Cell Biology
Motor Proteins (Kinesin & Dynein)
Two-legged molecular machines walk cargo along microtubule tracks — 8 nm per step, one ATP per step, ~800 nm/s
Motor proteins are two-headed molecular machines that haul cargo along microtubule tracks by walking hand-over-hand, taking 8 nm steps and burning one ATP per step. Kinesin walks toward the microtubule plus end — anterograde, toward the cell periphery and the axon terminal — at roughly 800 nm/s, while cytoplasmic dynein walks the opposite way, toward the minus end at the cell center, using the dynactin complex and a cargo adaptor. Each motor generates about 5 to 7 piconewtons of force, enough to drag a vesicle through crowded cytoplasm. Most cargoes carry both motor types at once and undergo a tug-of-war that sets their net direction. Defects in these motors cause Charcot-Marie-Tooth disease, lissencephaly, and primary ciliary dyskinesia.
- Step size8 nm (one tubulin dimer)
- Fuel1 ATP per 8 nm step
- Kinesin speed~800 nm/s (~100 steps/s)
- Stall force~5–7 pN per motor
- Kinesin directionPlus end (anterograde)
- Dynein directionMinus end (retrograde)
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What motor proteins are
Inside every one of your cells is a microscopic delivery network. Proteins built in the cell body have to reach the membrane, the synapse, the cell's far edge. Diffusion alone is hopeless over those distances — a vesicle would take years to wander down a meter-long motor-neuron axon by random jostling. So cells use motor proteins: nanometer-scale machines that physically walk along protein tracks, dragging cargo behind them and burning chemical fuel to do it. They are the only molecules in biology that convert the chemical energy of ATP directly into directed mechanical stepping, one stride at a time.
The two great microtubule motors are kinesin and cytoplasmic dynein. (A third family, myosin, does the same trick on actin filaments and powers muscle contraction — see the comparison below.) Kinesin and dynein both walk on microtubules: stiff, hollow, 25 nm-wide tubes built from alpha/beta-tubulin. Crucially, the microtubule is polarized — it has a chemically distinct "plus" end and "minus" end. Kinesin walks toward the plus end (outward, toward the cell periphery), and dynein walks toward the minus end (inward, toward the nucleus). The track sets the direction; the motor picks which way to go.
How the walk works, step by step
Conventional kinesin-1 is a dimer: two identical motor heads (each an ATPase, ~340 amino acids) joined to a long coiled-coil stalk that ends in a cargo-binding tail. The two heads behave like two feet. The mechanism is hand-over-hand, and it runs on a tightly gated chemical cycle:
- Both heads bound (or one bound), ATP arrives. The leading head is firmly attached to a beta-tubulin site. ATP binds to it.
- The neck linker docks. ATP binding triggers a small structural element called the neck linker (about 15 residues) to "zip" forward against the head. This is the power stroke — a conformational change, not a sliding motion.
- The rear head swings forward. Neck-linker docking throws the trailing head about 16 nm forward, so it overtakes the bound head and lands on the next binding site — 8 nm beyond it.
- The new front head binds; the old rear head hydrolyzes and releases. The roles swap. The previously front head hydrolyzes its ATP to ADP + Pi, releases phosphate, loses affinity, and detaches — becoming the new trailing head ready to swing.
The net result: the center of mass advances 8 nm per cycle, consuming one ATP per step. The two heads are coordinated ("gated") so that at least one stays bound almost the entire time — which is why kinesin is processive, taking ~100 consecutive steps (about 0.8 µm) before it lets go. Single-molecule fluorescence imaging by Yildiz and Selvin (2004) labeled one head and saw it take alternating 0 nm and 16 nm steps — the smoking gun for hand-over-hand over an inchworm crawl.
Cytoplasmic dynein is a completely different beast. It is enormous (~1.4 MDa) and belongs to the AAA+ ATPase family. Its motor domain is a ring of six AAA modules; ATP hydrolysis at one site (AAA1) drives a "linker" element to swing like a lever arm, while a long antiparallel coiled-coil stalk reaches out to a small microtubule-binding domain at its tip. Dynein on its own is barely processive; to walk long distances it needs the dynactin complex and a coiled-coil cargo adaptor such as BicD2 or Hook3 — the three together (dynein–dynactin–adaptor) form a stable, fast, minus-end-directed transport machine.
The track: microtubules and polarity
You can't understand the motors without the road. A microtubule is a hollow cylinder, usually 13 protofilaments wide, assembled from alpha/beta-tubulin heterodimers stacked head-to-tail. Each dimer is 8 nm long — which is exactly why kinesin's step is 8 nm: it binds equivalent beta-tubulin sites one dimer apart. The head-to-tail stacking makes the polymer asymmetric, giving the fast-growing plus end (beta-tubulin exposed) and the slow minus end (alpha-tubulin exposed).
In most cells the minus ends are anchored at the microtubule-organizing center (MTOC / centrosome) near the nucleus, with plus ends radiating outward. In an axon, microtubules are almost all oriented plus-end-out. So the polarity rule is simple and powerful: kinesin = outward/anterograde, dynein = inward/retrograde. Attach the right motor and your cargo goes the right way. The microtubule is also studded with a code of post-translational marks (acetylation, tyrosination, the "tubulin code") and decorated with microtubule-associated proteins like tau, which can act as roadblocks that motors must navigate around.
Kinesin vs dynein vs myosin
| Property | Kinesin-1 | Cytoplasmic dynein | Myosin II / V |
|---|---|---|---|
| Track | Microtubule | Microtubule | Actin filament |
| Direction | Plus end (anterograde) | Minus end (retrograde) | Plus (barbed) end |
| Step size | 8 nm | ~8–32 nm (variable) | 36 nm (myosin V) |
| ATP per step | 1 | 1–2 (variable coupling) | 1 |
| Speed | ~800 nm/s | ~1,000–1,500 nm/s (with dynactin) | ~350–500 nm/s (myosin V) |
| Stall force | ~5–7 pN | ~1–7 pN | ~2–3 pN (myosin V) |
| Size (dimer) | ~380 kDa | ~1.4 MDa | ~500 kDa |
| Motor family | P-loop (kinesin/G-protein fold) | AAA+ ATPase | P-loop (myosin/kinesin fold) |
| Needs cofactors to walk far? | No (autonomous) | Yes (dynactin + adaptor) | No (myosin V processive alone) |
| Classic job | Axonal anterograde transport | Retrograde transport, spindle, cilia | Muscle contraction, vesicle transport |
The numbers that matter
- Step size: 8 nm. Set by the 8 nm tubulin dimer repeat. Resolved by optical-trap interferometry (Svoboda, Schmidt, Schnapp & Block, 1993).
- Speed: ~0.8 µm/s for kinesin-1. At ~8 nm/step that's roughly 100 steps per second. Dynein–dynactin can hit ~1–1.5 µm/s.
- Force: ~5–7 pN stall force (kinesin). A piconewton is 10⁻¹² N. Measured directly with optical tweezers (Svoboda & Block, 1994).
- Energy: one ATP per step. ATP hydrolysis releases about 20 kBT (~80 zJ, ~50 kJ/mol under cellular conditions). Kinesin converts a large fraction of this into mechanical work, making it one of the most efficient motors known.
- Processivity: ~100 steps / ~0.8 µm per kinesin run. Cargoes pulled by teams of 3–8 motors travel far longer before falling off.
- Run distances in cells: micrometers to a full meter. A vesicle leaving a spinal motor-neuron cell body must travel up to ~1 m down the axon to a foot muscle — at ~1 µm/s "fast axonal transport" that's days; slow components creep at <10 mm/day.
- Track: 25 nm-wide microtubule, 13 protofilaments, dimers 8 nm long.
Where it shows up — neurons, mitosis, viruses, disease
- Axonal transport. Neurons are the extreme case: a single human motor-neuron axon can be a meter long. Kinesins carry fresh mitochondria, synaptic-vesicle precursors, and membrane outward (anterograde); dynein carries spent organelles, autophagosomes, and survival/injury signals back to the cell body (retrograde). Block this transport and the synapse starves.
- Cell division. Dynein and several kinesins build and operate the mitotic spindle — positioning the centrosomes, sliding microtubules, and pulling sister chromatids to opposite poles. Kinesin-5 (Eg5) cross-links and slides antiparallel microtubules; its inhibitor monastrol freezes cells in mitosis and is studied as an anticancer strategy.
- Cilia and flagella. A specialized axonemal dynein drives the beating of cilia and sperm tails by sliding adjacent microtubule doublets in the "9+2" axoneme. This is what sweeps mucus out of your airways and propels sperm.
- Viruses hijack the system. HIV, herpes simplex, adenovirus, and rabies all dock onto dynein to ride retrograde transport into the cell interior and reach the nucleus — turning the cell's own delivery trucks against it.
- Disease. KIF1B mutations cause Charcot-Marie-Tooth type 2A peripheral neuropathy. LIS1 and DYNC1H1 mutations cause lissencephaly ("smooth brain") and cortical malformations because newborn neurons can't migrate. Axonemal dynein-arm mutations (DNAI1, DNAH5) cause primary ciliary dyskinesia — chronic respiratory infections, situs inversus, and infertility. Failing axonal transport is implicated in ALS, Alzheimer's, and Huntington's disease.
The tug-of-war: how direction is decided
Here's the part textbooks often skip. Most cargoes — mitochondria, endosomes, lysosomes, mRNA granules — don't carry just one kind of motor. They carry both kinesin and dynein simultaneously, bound and ready. Watch a mitochondrion in a live axon and you'll see it lurch forward, stop, reverse, stop, reverse again. That's not indecision; it's a literal mechanical tug-of-war between plus-end and minus-end motor teams, with regulatory proteins, adaptors, and phosphorylation acting as the referee that tips the balance.
This bidirectional design is a feature, not a bug. It lets the cell redirect a cargo on the fly — recall a mitochondrion to a high-demand region, send a damaged organelle back for recycling, or pause delivery at a busy synapse — just by modulating which team currently wins, rather than by loading and unloading whole motors.
Common misconceptions
- "Motors slide along the track like a train on a rail." No — they step discretely. Each head detaches, swings, and re-binds a specific site 8 nm ahead. The motion is digital, not continuous, and was directly resolved as a staircase in optical-trap recordings.
- "ATP pushes the head forward like a piston." ATP doesn't shove anything. It binds and is hydrolyzed, and those chemical events bias conformational changes (neck-linker docking, lever-arm swing). The motor rectifies thermal motion — it's a Brownian ratchet biased by chemistry, not a steam engine.
- "Kinesin and dynein walk on different tracks." They walk on the same microtubules — just in opposite directions. Myosin is the one that uses a different track (actin).
- "Direction is set by the cargo." Direction is set by the motor + track polarity. The cargo is just luggage; swap the motor and the same cargo goes the other way.
- "Dynein walks fine by itself." Isolated cytoplasmic dynein is a poor, often diffusive walker. It needs the dynactin complex plus a cargo adaptor to become a fast, processive minus-end motor — a discovery that reshaped the field around 2014.
- "Motors are slow because cells are slow." ~800 nm/s is blistering at molecular scale — relative to its own ~10 nm body length, kinesin covers ~80 body-lengths per second, comparable to a cheetah scaled to molecular size, while navigating fluid that feels as thick as tar.
Frequently asked questions
How does kinesin actually walk hand-over-hand?
Conventional kinesin (kinesin-1) is a dimer with two identical motor heads joined by a coiled-coil stalk. Each head is an ATPase that binds the microtubule at a specific tubulin dimer. The cycle is coordinated so the two heads alternate: when the rear head hydrolyzes ATP and releases its phosphate, it detaches, the neck linker of the bound front head docks and swings forward, throwing the freed head 16 nm forward (past the still-bound head) to land on the next binding site 8 nm beyond it. The center of mass therefore advances 8 nm per step. Because the two heads are gated so that at least one is almost always bound, kinesin is processive — it takes about 100 consecutive steps (roughly 0.8 micrometers) before falling off. This 'hand-over-hand' mechanism, as opposed to an inchworm crawl, was confirmed by single-molecule fluorescence experiments (Yildiz and Selvin, 2004) that tracked one head taking alternating 0 nm and 16 nm steps.
What is the difference between kinesin and dynein?
Both walk on microtubules, but in opposite directions and with very different architectures. Kinesin-1 walks toward the microtubule plus end (anterograde transport — outward, toward the cell periphery and the axon terminal). Cytoplasmic dynein walks toward the minus end (retrograde transport — inward, toward the cell center and microtubule-organizing center). Kinesin is small (about 380 kDa as a dimer) and uses its compact motor heads and a docking neck linker; dynein is huge (about 1.4 MDa) and belongs to the AAA+ ATPase family, using a ring of six AAA domains, a long stalk that ends in a microtubule-binding domain, and a lever-arm-like linker. Kinesin works largely on its own; dynein needs the dynactin complex plus a cargo adaptor (such as BicD2 or Hook3) to become fully processive.
How much force can a single motor protein generate?
A single kinesin-1 motor produces a stall force of about 5 to 7 piconewtons — measured directly with optical tweezers (Svoboda and Block, 1994), which hold a cargo bead in a focused laser trap and measure how hard the motor pulls before it stops. Cytoplasmic dynein generates a comparable, somewhat more variable force of roughly 1 to 7 pN depending on load and ATP. These forces sound tiny — a piconewton is 10 to the minus 12 newtons — but at the scale of a 25 nm-wide microtubule and a vesicle a few hundred nanometers across, that is enough to drag cargo through crowded, viscous cytoplasm. Cargoes are usually pulled by teams of several motors, which raises the total force and the distance traveled before detachment.
Why is each step exactly 8 nanometers?
The microtubule track is built from alpha/beta-tubulin heterodimers stacked head-to-tail into protofilaments. Each tubulin dimer is 8 nm long, and kinesin's binding site is one dimer per step, so the motor lands on equivalent beta-tubulin sites spaced exactly 8 nm apart. The hand-over-hand mechanism throws the rear head 16 nm so it overtakes the bound head and lands on the next site, but the center of mass advances by 8 nm. This 8 nm quantization was first resolved with optical-trap interferometry in 1993 and is one of the most direct demonstrations that a protein takes discrete mechanical steps coupled to single ATP hydrolysis events.
Which diseases are caused by motor protein defects?
Because neurons have axons up to a meter long and depend entirely on motor-driven transport, they are hit hardest. Mutations in the kinesin gene KIF1B cause Charcot-Marie-Tooth disease type 2A, a peripheral neuropathy. Mutations in the dynein-regulator LIS1 (PAFAH1B1) and in cytoplasmic dynein itself (DYNC1H1) cause lissencephaly ('smooth brain') and other cortical malformations because neurons fail to migrate during development. Mutations in axonemal dynein arms (genes like DNAI1 and DNAH5) cause primary ciliary dyskinesia, in which cilia and sperm flagella can't beat — leading to chronic respiratory infections, situs inversus, and infertility. Impaired axonal transport is also implicated in ALS, Alzheimer's, and Huntington's disease.
How do motors know which direction to carry cargo?
Direction is set by the track and the motor, not by the cargo. Microtubules are polarized: the minus end is anchored at the microtubule-organizing center near the nucleus and the plus end points outward. Kinesin always walks toward the plus end and dynein always toward the minus end, so attaching a kinesin sends cargo outward and attaching a dynein brings it back. Most cargoes — mitochondria, endosomes, mRNA granules, virus particles — actually carry both kinds of motor at once and undergo a back-and-forth tug-of-war; regulatory proteins, adaptors, and phosphorylation tip the balance to set the net direction. This is why a single mitochondrion in an axon can repeatedly reverse course as it is handed between competing motor teams.