BIOL 2021 Lecture Notes - Lecture 15: Microtubule Organizing Center, Tubulin, Intermediate Filament
13 May 2018
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• Fig 16-32C&D diagram of sarcomere
• Mysoin in non-muscle cells
• Panel 16-1 actin in stress fibers (a very important panel, you gotta know
everything on here!)
o Myosin II + actin > contractile bundles.
o Stress fibers provide mechanical support and contraction in migrating
cells. Nothing like the muscle, but the movement is like in muscles.
• Fig 16-40 myosin family
o Mano other myosins, many functions unknown.
• Microtubules
• Microtubule structure
• Panel 16-1 microtubules (part 1)
o All eukaryotes, conserved.
o 25nm thick.
o Rigid, hollow tubes.
o Attached to MTOC (microtubule organizing center).
• Panel 16-1 microtubules (part 2)
o Ex: cilia, Golgi localized on microtubules, ciliated protozoa (a single cell
animal, covered in cilia) part IV).
o Part 3: the microtubules are in green.
• Fig 16-42 microtubule structure
o Microtubule subunits - tubulin.
o Multiple genes in animals.
o 2 globular proteins = alpha and beta tubulin, the alpha and beta bind to
each other through non covalent binding.
o Each subunit binds GTP. alpha GTP never leaves, it’s trapped.
o Beta GTP hydrolyzes and exchanges.
o Protofilament = alternating alpha and beta subunits.
o Microtubule - 13 protofilaments in parallel polarity: alphae = minus end,
beta = plus end.
o Plus end grows and shrinks faster than minus end.
• Microtubule polymerization
o Panel 16-2 nucleotide hydrolysis.
o We have a GTP here which will add to the + end. Will hydrolyze to the
GDP and will fall off.
• Panel 16-2 dynamic instability (important in tubulins, vs treadmilling in actin)
o At intermediate concentrations of tubulin = dynamic instability.
o Rapid growth when subunits at end are GTP = “GTP cap”.
o Rapid shrinking from plus end when GTP is all hydrolyzed = catastrophe
(catastrophic collapse of microtubules, the subunits don’t fall off one by
one).
o Filament stays the same length even though they are dynamic (not sure
about this, do they stay the same length?).
o Many but not all microtubules grow from centrosomes.
• Video 16-8 microtubule dynamics
o Requires constant supply of energy.
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