Kinesin

Focus: Kinesin family of motor proteins; comparison to dynein (described metaphorically as the “mean little fellow” that creates problems for hardworking kinesin “John”).
Role: Transport cargo (e.g., vesicles) along microtubules inside the cell.

Three-part organization:
- Globular head (motor domain)
- Attaches directly to microtubules.
- Contains the catalytic site for ATP binding and hydrolysis.
- Coiled-coil helix (stalk/neck linker)
- Long, rod-like domain.
- Connects the two motor heads to the tail region.
- Provides flexibility and spacing so heads can “step.”
- Light-chain region (tail)
- Binds cargo (e.g., vesicles, protein complexes, organelles).
- Physically touches and secures whatever is being transported.

Forward motion described as a “hand-over-hand” walk:
- Leading head binds a new β-tubulin subunit.
- Trailing head then detaches, swings forward, and binds the next β-tubulin.
Critical rule: Heads bind β-tubulin only; they never interact with α-tubulin.
Chemical energy conversion: ATP \rightarrow ADP + P_i + \text{energy}
- Hydrolysis in the bound head triggers conformational change that swings the trailing head forward.

Kinesins are a family of proteins—there isn’t just one universal kinesin.
Each kinesin has: globular head (motor), coiled-coil stalk (linker), light-chain tail (cargo attachment).
Motion is strictly β-tubulin to β-tubulin, ensuring directional, coordinated steps along the microtubule lattice.
ATP hydrolysis is inseparable from movement; without ATP, kinesin heads cannot detach/reattach properly.

Understanding kinesin dynamics is crucial for:
- Intracellular transport studies (neurotransmitter vesicles, organelle positioning, mitotic spindle assembly).
- Drug targeting (cancer therapeutics aimed at mitotic kinesins).
Contrast with dynein:
- Dynein moves toward the microtubule’s minus end (opposite polarity) and often competes or cooperates with kinesin.
- Transcript humorously frames dynein as “mean,” causing obstacles for kinesin’s forward, plus-end-directed march.