VL 29 (Ralph Gräf)

Question 1

Cytoskleton

Answer 1

• Aktin (6 nm)
• Intermediate filaments (10 nm)
• Microtubules (24nm)

Question 2

Shared properties of actin and tubulin:

Answer 2

• self-organization from simple building blocks
• nucleotide cleavage
• polarity
• main elements of cellular dynamics (motility, transport)

Question 3

Polymerization of actin, tubulin filaments:

Answer 3

Nucleation:
→nuclei formation; needed time = incubation period
* low g-actin affinity
* nuclei formed → g-actin affinity rises → faster attachement

Treadmilling:
* (+): assembly; higher g-actin affinity
* (-): disassembly

ATP(Actin)/GTP(Tubulin) Caps:
* assembly rate > hydrolysis rate→cap building
* c(g-actin/tubulin) > CC

Question 4

Actin-binding proteins

Answer 4

• regulate nucleation, assembly, disassembly
• build or sever supramolecular structures (bundles/networks)
• determine stability, elasticity and rigidity of cells

actin-specific drugs:
* Phalloidins inhibit actin filament disassembly by locking adjacent actin SU together
* Cytochalasin binds barbed end → prevent actin filament (dis)assembly
* Latrunculin A binds g-actin → inhibit their polymerization; promotes filament disassembly
* Jasplakinolides stabilizes g-actin, enhancing filament nucleation + assembly

Question 5

Actin-binding proteins in polymerization of actin filaments:

Answer 5

Question 6

Orientation of microtubules in various cell types

Answer 6

• Epithelia: short MTs bound to centrosome; majority in periphery
• Neurons
  –> axon: (+)-ends towards synapwe
  –> dendrites: (+/-)-ends towards soma
• s. pombe: parallel MT orientation; (+)-ends towards periphery; nucleus with MT-association proteins
• Myotubes: random orientation; parallel
• Plants: parallel; periphery (plasmalemma)

Question 7

Micrtotubule dynamics

Answer 7

Lower GTP-tubulin dimer → shrinkage,
because hydrolysation rate catches up with assembly rate

Leads to dynamic instability

Question 8

Functions of MT-associated proteins:

Answer 8

Question 9

MT-associated proteins:

Answer 9

• regulate nucleation, (dis)assembly
• regulate contacts of MT ends
• mediate contacts with actin-cytoskeleton, cell cortex

Question 10

Centrosome and miotic spindle

Answer 10

Requirement of tubulin, actin systems
* MTs require for spindle formation
→ chromosome segregation
* Actin/myosin for cleavage furrow formation during cytokinesis (contractile ring)

Composition of animal centrosomes:
* centrosome
–> no membrane
–> > 100 proteins
* centriols
–> cylinders
–> 9 MT triplets
–> size: ~0.5x0.2 μm
–> in most cells, all cells with cilia/flagellae

Question 11

Centrioles and basal bodies of cilia and flagellae are
structurally identical

Answer 11

• flagellae: long cilia
• basal bodies: centriole

Question 12

Model of Plk4-induced procentriole formation:

Answer 12

• Cep192/152-binding → recruits kinase Plk4
• Plk4 phosphorylates GCP6 in γ-TuRC
• recruitment: SAS4-6, Cep135 for cartwheel
• recruitment: γ,δ,ε-tubulin, CP110 (MT capping protein)
• CP110-dissociation → cilia/flagella