became known as the sliding filament theory. Hugh Huxley and Jean Hanson showed that muscles contain overlapping sets of filaments that do not themselves change much in length. Instead, these filaments slide past each other when the muscle shortens.1PubMed Central. Hugh E. Huxley: the compleat biophysicist The idea was revolutionary because earlier models assumed that the protein filaments compressed or folded like an accordion. Evidence for the correct interpretation had actually been available since the mid-1800s, but it took a century before anyone assembled the pieces.2PubMed Central. Muscle contraction: Sliding filament history, sarcomere dynamics and the two Huxleys
Inside a muscle cell, the basic contractile unit is called a sarcomere. Picture it as a tiny compartment where thin actin filaments are anchored at each end and thick myosin filaments sit in the middle, overlapping with the actin from both sides. When the muscle contracts, myosin heads reach out, grab the actin filaments, and pull them inward, effectively dragging the two ends of the sarcomere closer together. Multiply that by millions of sarcomeres lined up end to end, and you get a muscle that shortens with real force. The filaments themselves stay the same length throughout. All the shortening comes from the increased overlap as they slide past one another.3Annual Review of Biophysics. The Sliding Filament Theory Since Andrew Huxley: Multiscale and Multidisciplinary Muscle Research
The Power Stroke and the ATP Cycle
The molecular action that converts chemical energy into movement is called the power stroke. A myosin head starts in a “cocked” position, primed with energy from a molecule of ATP that has already been split. When this cocked head binds to an actin filament, a small rotation at the actin-binding interface puts strain on structures deep within the myosin molecule. That strain drives a larger conformational shift, a kind of lever-arm swing, that yanks the actin filament forward.4Cytoskeleton. The myosin start‐of‐power stroke state and how actin binding drives the power stroke After the stroke is complete, the myosin head is stuck tightly to actin in what researchers call the “rigor” state. It cannot let go on its own.
This is where ATP comes in again. A fresh ATP molecule binds to the myosin head, and even though the ATP binding site is a considerable distance from the actin-binding face, it triggers a structural change that pries the myosin loose.5PubMed Central. Structural mechanism of the ATP-induced dissociation of rigor myosin from actin Once free, the myosin splits that ATP, recocks itself, and is ready to grab actin again further along the filament. The whole cycle, grab, pull, release, recock, repeats many times per second in an active muscle. When ATP runs out entirely, myosin heads lock onto actin and cannot detach. That is what happens in rigor mortis after death: without fresh ATP, the crossbridges are permanently stuck.
How Calcium Switches Contraction On and Off
Having a motor protein constantly grabbing at actin would be a problem. Your muscles need to relax most of the time, so the system has a gatekeeper. In skeletal and cardiac muscle, two regulatory proteins, tropomyosin and troponin, sit along the actin filament and physically block the sites where myosin would attach.
When calcium levels inside the muscle cell are low, a part of troponin called TnI anchors itself to actin and pushes tropomyosin into a position that covers the myosin-binding sites. This is the “off” state.6PubMed Central. Structural basis for the regulation of muscle contraction by troponin and tropomyosin When a nerve signal triggers the release of calcium from storage inside the cell, calcium ions bind to troponin. This binding causes TnI to release its grip on actin, and another part of troponin then nudges tropomyosin out of the way, exposing the myosin-binding sites so crossbridge cycling can begin.7PubMed Central. Structural basis for the activation of muscle contraction by troponin and tropomyosin When calcium is pumped back into storage, tropomyosin slides back over the binding sites and contraction stops.8PubMed Central. Mechanism of the calcium-regulation of muscle contraction–in pursuit of its structural basis
The system is remarkably fast. Calcium release and reuptake happen in milliseconds, which is why you can flutter your fingers or maintain a steady heartbeat without conscious effort. The troponin-tropomyosin switch is essentially a molecular gate controlled by calcium concentration.
Smooth Muscle Uses a Different Switch
The walls of your blood vessels, your intestines, your airways, and your bladder all contain smooth muscle. This tissue contracts more slowly and can sustain contraction for long periods, which makes sense given its jobs: maintaining blood pressure, pushing food through your gut, controlling airflow. Smooth muscle cells lack troponin entirely. Instead, they regulate contraction through myosin itself.
When calcium enters a smooth muscle cell, it binds to a small protein called calmodulin. The calcium-calmodulin complex then activates an enzyme, myosin light-chain kinase, which attaches a phosphate group to one of myosin’s light chains. This phosphorylation event is what allows myosin to interact with actin and start contracting.9PubMed. Calmodulin and the regulation of smooth muscle contraction When calcium drops, the phosphate is removed by another enzyme, and contraction winds down.10PubMed Central. Early-Onset Hypertrophic Cardiomyopathy Mutations Significantly Increase the Velocity, Force, and Actin-Activated ATPase Activity of Human β-Cardiac Myosin So where skeletal muscle controls the actin side of the equation, smooth muscle controls the myosin side. The end result, calcium-dependent contraction, is the same, but the regulatory wiring is different.
Fast-Twitch and Slow-Twitch Muscle Fibers
Not all skeletal muscle fibers are the same, and much of the difference comes down to which version of myosin they carry. Human muscle fibers contain distinct mixtures of myosin heavy and light chain variants. Slow-twitch fibers (type I), the kind that dominate endurance muscles, express particular heavy chains called MYH6 and MYH7 along with a specific light chain. Fast-twitch fibers (type IIx) contain a different set of heavy chains and essentially lack the slow-twitch markers.11PubMed Central. Myosin content of individual human muscle fibers isolated by laser capture microdissection
These molecular differences translate directly into performance. Slow-twitch fibers contract less forcefully but can keep going for a long time without fatiguing. Fast-twitch fibers generate more power but tire quickly. The ratio of fiber types varies between individuals and between muscles within the same person. Your calf muscles, for example, tend to be rich in slow-twitch fibers because they work all day keeping you upright, while the muscles you use for jumping or sprinting have a higher proportion of fast-twitch fibers. Training can shift the balance somewhat, but your genetic starting point matters a lot.
Actin and Myosin Outside of Muscle
Actin and myosin are not exclusive to muscle tissue. Every cell in your body uses actin filaments as part of its internal skeleton, and most cells also contain non-muscle forms of myosin. These proteins do fundamentally the same thing in non-muscle cells as they do in muscle: they generate force by sliding past each other. The applications, though, are strikingly varied.
During cell division, an actomyosin ring assembles at the middle of the cell and physically pinches the cytoplasm in two, splitting one cell into two daughter cells.12PubMed Central. Central Role of the Actomyosin Ring in Coordinating Cytokinesis Steps in Budding Yeast In migrating cells, like immune cells chasing bacteria or embryonic cells moving into position during development, actin polymerization at the leading edge pushes the membrane forward while myosin at the rear contracts the cell body, creating a kind of crawling motion.13Molecular Biology of the Cell. Comparative Maps of Motion and Assembly of Filamentous Actin and Myosin II in Migrating Cells Myosin V, a different family member from the muscle myosin, acts as a cargo transporter, ferrying vesicles and organelles to specific destinations within the cell.14PubMed. Motor proteins: myosin V–the multi-purpose transport motor
Cells also use actin-myosin interactions to sense the physical properties of their surroundings. Bundles of actin and non-muscle myosin II called stress fibers act like tension cables that probe how stiff or soft the surrounding tissue is. This mechanical feedback influences cell behavior in profound ways, helping determine whether a stem cell becomes a bone cell, a muscle cell, or a neuron, and whether a cancer cell stays put or starts migrating.15PubMed Central. Actomyosin stress fiber mechanosensing in 2D and 3D
How Actin Networks Are Built and Remodeled
Actin filaments are not static structures. They are constantly assembling and disassembling, which lets cells reshape themselves on the fly. Individual actin molecules (called monomers, or G-actin) polymerize end to end into filaments (F-actin), and these filaments can organize into wildly different architectures depending on what the cell needs. At a migrating cell’s leading edge, actin forms a branched meshwork in a flat sheet called a lamellipodium, with filaments oriented at roughly 45° to the direction of movement.16Cell. Molecular Mechanism of Cell Locomotion In finger-like protrusions called filopodia, actin is bundled into tight parallel arrays. In contractile structures, actin filaments run antiparallel to one another so myosin can slide between them, much like a miniature version of the sarcomere.17Physiological Reviews. Actin Dynamics, Architecture, and Mechanics in Cell Motility
The construction of new actin branches depends on a molecular machine called the Arp2/3 complex, which nucleates new filament branches off the sides of existing ones. Arp2/3 does not work alone; it requires activation signals from upstream proteins. Researchers have found that another set of actin-building proteins called formins are recruited to sites of actin assembly before Arp2/3 arrives, and the two systems can compete with each other for the same upstream regulators. This interplay, a kind of molecular tug-of-war, shapes the timing and geometry of actin waves that sweep across the cell surface.18PubMed Central. Competition and synergy of Arp2/3 and formins in nucleating actin waves
When Actin or Myosin Goes Wrong
Because actin and myosin are so central to muscle function, mutations in the genes that encode them can cause serious disease. Hypertrophic cardiomyopathy (HCM), the most common inherited form of heart disease, is linked to over a thousand mutations, many of them in the gene for beta-cardiac myosin. Some of these mutations make individual myosin heads weaker but simultaneously destabilize a resting state called the super relaxed state, freeing more myosin heads to engage at once and producing a net hypercontractile effect. One well-studied mutation, P710R, reduced the step size of the myosin motor in single-molecule experiments yet caused cardiomyocytes carrying the mutation to generate significantly more force overall, along with hypertrophy and cytoskeletal remodeling.19PubMed Central. Hypertrophic cardiomyopathy β-cardiac myosin mutation (P710R) leads to hypercontractility by disrupting super relaxed state
Early-onset forms of HCM can be even more severe at the molecular level. Two mutations associated with childhood-onset disease, D239N and H251N, increased the speed, force, and ATP-consumption rate of beta-cardiac myosin by 20% to 90% compared to normal myosin.10PubMed Central. Early-Onset Hypertrophic Cardiomyopathy Mutations Significantly Increase the Velocity, Force, and Actin-Activated ATPase Activity of Human β-Cardiac Myosin The heart muscle effectively works too hard, thickening the walls of the ventricles in ways that impair filling and can lead to dangerous arrhythmias.
Actin mutations cause HCM too, though they are rarer. Researchers have identified single amino acid changes in the cardiac actin gene that disrupt the sites where actin molecules contact each other or where actin contacts myosin. Depending on which interaction is affected, the result can be either thickening of the heart wall or progression to heart failure.20PubMed. Inherited and de novo mutations in the cardiac actin gene cause hypertrophic cardiomyopathy
Drugs That Target the Actin-Myosin System
The molecular details of the crossbridge cycle have opened the door to drugs designed to dial muscle contraction up or down at its source. Two broad strategies are currently in play: myosin inhibitors and myosin activators.
Blebbistatin is a widely used research tool that blocks actin-myosin interaction directly. In mouse cardiac tissue, it inhibits contraction with an inhibitory constant in the low micromolar range, and its potency against organized muscle filaments closely matches what is seen with isolated myosin fragments in a dish.21PubMed. Blebbistatin specifically inhibits actin-myosin interaction in mouse cardiac muscle Blebbistatin has also shown up in virology research, where blocking actin-myosin function suppressed the replication of duck enteritis virus both in cell culture and in live animals, highlighting how even viruses depend on the host cell’s actin-myosin machinery.22International Journal of Molecular Sciences. Proteomic Screening for Cellular Targets of the Duck Enteritis Virus Protein VP26 Reveals That the Host Actin–Myosin II Network Regulates the Proliferation of the Virus
On the clinical side, mavacamten and blebbistatin-type molecules work differently at the structural level. Mavacamten pushes myosin into the energy-saving super relaxed state by binding at a different site than blebbistatin, while blebbistatin appears to produce a distinct “ultra-relaxed” conformation. X-ray scattering studies show the two inhibitors cause myosin to adopt different shapes, with mavacamten producing a more compacted structure.23bioRxiv. TWO CLASSES OF MYOSIN INHIBITORS, BLEBBISTATIN AND MAVACAMTEN, STABILIZE β-CARDIAC MYOSIN IN DIFFERENT STRUCTURAL AND FUNCTIONAL STATES Mavacamten has since been approved for treating obstructive HCM, where the excessive myosin activity needs to be toned down.
The opposite strategy, boosting myosin activity, targets heart failure. Omecamtiv mecarbil is a selective cardiac myosin activator that has been tested in large clinical trials. In a trial of over 8,000 patients with heart failure and reduced pumping capacity, those receiving the drug had a modestly lower rate of heart-failure events or cardiovascular death compared to placebo over roughly 22 months of follow-up.24PubMed. Cardiac Myosin Activation with Omecamtiv Mecarbil in Systolic Heart Failure The concept is appealing: rather than propping up a failing heart with drugs that change blood pressure or fluid balance, you directly coax the motor protein to work more efficiently. In practice, the therapeutic window has been tricky. Higher doses can cause problems with heart filling and energy balance, and no cardiac myosin activator has yet achieved regulatory approval.25PubMed Central. Cardiac Myosin Activators in Heart Failure: Experimental Advances Amid Clinical Uncertainty The field is still working out how to harness the crossbridge cycle pharmacologically without tipping the balance too far in either direction.
Why the Actin-Myosin System Matters Beyond Biology Class
It is easy to treat actin and myosin as textbook characters that appear in a chapter on muscle contraction and then vanish. In reality, these two proteins intersect with a surprising range of practical concerns. Exercise physiology is essentially a story about how training shifts the expression of different myosin isoforms, remodels actin networks, and changes the calcium-handling machinery in muscle fibers. Physical therapy after injury or surgery relies on rebuilding functional sarcomeres and restoring the crossbridge cycle in atrophied muscle. Cardiac medicine increasingly frames heart disease in terms of specific myosin mutations and their biomechanical consequences, moving toward treatments that target the motor protein directly rather than patching downstream symptoms.
Even fields like wound healing and cancer biology depend on a working understanding of how non-muscle actin and myosin drive cell migration. A tumor cell that metastasizes is, at the molecular level, crawling through tissue using the same actin-polymerization and myosin-contraction toolkit that a healthy immune cell uses to chase down a pathogen. The difference lies in the signals that activate the machinery, not in the machinery itself. That convergence is why actin and myosin keep showing up at the center of research problems that seem, on the surface, to have nothing to do with muscle.