slide set 13 Flashcards
microtubules can
rapidly reorganize in response to cellular cues
microtubules are….
- required for intracellular organization
- responsible for chromosome movement in mitosis
- involved in long range vesicle transport
- involved in directional persistence in cell motility
what do microtubules assemble from
tubulin dimers
alpha and beta tubulin
microtubule polarity
start with a very stable microtubule structure (axoneme) and introduce purified tubulin
dynamic instability of microtubule assembly/disassembly
microtubules switch between a state of assembly and a state of disassembly
dynamic instability diagram GTP GDP
regulation of microtubule assembly
a series of proteins regulate microtubule assembly in time and space
nucleators
help determine organization of the array
gamma-tubulin
present in microtubules in smaller concentrations than alpha and beta tubulins
involved in nucleation of microtubule growth in yeast to humans
microtubules are nucleated from a specific intracellular location (microtubule-organizing center - MTOC)
gamma-tubulin is most enriched in MTOC
nucleation sometimes depends on gamma tubulin ring complex Y-TuRC
within Y-TuRC, 2 accessory proteins bind directly to gamma-tubulin and several other proteins to create a spiral ring (this acts as a template for the microtubule)
microtubule nucleation by gamma-tubulin ring complex
A. 2 copies of gamma tubulin associate with a pair of accessory proteins to form the gamma-tubulin small complex (TuSC)
Y-TuRC nucleates minus end of microtubule
centrosome structure
complex with centrioles at center
centrioles are surrounded by proteins that hold the ring complex nucleation sites
centrioles = arrangement of microtubules into stable cylinders
centriole is major MTOC of animal cells
do microtubule arrays need a centrosome to organize themselves?
NO!
+TIPs protein
remains associated with growing plus ends and can link them to other structures such as membranes
EB1 binds microtubule plus ends
balance of regulators at MT plus end tips
various proteins control transition between microtubule growth and shrinkage
catastrophe factors (kinesin-13) bind to microtubule ends and pry them apart, promoting depolymerization
MAPs (ex: XMAP215) stabilizes the end of a growing microtubule
XMAP215 binds tubulin dimers and delivers them to the microtubule plus end (increases growth rate and suppresses catastrophes)
balance of regulators at MT plus end tips
balance of regulators regulate the length and organization of the microtubule array
how kinesins move
- rear head of kinesin detaches from tubulin binding site, passes the partner motor domain, and then rebinds to the next available tubulin binding site (hand over hand motion)
- can travel long distances
- rear or lagging head is tightly bound to microtubule and ATP
- front or leading head is loosely bound to the microtubule with ADP in its binding site
- forward displacement of rear motor domain is driven by the dissociation of ADP and binding ATP in the leading head
- binding of ATP to motor domain causes a small peptide (neck linker) to shift from rearward to frontward conformation
- this pulls rear head forward once it has detached from the microtubule with ADP bound (detachment requires ATP hydrolysis and Pi release)
- kinesin is now ready for next step (repeat the process)
2 ways to walk on a MT
kinesin walks, dynein uses inchworm
dynactin
dynein works in complex with dynactin and links to vesicles via Arp1 filaments
