A molecular catwalk - MEDICINE

Every day, your cells have to move many molecules from one site to another. Have you wondered how they do this?

Your cells use the help of two versatile motor proteins, called kinesin and dynein. These motor proteins "walk" along a molecular catwalk, which is made of proteins called "microtubules". Using this catwalk, the motor proteins can carry loads several times their size from one site to another.
Kinesins moving along microtubules usually carry cargo such as organelles and vesicles from the center of a cell to its periphery.

How do kinesin and dynein work?
Both motor proteins have two 'hands' and two 'feet'. The hands and feet are 'sticky' - as sticky as post-it notes. Precious cargo, like proteins, carbohydrates and enzymes, which need to be transported around the cell, are first enclosed in membrane 'sacks'. Using their hands, the motor proteins grab these large bundles as they float around the cell.  The sticky patches on the 'hands' prevent the bundle from falling off as the motor proteins set off on their journey.

Now comes the amazing part.  Just as a human can walk by placing one foot in front of the other, the motor proteins start 'walking' along the microtubules by swinging one foot in front of the other, much like a trapeze artist walking on a high wire. Some scientists say that the motor proteins "walk like a drunken sailor"! ;)

Each step uses a molecule of energy. The motor proteins need 125,000 steps to move 1mm along the catwalk - that is a lot of energy!

The astonishing thing is that the feet never walk backwards, only forwards! Once it reaches the other side, it drops its cargo off, jumps off the microtubule, swims back to the other end, and the whole process repeats again.

Journal article:http://science.sciencemag.org/content/279/5350/519.full
https://www.ncbi.nlm.nih.gov/books/NBK21710/
http://www.nature.com/nrm/journal/v10/n10/full/nrm2774.html

References:http://oregonstate.edu/instruction/bi314/summer08/cytotwo.html

Source:http://www.planet-science.com/categories/over-11s/human-body/2012/01/a-molecular-catwalk.aspx

Animation:
Kinesin protein walking on microtubule extracted from The Inner Life of a Cell by Cellular Visions and Harvard.
Corina Marinescu