The Microtubule Track
Microtubules are hollow cylinders made of 13 protofilaments, each a chain of
α/β-tubulin dimers. Kinesin walks along a single protofilament, stepping from
one β-tubulin to the next. The 8nm step size matches exactly one tubulin dimer.
Hand-Over-Hand Walking
Kinesin has two motor domain "heads" connected by flexible neck linkers to a
coiled-coil stalk. The heads alternate: one stays bound while the other swings
forward in an arc, landing 16nm ahead (net 8nm progress per head).
The ATP Cycle
1. ATP Binding
Neck linker docks, swinging rear head forward
2. Hydrolysis
ATP → ADP + Pi while new head searches for binding site
3. Pi Release
Front head binds, rear head releases from track
4. ADP Release
New binding allows ATP to enter, restarting cycle
Processivity
Kinesin is highly processive - one motor can take ~100 steps before detaching.
This is crucial for long-distance transport. The "gating" mechanism ensures
one head is always bound, preventing the cargo from diffusing away.
Cellular Functions
Vesicle Transport
Moves secretory vesicles, endosomes, and lysosomes
Mitosis
Moves chromosomes during cell division
Key Numbers
- • Step size: 8 nm (one tubulin dimer)
- • Speed: ~800 nm/s (100 steps/s)
- • Force: ~6 pN (can carry organelles)
- • Efficiency: ~50% (mechanical/chemical energy)
- • Run length: ~1 μm before detaching