Lecture Set 15

Question 1

Describe the structure of microtubules

Answer 1

alpha-beta tubulin (globular protein) heterodimer, forms protofilaments that then form sheets that end up wrapping around (13 subunits) to create a microtubule
alpha tubulin has GTP, beta tubulin can have GTP or GDP

Question 2

What are the two types of general microtubules, what are the three classes of microtubules in mitotic spindles?

Answer 2

Axonemal = in flagella and cilia, stable, long-lived
Cytoplasmic = unstable, shortlived

1) Astral = project towards cell cortex, help orient spindle
2) kinetochore = connect to chromosomes via kinetochore
3) polar = interdigitate at + end, help to push spindle poles apart

Question 3

How does elongation work? Describe the difference between GTP and GDP sheets

Answer 3

GTP hydrolyzed to GDP in beta tubulin when dimer is attached, addition only occurs at + end
GTP cap helps stabilize microtubule, since GDP has a less strong bond and ends up being curved
GDP filament depolymerizes much quicker

Question 4

Define dynamic instability

Answer 4

Process of alternating between growth and shrinkage, sudden conversion between two phases
shrinkage is much more rapid than elongation

Question 5

How does tubulin concentration affect dynamic instability?

Answer 5

at high concentration, addition rate > GTP hydrolysis so GTP cap is added
at low concentration, GTP hydrolyzed, which then promotes disassembly

Question 6

Describe the structure of a microtubule-organizing centre (MTOC), give some examples

Answer 6

collection of microtubule-associated proteins, used to nucleate and organize MTs
centrosome = interphase cells (contains centrioles)
basal body = cilia/flagella
spindle poles = mitotic cells

Question 7

How do microtubules emanate from the centrosome

Answer 7

γTURCS (tubulin ring complex) provides nucleation site on centrosome for γtubulin to attach to
- end attached to MTOC

Question 8

what is a centriole

Answer 8

short cylinder of modified MT + accessory proteins

Question 9

How are microtubules regulated by binding proteins?

Answer 9

1) MAPs (microtubule associated proteins)
increase stability
2) + TIP proteins (+ end tubulin interacting proteins) stabilize
3) Microtubule destabilizing proteins = kinesin-13 (ATPase) or Stathmin binds to curved protofilaments and enhances dissociation (inactivated through phosphorylation)
4) Taxol = prevents change from GTP to GDP

Question 10

How do microtubule motor proteins work?

Answer 10

Kinesins = walking, move towards + end
dyneins = associate with dynactin complex, move towards - end through nucleotide hydrolysis and conformational changes
high cAMP leads to increased kinesin activity