motor proteins (myosin, kinesin, dynein)
/ MOH-ter PROH-teen; MY-oh-sin, ky-NEE-sin, DY-neen /
Imagine a tiny porter that walks along a track on two legs, taking one step at a time and dragging a heavy box behind it. Motor proteins are exactly that, only at molecular scale. They are proteins that convert chemical fuel into movement, literally walking step by step along cytoskeletal filaments while towing cargo. They are how a cell delivers materials over distances far too large for random drifting to handle.
There are three main families. Myosins mostly walk along actin microfilaments and produce, among other things, muscle contraction and the pinching of a dividing cell. Kinesins walk along microtubules, generally toward the plus end (often outward, toward the cell edge), carrying vesicles and organelles. Dyneins also walk along microtubules but in the opposite direction, toward the minus end (often inward, toward the cell center), and a specialized dynein bends cilia and flagella. Each step is powered by the energy released when the motor splits the fuel molecule ATP, which changes the protein's shape and swings one leg forward.
Motor proteins matter because diffusion alone is hopelessly slow inside a large cell — a vesicle drifting at random might never reach the far end of a nerve cell. By walking purposefully along filament tracks, motors deliver cargo to the right place quickly and reliably. When motors fail, cargo piles up; several nerve diseases involve broken motor transport along the long axons of neurons.
When a chameleon changes color, kinesin and dynein motors slide pigment granules outward or inward along microtubules to darken or lighten its skin.
Opposing motors on the same track let a cell move cargo in either direction.
A motor's direction depends on the motor type, not the track: kinesin and dynein both ride microtubules but walk opposite ways. Motors do not pull track toward themselves; they walk along a fixed track.