Cytoskeleton (Microtubules)

Question 1

Microtubules

Answer 1

alpha and beta heterodimers that form hollow, tube-like cylinders

Question 2

GTP and tubulin

Answer 2

~ a’ and B’ tubulin dimers bind GTP = form microtubules
~ B’-tubulin hydrolyses GTP–>GDP (after assembly) and exchanges GDP–>GTP (after disassembly)
~ GTP bound to a-tubulin is never hydrolysed

Question 3

Protofilament

Answer 3

long line of a’-b’ tubulin dimers

many makeup a microtubule

Question 4

Doublet MT

Answer 4

two rings

found in: cilia and flagella

Question 5

Triplet MT

Answer 5

three joined rings

found in: basal bodies and centrioles

Question 6

Microtubule organising centres (MTOCs)

Answer 6

~ nucleate and organise microtubules
~ most MT originate here
~ -ve ends of MT remain associated with the MTOC
~ found @ spindle poles, basal bodies, centrosome or stand alone ones

Question 7

Basal body

Answer 7

a cylindrical organelle, within the cytoplasm of flagellated and ciliated cells, contains microtubules and forms the base of a flagellum or cilium
~ identical in internal structure to a centriole

Question 8

Centrosome

Answer 8

pair of 9, triplet MT arranged at right angles to one another

Question 9

MT polarity

Answer 9

~ crown of a-tubulin @ -ve end
~ crown of B-tubulin @ +ve end
~ -ve ends associated with MTOCs

Question 10

MT motors

Answer 10

~ can read the polarity of MT

~ transports cargo towards +/-ve end

Question 11

Y-tubulin

Answer 11

~ MT are nucleated by Y-tubulin ring complexes –> located in the pericentriolar material of an MTOC
~ provides a binding site for aB-tubulin dimers
~ provides a template for nucleating MT formation

Question 12

Cc + MT

Answer 12

~ tubuli dimers assemble above the Cc
~ Cc is different at each end
\+ve = lower
-ve = higher
~ tread milling is similar to actin filaments

Question 13

Treadmilling + MT

Answer 13

dimer leaves at -ve end
dimer joins at +ve end
FUN FACT: MT grow more quickly from +ve end

Question 14

Dynamic instability

Answer 14

MT alternate between growing and shortening

~ explained by GTP cap model

Question 15

GTP cap model

Answer 15

B’-GTP’-tubulin adds to the end of the growing MT
~ MT will continue to grow as long as B’-GTP’-tubulin ‘cap’ is at end of molcule
~ eventually hydrolysed to B’-GDP’-tubulin, has no effect as long as inside the tubule
~ GDP exposed at end = MT shortens

Question 16

Microtubule associated proteins (MAPs)

Answer 16

~ bind to MT and regulate assembly
~ increase MT stability
~ crosslink MT into bundles
e.g. Tau and MAP2 (regulates spacing between MT)

Question 17

End binding MT proteins

Answer 17

+TIPs (e.g. EB1)
Kinesin 13
Stathmin

Question 18

EB1

Answer 18

~ transports other +TIPs which effect dis/assmebly

~ recognise specific +ve end structure

Question 19

Kinesin 13

Answer 19

~ helps disassembly
~associated with +ve end
~ requires ATP–>ADP to release dimer once bound

Question 20

Stathmin

Answer 20

~ binds to curved protofilaments and increases disassembly
~ protofilmants curve during disassembly/peel away from bundle
~ associates with +ve end