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A motor protein like kinesin-1 cannot use an IF as a track because it lacks polarity.

Can someone elaborate on this answer?

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Just a guess: the IF doesn't have directionality. It would be like a single-lane, but two-way road. – MCM May 29 '14 at 20:17

Polarity essentially means a plus end (growing/polymerising) and a minus end (depolymerising). Look at microtubule (MT) ( So molecules such as Kinesin-1 ( use the hetero-dimeric nature of MT (alpha and beta subunits) and its polarity to orient themselves and move directionally along MT in an energy dependent manner.

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