Lecture 26 – Composition of Microtubules
• Tubulin, MAPs (Microtubule-Associated Proteins)
• Composed of microtubules
• ATP was required for activity
• Outside arm (dynein) of microtubule was found to be anATPase, arms move relative to each
other causing movement of cilium.
• To demonstrate this, microtubules were isolated, place on a slide, beads were placed on top of
the microtubules, and then ATP and Mg were added. This resulted in the bead moving along the
• The motor protein responsible for this movement was isolated by making a column ofATP-
gamma S, which bound to this motor protein, which was called kinesin, which was anATPase,
and was found to be responsible for this movement.
• Amicrotubule has two kinesin proteins attached to its end.
Kinesin (see film at http://www.youtube.com/watch?v=YAva4g3Pk6k )
• Kinesin is a dimer, basically consisting of two catalytic cores, two neck linkers, and a coiled-
• The two kinesin protein motor heads (composed of the catalytic cores and the neck linkers)
continue in step wise fashion over each other
1. Initially both motor heads are bound toADP. One catalytic core binds tighlty to the
active site on the microtubule, this binding causes it to release its ADP.
2. ATP quickly fills the nucleotide binding site left byADP.
3. This exchange causes the necklinker to “zipper” to the catalytic core.
4. The shortening of the neck linker slings the unbound motor head forward and close to
the next active site on the microtubule.
5. The attached head hydrolyzes ATP and releases the phosphate, which causes its
necklinker to “unzip.”
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6. As the neck linker of the first “slackens,” the first motor head also is releasing from the
actin, and the second motorhead is bind