BIO230H1 Lecture Notes - Lecture 2: Tubulin, Centrosome, Microtubule
Lecture Two: Cytoskeleton Networks
Microtubules:
• Inherently polarized – have different ends
• Each protofilament is made of heterodimers of
the monomeric proteins (alpha and beta tubulin)
• Tubulin monomers bind and hydrolyze with
GTP (shown in red)
• Each heterodimer is asymmetric (bound to 2 GTP)
• The heterodimers assemble head to tail forming
polarized filaments
Protofilament:
• Minus end – end that has alpha tubulin
• Plus end – end that has beta tubulin
• 13 parallel protofilaments will form a
microtubule
• After the heterodimers have been in the protofilament
for a while they will cut GTP to GDP (T form to D form)
o IF there are GDP bound heterodimers at the ends they will dissociate
y-Tubulin Complexes:
• They nucleate microtubules – bind to the
minus end (prevent dissociation)
• Y-tubulin binds tubulin heterodimers
assembling protofilaments into tubes
• Y-tubulin nucleates microtubules
microtubules at their minus end
• Plus end grow away from nucleation
sites
• y-tubulin often associates with large
microtubule organizing centers (MTOCs)
o ex. The centrosome – contain two centrioles (at right angels) surrounded by
hundreds of proteins with y-tubulin nucleation sites on the surface
Microtubule Movement by Dynamic Instability:
• Single microtubules switch between growing and shrinking
o Known as dynamic instability
▪ The minus end is usually anchored, so the plus end grows/shrinks
find more resources at oneclass.com
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• Growing microtubules have a protective
cap of GTP-bound tubulin
• If GTP hydrolysis is faster than subunit
addition the cap is lost and catastrophe occurs
o Depolymerization is ~100x faster at an
exposed GDP end
• Regaining a GTP cap rescues growth
Microtubule Networks Can Form a Coordinate System on their Own:
• A purified centrosome was mixed with purified tubulin
subunits and GTP in an artificial membrane-bound container
o On the centrosome you will begin forming microtubules
• It moves to the center of the container as microtubule plus
ends push on the outer membrane
o Minus ends will centralize and the plus ends will grow
outwards
This may contribute to microtubule organization in cells
(but many regulatory proteins are also involved)
centrosomes are typically near the nucleus, which is located in the center of the cell
(not always but usually)
Motors Move Cargo Through the Microtubule Networks:
• Motor activity is polarized
o Dynein moves to microtubule minus end
o Kinesin moves to microtubule plus end
• Anchoring proteins are involved to anchor the
vesicle to dynein
Square = GTP bound - Circle = GDP bound
find more resources at oneclass.com
find more resources at oneclass.com
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BIO230H1 Full Course Notes
Verified Note
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Document Summary
Inherently polarized have different ends: each protofilament is made of heterodimers of the monomeric proteins (alpha and beta tubulin, tubulin monomers bind and hydrolyze with. Gtp (shown in red: each heterodimer is asymmetric (bound to 2 gtp, the heterodimers assemble head to tail forming polarized filaments. The centrosome contain two centrioles (at right angels) surrounded by hundreds of proteins with y-tubulin nucleation sites on the surface. Microtubule movement by dynamic instability: single microtubules switch between growing and shrinking, known as dynamic instability, the minus end is usually anchored, so the plus end grows/shrinks, growing microtubules have a protective cap of gtp-bound tubulin. If gtp hydrolysis is faster than subunit addition the cap is lost and catastrophe occurs: depolymerization is ~100x faster at an exposed gdp end, regaining a gtp cap rescues growth. Microtubule networks can form a coordinate system on their own:
