User:Jenn545

Motor proteins travel in a specific direction along a microtubule. Microtubules are polar; meaning, the heads only binds to the microtubule in one orientation, while ATP binding gives each step its direction through a process known as neck linker zippering.

It has been previously known that kinesin move cargo towards the positive (+) end of a microtubule, also known as anterograde transport/orthrograde transport. However, it has been recently discovered that in budding yeast cells kinesin Cin8 (a member of the Kinesin-5 family) can move toward the negative end as well, or retrograde transport. This means, kinesin has the novel ability to switch directionality. It should be noted that kinesin, so far, has only been shown to move toward the negative end when in a group, with motors sliding in the antiparallel direction in an attempt to separate microtubules. This dual directionality has been observed in identical conditions where free Cin8 molecules move towards the minus end, but cross-linking Cin8 move toward the plus ends of each cross-linked microtubule. One specific study tested the speed at which Cin8 motors moved, their results yielded a range of about 25-55nm/s, in the direction of the spindle poles. On an individual basis it has been found that through the use of ionic conditions Cin8 motors can become as fast as 380nm/s, a notable jump. This tells us that Cin 8 can easily change directions on a microtubule, and in turn led to the plus end movement of kinesin on a microtubule. It is suggested that this unique ability is a result of coupling with other Cin8 motors and helps to fulfill the role of dynein in budding yeast. This discovery in kinesin-14 family proteins (such as Drosophila melanogaster NCD, budding yeast KAR3, and Arabidopsis thaliana ATK5) allows kinesin to walk in the opposite direction, toward microtubule minus end. This is not typical of kinesin, rather, an exception to the normal direction of movement.

Another type of motor protein, known as dyneins, move towards the minus end of the microtubule. Thus, they transport cargo from the periphery of the cell towards the center. An example of this would be transport occurring from the terminal boutons of a neuronal axon to the cell body (soma). This is known as retrograde transport.