Kinesin

Kinesin is a family of motor proteins that carry cargo along microtubules, mostly toward the plus end, which usually points to the cell periphery. The first kinesin was discovered in 1985 by Ronald Vale and colleagues in the giant axon of the squid.

Kinesin-1 has two motor "heads" joined by a coiled stalk, and it moves by a hand-over-hand step in which each head takes turns to move forward, covering 8 nanometers, the spacing of the tubulin units along a microtubule. Each step uses one molecule of ATP, and the motor moves at up to about 800 nanometers per second. It can take hundreds of steps before letting go.

Kinesins transport vesicles, mitochondria, mRNA and proteins, such as the movement of cargo down the long axons of neurons. Other members of the family, such as kinesin-5, push the two halves of the spindle apart during mitosis, and some kinesins help to regulate the length of microtubules. Humans have around 45 kinesin genes.

Dynein carries cargo in the opposite direction, toward the minus end, and the two motors often share the same cargo. Mutations in kinesin genes are linked to hereditary spastic paraplegia, to some neuropathies and to defects in cilia. Kinesin-5 inhibitors have been tested as anticancer drugs, and the motor is a standard subject for biophysics because single molecules can be watched.

• Dynein — the opposing motor
• Microtubule — its track
• Cytoskeleton — the network it works in