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November 30, 2008

Walking Proteins Move Cargo

Kinesin dimer / David S. Goodsell, Scripps Research Institute

One way to transport objects inside a cell is by the use of Kinesin motor proteins. Three things are required: a motor to move the cargo, a track along which to move it, and the cargo itself that needs to be delivered.

Motor Proteins

Proteins are essential components of cells that are involved in a number of processes. They are formed by linking amino acid molecules together in chains. The sequences of amino acids needed to build proteins are specified by genes as part of the genetic code.

Motor proteins are a special form of protein that do physical work. Remarkably, these proteins have the ability to move along surfaces, transport cargo that is attached to them, or produce force. They use the chemical Adenosine Triphosphate (ATP) as an energy source to power their movement. The motor protein myosin, for example, is involved in the contraction of muscle fibers in animals. Dynein is a motor protein that is found in flagella, the long tail-like structure that projects from certain types of cells like sperm to help the cell move.


Microtubules are one of the components of the cytoskeleton, the infrastructure that supports the cell. They are made by linking repeating units of the tubulin protein together. The resulting chain is then curled into a hollow cylindrical shape. They can grow or shrink to produce force, and also serve as conduits along which other cellular components can be transported.

Kinesin Cargo Transportation

Small molecules in the cell can move to where they are needed by the process of diffusion. However, larger molecules that are synthesized in the cell body are transported by motor proteins to their destinations. Kinesins are a type of motor protein that use microtubule tracks to walk along. 

Two intertwined chains with globular heads on one end form a Kinesin dimer. To move, the heads repeatedly attach and detach to the tubulin units of the microtubule track, moving everything forward in a hand-over-hand fashion. The opposite ends of the dimer drag the cargo along that they are attached to.

Thanks to Nested Universe reader Faris Naji for inspiring this topic, and discovering the attached video which shows the Kinesin protein in action.