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Unformatted text preview: motor proteins is ciliary dynein which looks and functions much like the cytoplasmic dynein we have already seen. The subunit of microtubules is a tubulin heterodimer called -tubulin . Each or monomer has a binding site for one molecule of the nucleotide GTP. The GTP that binds to an -tubulin monomer is trapped at the dimer interface and thus does not thereafter exchange nor is it hydrolyzed to GDP. The GTP that binds to a -monomer is exposed and thus can be hydrolyzed or exchanged and thus can be either GDP or GTP. The significance of this is that a dimer with two molecules of GTP is more stable than a dimer with only one GTP. The most recently assembled portion of the microtubule with the greatest proportion of GTP is the most stable and thus will grow more readily than the older part. This is what makes the end furthest from the MTOC the plus end....
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- Winter '07