19 microtubule growth depends on tubulin

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19
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Microtubule growth depends on tubulin concentration 20
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Treadmilling conditions C c (+ end) < actual concentration < C c (- end) Microtubules undergo treadmilling at steady state 21
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Microtubules undergo dynamic instability Catastrophe- spontaneous depolymerization; rescued by addition of fresh GTP-bound tubulin dimers Dynamic instabiliy- alternating growing and shrinking, not steady state treadmilling 22
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Dynamic instability occurs in cells 23
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GTP cap on (+) end holds MT together GTP in β -subunit is hydrolyzed after binding to a new α -subunit β -subunit at (+) end is GTP-bound until a new dimer binds GTP-bound tubulin is straight and stable GDP-bound tubulin is bent and unstable Energy from GTP hydrolysis is stored in MT When cap is lost, MT depolymerize catastrophe 24
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Microtubule assembly requires help Spontaneous nucleation is rare All microtubules in a cell originate from M icrot ubule O rganizing C enters (MTOC) MTOC binds minus end of MT Organizes entire microtubule network Microtubule associated proteins (MAP) enhance assembly by stabilization 25
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Centrosomes organize MT in resting and dividing cells Centrosome are in vivo MTOC nucleate assembly of MT, anchor and regulate (-) end dynamics made up of a pair of centrioles; align perpendicular to one another centrioles made up of 9 sets of triplet MT surrounded by pericentriolar material γ -tubulin major component of pericentriolar material; binds to α -tubulin 26
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Use of drugs affecting microtubule stability reveal in vivo dynamics Examples of depolymerizing agents Colchicine- promotes depolymerization by binding free α / β dimers Example of stabilizer Taxol- stabilizes MT Colchicine treated cell After colchicine washed out 27
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Microtubule associated motor proteins Anterograde Retrograde Anterograde- (+) directed Retrograde- (-) directed Move organelles Endocytosis and exocytosis Chromosome segregation 28
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Kinesins are (+) directed MT motors Dimer- 2 heavy and 2 light chains Require ATP hydrolysis for function 4 regions Head- binds MT and ATP; contains motor domain Linker- critical for forward motility Stalk- dimerization domain Tail- binds to receptor in cargo membrane Walk hand-over-hand along MT; bind to β -subunit 29
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Kinesins “walk” on microtubules 30
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Steps to Kinesin Walking Binding of head to beta-tubulin induces loss of ADP Nucleotide free state binds tightly to MT
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