microtubules Flashcards
what is the structure of a single microtubule
13 protofilaments forming a 25nm diameter cylindrical hollow tube
structure of a tubulin dimer
alpha tubulin and beta tubulin dimer
non exchangeable GTP bound to alpha tubulin
exchangeable GDP bound to beta tubulin (can be exchanged with free GTP)
where are microtubules nucleated from
the microtubule organising centre (MTOC)
example of MTOCs
centrosomes, spindle poles during mitosis, basal body in flagella/cilia , in axons
what is needed for nucleation of microtubules
gamma tubulin ring complex (y-TuRC) in the peritubular material nucleates assembly of microtubules
structure of a centrosome in animal cell
a pair of centrioles (9 sets of triplet microtubules) surrounded by peritubular material
why do microtubules have intrinsic polarity
all the alpha-beta tubulin dimers are orientated the same way
what is the primary determinant of whether microtubules grow or shrink
GTP hydrolysis
structure of a growing microtubule
GTP cap forming a blunt end with ADP tubulin behind it
what happens when GTP cap is hydrolysed
GDP bound beta tubulin is exposed and formerly blunt ended microtubules will curl, a catastrophe will happen resulting in rapid disassembly
why doesn’t a disassembling microtubule fully disassemble
there are islands of GTP-tubulin along the length of the disassembling microtubule. once these are encountered, disassembly pauses and rescue is provoked
what is dynamic instability
rapid alterations between the growing and shrinking states of a microtubule by catastrophe and rescue events
how are microtubules stabilised
side binding proteins stabilise MTs
Tau and MAP2 affect spacing between MTs
+TIPs capture the (+) end and link them to membranes
how are microtubules destabilised
kinesin 13 binds and curves (+) end into GDP-beta tubulin conformation
stathmin also binds and curves protofilament ends
both enhance the frequency of catastrophes
examples of post translational modifications to stabilise tubulin
acetylation of lysine residues on alpha tubulin
enzyme removal of c-terminal tyrosine residues on alpha tubulin
the 2 classes of motor proteins in microtubules
kinesins and dyneins
why are microtubules important in the context of transport
they transport materials down the axon, from cell body to the axon terminal, replacing the materials lost to exocytosis into synapses
how does kinesin 1 work
kinesin 1 drives movement of vesicles to (+) end of microtubule
it has 2 head domains, linker domain and binds a vesicle with stalk to tail domains
ATP binds to head domains and ATP hydrolysis makes them walk down the microtubule
what does dynein do
dynein drives movement in retrograde direction towards (-) end of microtubule
it requires dynactin to link it to its cargo
what causes the cargo towards (-) end
dynein power stroke when linker straightens as a result of Pi release
what are the 3 types of microtubules growing from the spindle pole MTOC
kinetochore- attaching to chromosomes
polar-overlapping in the middle of the spindle
astral- point outwards towards the cell cortex
