Cytoskeleton & Actin

Question 1

What are the THREE filament systems in the Cytoskeleton?

Answer 1

1. Microfilaments (Actin)
2. Microtubules
3. Intermediate Filaments

Question 2

Which motor protein is comprised of Microfilaments (Actin)?

Answer 2

Myosin

Question 3

Which motor proteins are comprised of Microtubules?

Answer 3

• Kinesin
• Dynein

Question 4

Does Actin bind ATP or GTP?

Answer 4

ATP

Question 5

Does αβ-Tubulin bind ATP or GTP?

Answer 5

GTP

Question 6

What is the atomic mass of G-Actin (Actin Monomer)?

Answer 6

43kDa

Question 7

TRUE OR FALSE:
G-Actin is a highly conserved sequence

Answer 7

True

Question 8

How many Actin genes are present in humans?

Answer 8

6
* 4 muscle
* 2 non-muscle

Question 9

What is the structure of G-Actin?

Answer 9

• 4 subunits
• ATP binding cleft on one side

Question 10

What is the structure of Actin Filaments (F-Actin)?

Answer 10

2 strands of subunits twisted into a helical filament

Question 11

How many subunits do helical repeats have and what is its length?

Answer 11

• 28 subunits
• Spans 72nm

Question 12

TRUE OR FALSE:
ATP-binding clefts on F-Actin point in opposite directions?

Answer 12

False.
They all point in the SAME direction

Question 13

Which end of F-Actin is the ATP-binding cleft?

Answer 13

(-) end

Question 14

TRUE OR FALSE:
G-Actin can self assemble?

Answer 14

True.
Self assemble in the presence of MG++ and physiological ionic strength

Question 15

How many G-Actin molecules are needed to “seed” or nucleate actin polymerisation?

Answer 15

3

Question 16

Which end does Actin polymerisation occur at?

Answer 16

Both ends

Question 17

Which end of Actin is ATP-Actin addition fastest?

Answer 17

(+) end

Question 18

Why does ATP-Actin addition occur faster at the (+) end?

Answer 18

Critical concentration is LOWER for the (+) end

Question 19

What is Threadmilling in regards to Actin?

Answer 19

Net addition of actin at (+) end and loss at (-) end

Question 20

How is Threadmilling powered?

Answer 20

ATP Hydrolysis
* After addition to (+) end, ATP is hydrolysed to ADP + Pi

Question 21

Which end of the Actin filament has ATP-Actin and which end has ADP-Actin?

Answer 21

• (+) end has ATP-Actin
• (-) end has ADP-Actin

Question 22

What is the function of Monomer Binding Proteins in Actin Filaments?

Answer 22

Binds soluble actin and either enhances or decreases its availability to form filaments

Question 23

What is the function of Filament Stabilising or Destabilising Proteins in Actin Filaments?

Answer 23

Regulates filament stability

Question 24

What is the function of Capping Proteins in Actin Filaments?

Answer 24

Bind to the ends of actin filaments and block assembly / disassembly

Question 25

What are TWO examples of Capping Proteins in Actin Filaments?

Answer 25

1. **CapZ** binds (+) end 2. **Tropomodulin** binds (-) end

Question 26

What is the function of Actin Filament Nucleating Proteins in Actin Filaments?

Answer 26

Catalyse the formation of new filaments

Question 27

How many Myosin genes are there in the human genome?

Answer 27

40

Question 28

Which Myosin protein forms bipolar filaments?

Answer 28

Myosin II

Question 29

What is the function of Formins in Actin Filament assembly?

Answer 29

Assembly of **unbranched** filaments

Question 30

What is the function of Arp2/3 complexes in Actin Filament assembly?

Answer 30

Assembles **highly branched** filament networks

Question 31

How do Formins work?

Answer 31

* Formin dimer nucleates filament assembly by binding two actin monomers * Remains bound to the (+) end (preventing CapZ binding) even as futher monomers are added

Question 32

How many subunits does Actin Related Proteins 2 & 3 (Arp 2&3) have?

Answer 32

Seven subunits

Question 33

How does the Actin Related Proteins 2 & 3 (Arp 2&3) work?

Answer 33

* Arp 2/3 binds **to the sides** of existing actin filaments * **Upon activation**, Arp2/3 adopts a conformation that resembles the **(+) end** of an actin filament * Acts as a nucleus for filament assembly * (-) end remains attached to Arp 2/3 * (+) end grows at 70° angle

Question 34

Where does cell migration occur?

Answer 34

* Wound healing * Embryonic development * Cancer mestastasis

Question 35

What are the FOUR stages in cell migration?

Answer 35

1. Extension 2. Adhesion 3. Translocation 4. De-adhesion and Endocytic Recycling

Question 36

What are Lamellipodia?

Answer 36

Sheet like protrusions

Question 37

What are Filopodia?

Answer 37

Spiky / finger like protrusions

Question 38

What happens at the Leading Edge in cell migration?

Answer 38

Actin polymerisation mediated by Arp 2/3 that pushes the plasma membrane forward

Question 39

What THREE small GTPases play a key role in the regulation of the actin cytoskeleton?

Answer 39

* Rho * Rac * Cdc42