Regulation of Actin Polymerisation

Question 1

What are the advantages of capping protein?

Answer 1

1. Limits length of growing branches so filaments are more effective
2. Means actin polymerisation can be favoured at the plasma membrane where Arp 2/3 is activated

Question 2

What family of proteins are nucleation promoting factors in?

Answer 2

WASp

Question 3

Give two examples of nucleation promoting factors?

Answer 3

WASP and WAVE

Question 4

How is WASP inhibited?

Answer 4

Intrinstically inactive due to autoinhibition by CRIB domain which binds to the VCA domain

Question 5

How is WASP activated?

Answer 5

By Cdc42 which binds to CRIB and releases the VCA domain to bind Arp2/3 complex

Question 6

How is WAVE activated?

Answer 6

Rac1 binds to Nap/Pir and releases them from WAVE which becomes activated and released so VCA domain can activate Arp2/3

Question 7

What do GEFs do?

Answer 7

Exchange GDP on inactive GTPase for GTP thus activating them

Question 8

What is the advantage of having more GEFs and GAPs than GTPases?

Answer 8

• Not all cells produce all GEFs and GAPs
• Allows for regulation to be fine tuned

Question 9

What is a GAP?

Answer 9

GTPase activating protien

Question 10

What is the effect of GAPs?

Answer 10

They help promote hydrolysis of GTP to GDP thus inactivating the GTPase

Question 11

The more actin stress fibres the more slow or fast cell movement?

Answer 11

Slow

Question 12

What is the MTOC?

Answer 12

Microtubule organising centre

Question 13

Where is the MTOC found?

Answer 13

Just ahead of the nucleus

Question 14

What is the role of Cdc42 in cell polarisation?

Answer 14

Migrating cell polarised where Cdc42 is activated and stabilises microtubules

Question 15

How does Cdc42 stabilise microtubules?

Answer 15

• Cdc42 is associated with IQCAP which binds to CLIP170
• CLIP170 is found at the edge of microtubules and stabilises them

Question 16

What does downstream signalling of Rho control?

Answer 16

Stress fibres

Question 17

How does mDia regulate actin polymerisation?

Answer 17

• Associates with growing ends of actin filaments and opens them up
• This allows the actin monomer to bind
• This bind, close, open is repeated

Question 18

What does mDia do?

Answer 18

Regulates actin polymerisation

Question 19

What happens to lamellipodia in WAVE knockout mice with constitutively active Rac1?

Answer 19

Lamellipodia are not made

Question 20

What does inhibition of Rac1 do?

Answer 20

• Prevents lamellipodium formation and inhibits cell migration

Question 21

Can cells still move in the abscence of Cdc42?

Answer 21

Yes

Question 22

What happens to cells with RhoA inhibited?

Answer 22

They exhibit long tails incapable of retraction and detachment

Question 23

What do focal adhesions contain?

Answer 23

Transmembrane Integrins

Question 24

What happens to focal adhesions during cell movement?

Answer 24

1. Focal complexes form at base of lamellipodia
2. Complexes remain stationary wrt substrate and new complexes form
3. Focal complexes move to rear or Focal complexes are turned over

Question 25

What is the role of transmembrane integrins in focal adhesions?

Answer 25

* Extracellular domains bind matrix * Intracellular domains link to actin cytoskeleton