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I know that kinesin motor proteins move towards the positive (beta) end of the microtubule, while dyenin motor proteins move towards the negative (alpha) end of the microtubule. However, because the microtubule is composed of repeating alpha and beta units in ABABABABAB fashion (as shown in the picture below), what causes motor proteins to move to the positive rather than the negative pole?

For example, if one "leg" of the kinesin is bound to a beta tubulin while the other leg is up and ready to bind again, what determines which alpha tubulin it binds to (since there is alpha in front and in back of the beta tubilin it is bound to)?

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While you're entirely correct that the ABABAB sequence is symmetric to a motor protein cruising along, the individual $\alpha$ and $\beta$ monomers themselves are not symmetric along the axis of movement (or any axis at all, as far as I can tell). This produces a polarity that can be seen in the figure below, from the original 1998 paper describing it.

X-ray crystallography from the 1998 nature article

The figure you use above to display the microtubule is slightly misleading in this way; it shows the subunits as blobby spheres. However, the motor proteins are able to distinguish the top of these molecules from the bottom, seeking a unique set of loops and helices to attach to. As it walks, then, the motor protein can only bind to one side (pole) of the subunit and is unable to walk backwards.

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