lec 8- microtubules Flashcards
(30 cards)
what are microtubules?
-stiff polymer tubes of protein called tubulin
-25 nm in diameter
-up to 20 micro meters in length in cells
how was the microtubule discovered?
-by accident
-Kieth Porter used a different chemical (glutaraldehyde) to fix cultured cells prior to processing them for electron microscopy, but forgot to cool the fixative
-microtubules disassemble at 4 degrees so not cooling it let them not disassemble and allowed for them to be viewed
at what temp to microtubules assemble?
37 C
how are microtubules assembled?
-formed by a and B tubulin heterodimers that assemble head to tail into 13 filaments (in mammals)
-they assemble then seal into a tube
how many protofilaments in a microtubule in worms?
11-15
what is seen when following a row of subunits around the microtubule?
3 rows up at a seam that is off-set
are a-tubulin beside a-tubulin and B-tubulin beside B-tubulin?
yes, but at 1 part they mismatch
when is the seam in the microtubule generated?
when the microtubule seals into a tube
what gives the microtubules intrinsic polarity?
the a and B tubulin heterodimer head-to-tail arrangement
what does the intrinsic polarity of the microtubule allow for?
-one end will grow faster (+) than the other (-)
-a polar filament can be used for the directional movement of cargo by molecular motors
how is the polarity of a microtubule different in a flat cell and a epithelial cell?
flat: - end is towards center, + faces towards membrane
epithelial: - end is towards membrane, + is towards center
what do a and B tubulin associate with?
GTP
what happens when a and B tubulin form their heterodimers?
the a-tubulin-GTP is locked between alpha and beta monomers, the sandwiched GTP does not interact with GTP in the cytoplasm
what does GTP do for microtubules?
stablilizes them, but it is hydrolyzed to GDP rapidly, causing the to become unstable
what does the stable GTP rapidly changing to GDP cause in a microtubule?
it causes stable B-tubulin-GTP to be at the poles and unstable B-tubulin GDP within the core
what is dynamic instability?
when the conversion of GTP-tubulin to GDP-tubulin is too fast, causing high instability and catastrophy
who discovered dynamic instability?
Timothy Mitchison
if GTP is not capped on a microtubule, what happens?
catastrophy and disassembly of microtubules by peeling
how are microtubules assembled at the beginning?
-nucleation happens by short protofilaments that assemble head-to-tail between a/B tubulin dimers, but then the make small sheets through parallel interprotofilament interactions
-then lateral and longitudinal tension causes microtubule closure into tube
how does GTP hydrolysis cause microtubule disassembly?
-the structure changes
-GTP hydrolysis causes compaction at the longitudinal interface of the tubulin dimers
-it also causes a conformational change in the alpha-tubulin subunit
-this all puts strain on the microtubules lattice ultimately causing disassembly
does the age of the microtubule matter when it comes to disassembly?
yes, older microtubules experience more catastrophies which suggests more defects at the ends
how do microtubules know where to stop Depolymerizing?
not ll the GTP gets hydrolized, some remain in the microtubules lattice
why do protofilaments peel back and not just disintegrate during assembly?
-because the GTP-bound conformation of tubulin forces the protofilaments to peel and curve away from one another, then the energetics of the bending causes them to peel back
-the GTP cap locks the tubulin into a straight conformation
