The Cell Cytoskeleton

Question 1

Why is a cell cytoskeleton used?

Answer 1

Maintains: Structure and position of organelles
Cell shape and asymmetry
Cell Polarity
For intracellular and whole cell movement

Question 2

What composes the cytoskeleton?

Answer 2

Actin microfilaments - lamellipodia, filopodia and stress fibres.
Microtubules from the MTOC
Intermediate filaments

Question 3

What are the two forms of actin?

Where do association and dissociation occur?

Answer 3

G-Actin and F-Actin

Association occurs at the + end of F-actin and dissociation occurs at the - end.

Question 4

What is the minimum number of G-actin molecules need for nucleation?

Answer 4

3

Question 5

What is critical concentration (Cc)?

And how much?

Answer 5

Cc is the minimum concentration needed for the G-actin monomer to begin to form the filament F-actin.
200uM for F-Actin formation.

Question 6

Why does the treadmilling effect in actin occur?

Answer 6

When G-Actin is added to the + end of F-actin, ATP hydrolysis occurs and decreases the critical concentration at the + end, meaning more association happens.
ATP hydrolysis powers treadmilling by affecting the Cc.

Question 7

Which two proteins can regulate the CAPPING process of actin dynamics and how do they work?

Answer 7

CapZ - Binds the + end with very high affinity. Caps newly formed + ends, can be inhibited by PIP signalling.
Tropomodulin - Binds the - end when filaments need to be stabilised, stops dissociation.

Question 8

What catalyses the NUCLEATION reaction in actin formation?

Answer 8

Formin - for growth and protection from capping proteins

Arp2/3 Complex - for actin branching

Question 9

How does Formin work?

Answer 9

FH2 subunit - brings two G-actin molecules together

FH1 subunit - binds profilin-Gactin-ATP complex and FH2 and adds it to the growing filament.

Question 10

What are the 4 functions of RhoGTPase in the cell?

Answer 10

1. Contraction - regulates smooth muscle contraction.
2. Phagocytosis - controls actin polymerisation to promote particle internalisation.
3. Secretion - Microtubule re-orientation for secretion.
4. Cell Division - Promote Cyclin D transcription for separation of daughter cells.

Question 11

How are microtubules formed?

Answer 11

Alpha or beta tubulin monomers add to the + end of the filament. 13 filaments assemble into a sheet. GTP hydrolysis regulates the structure.
Nucleation doesn’t occur because it’s not energetically favourable - - end anchored to the MTOC.

Question 12

Why are microtubules dynamically unstable?

Answer 12

GTP hydrolysis of beta tubulin monomers occurs inside the filament.
Catastrophe / rescue occurs when they get too long / too short.

Question 13

What binds to microtubules for organelle transport?

Answer 13

Kinesin motors (Anterograde, towards +end)
Dynein motors (Retrograde, towards -end)

Question 14

What are intermediate fibres used for and why can they not be used for organelle transport?

Answer 14

Used for cell structural integrity as they have greater tensile strength.
No polarity, so cannot be used for organelle transport.

Question 15

What 2 drugs can be administered to effect microtubule polymerisation?

Answer 15

TAXOL - stabilises microtubules and inhibits depolymerisation.
COLCHICINE - inhibits polymerisation

Question 16

Cofilin

Answer 16

Binds F-actin between Actin-ADP subunits to cause severing, leading to more + and - ends.

Question 17

Profilin

Answer 17

Binds G-actin to convert Actin-ADP to Actin-ATP.
Causes polymerisation at + end.

Also a capping protein, binding at the + end to stop polymerisation. Can be removed by PIP2 signalling.