microtubules

Question 1

what is the structure of a single microtubule

Answer 1

13 protofilaments forming a 25nm diameter cylindrical hollow tube

Question 2

structure of a tubulin dimer

Answer 2

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)

Question 3

where are microtubules nucleated from

Answer 3

the microtubule organising centre (MTOC)

Question 4

example of MTOCs

Answer 4

centrosomes, spindle poles during mitosis, basal body in flagella/cilia , in axons

Question 5

what is needed for nucleation of microtubules

Answer 5

gamma tubulin ring complex (y-TuRC) in the peritubular material nucleates assembly of microtubules

Question 6

structure of a centrosome in animal cell

Answer 6

a pair of centrioles (9 sets of triplet microtubules) surrounded by peritubular material

Question 7

why do microtubules have intrinsic polarity

Answer 7

all the alpha-beta tubulin dimers are orientated the same way

Question 8

what is the primary determinant of whether microtubules grow or shrink

Answer 8

GTP hydrolysis

Question 9

structure of a growing microtubule

Answer 9

GTP cap forming a blunt end with ADP tubulin behind it

Question 10

what happens when GTP cap is hydrolysed

Answer 10

GDP bound beta tubulin is exposed and formerly blunt ended microtubules will curl, a catastrophe will happen resulting in rapid disassembly

Question 11

why doesn’t a disassembling microtubule fully disassemble

Answer 11

there are islands of GTP-tubulin along the length of the disassembling microtubule. once these are encountered, disassembly pauses and rescue is provoked

Question 12

what is dynamic instability

Answer 12

rapid alterations between the growing and shrinking states of a microtubule by catastrophe and rescue events

Question 13

how are microtubules stabilised

Answer 13

side binding proteins stabilise MTs
Tau and MAP2 affect spacing between MTs
+TIPs capture the (+) end and link them to membranes

Question 14

how are microtubules destabilised

Answer 14

kinesin 13 binds and curves (+) end into GDP-beta tubulin conformation
stathmin also binds and curves protofilament ends
both enhance the frequency of catastrophes

Question 15

examples of post translational modifications to stabilise tubulin

Answer 15

acetylation of lysine residues on alpha tubulin

enzyme removal of c-terminal tyrosine residues on alpha tubulin

Question 16

the 2 classes of motor proteins in microtubules

Answer 16

kinesins and dyneins

Question 17

why are microtubules important in the context of transport

Answer 17

they transport materials down the axon, from cell body to the axon terminal, replacing the materials lost to exocytosis into synapses

Question 18

how does kinesin 1 work

Answer 18

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

Question 19

what does dynein do

Answer 19

dynein drives movement in retrograde direction towards (-) end of microtubule
it requires dynactin to link it to its cargo

Question 20

what causes the cargo towards (-) end

Answer 20

dynein power stroke when linker straightens as a result of Pi release

Question 21

what are the 3 types of microtubules growing from the spindle pole MTOC

Answer 21

kinetochore- attaching to chromosomes
polar-overlapping in the middle of the spindle
astral- point outwards towards the cell cortex