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Actin Flashcards

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1
Q

What are the main structures that involve actin in the cell?

A

Leading edge
stress fibres
filopodia

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2
Q

What drugs destroys actin cytoskeleton?

A

Latrunculin A

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3
Q

How many genes do humans have for actin?

A

Six

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4
Q

What are the six isoforms of actin in vertebrates?

A

4 isoforms in muscles cells

2 (beta and gamma) in non-muscle cells

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5
Q

What are the functions of 3 of the isoforms of actin?

A

alpha-actin is associated with contractile structures

beta-actin is enriched in the cell cortex and leading edge

gamma-actin accounts for filaments in stress fibres

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6
Q

What are the two forms of Actin?

A

G-actin: globular monomer

F-actin: linear chain of G-actins

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7
Q

What does each actin molecule contain?

A

Mg2+ ion complexed with either ATP or ADP inside the ATP-binding cleft

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8
Q

Describe structure of G-Actin

A

Divided by a central cleft and into two equal sized lobes and has 4 subdomains

Atp binds at the bottom of the cleft and contacts both lobes

N and C- terminus reside in subdomain I

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9
Q

What are the ends of actin filaments?

A

+ Barbed end

- Pointed end

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10
Q

Describe process of actin polymerisation

A

1) Nucleation: ATP-G monomers slowly form stable trimers(nucleus). This is a rate limiting step.
2) Elongation: Stable nuclei rapidly elongate by addition of subunits to both ends of the filament
3) Steady state : the filament ends are in a steady state with G-actin (treadmilling)

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11
Q

What is critical concentration ?

A

It is the concentration of G-actin monomers needed to achieve polymerisation of actin into filaments. Below this conc. no polymerisation will happen. After this conc. polymerisation will happen until a steady state is reached

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12
Q

Why is there a difference in actin end polymerisation?

A

Plus end polymerises faster because the critical concentration needed is much lower compared to that needed by the minus end

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13
Q

Actin tread milling is powered by …

A

ATP hydrolysis

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14
Q

What are the three regions of an actin filament?

A

The ATP-actin region, the ADP-Pi-actin region and the ADP-actin region

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15
Q

What happens during hydrolysis of ATP and release of Pi?

A

Actin undergoes a conformational change

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16
Q

Describe cofilin- profilin cycle

A
  1. Cofilin binds to the ADP-actin region of the filament. It binds by bridging two monomers, creating a twist that facilitates breaking of filament
  2. ADP-actin then dissociate singularly and cofilin is replaced by profilin.
  3. When profilings binds, it opens the cleft so ADP is released and ATP comes in , creating a profilin-ATP-G-actin
  4. profilin-ATP-G-actin
    cannot bind to minus end cause that part of the monomer is occupied by profilings, but it can binds to plus end
  5. profilin-ATP-G-actin binds to plus end and profilings dissociates
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17
Q

What is the role of Thymosin-Beta4

A

It binds to ATP-G-actin inhibiting its addition to the filament.
This is especially needed in platelets to facilitate a massive polymerisation event during clotting process

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18
Q

What are capping proteins?

A

Proteins that specifically bind to one end and stabilise it stopping it from shrinking or growing

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19
Q

What is a + capping protein?

A

CapZ. Contains two subunits that bind tightly

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20
Q

What is a - capping protein?

A

Tropomodulin. Abundant in cells that require stable actin filaments such as red blood cells and muscle.

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21
Q

What is gelsolin?

A

When it binds to ca2 it undergoes a conformational change that makes it bind to the side of the filament, breaking the helix.

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22
Q

What does Arp 2/3 stand for ?

A

Actin Related Proteins

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23
Q

What does Arp 2/3 activity lead to ?

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A

Branching filaments in structures like the leading edge of motile cells

24
Q

What is the angle of actin branches ?

25
Which proteins enhance Arp2/3 activity?
Nucleation Promoting Factors (NPF's)
26
What are the three steps of filament branching ?
1) Arp 2/3 complex binds to the site of an existing actin filament 2) An activating protein such as WASp binds and induces a conformational change that greatly increases the complexes ability to bind two actin monomers, forming a (+) end equivalent. 3) The new stable seed allows filament formation to follow.
27
What does cytoskeletal organisation respond to ?
To the environment by signal transaction pathways
28
What is Wasp?
The NPF that regulates Arp 2/3
29
What is Wasp controlled by ?
By a signalling molecule called cdc42 which target the Rho Binding Domain (RBD) on Wasp
30
What superfamily of proteins does cdc42 belong to ?
Ras GTPase
31
Describe the inactive state of WASp
RBD domain is hidden by a bending of the region of wasp that contains G-actin binding domain
32
What is the purpose of G-actin binding domain in Wasp?
To facilitate nucleation by bringing in the "third" subunit
33
What is one disease caused by defects in WASp?
Wiskott-Aldrich syndrome
34
What happens upon activation of Wasp by cdc42?
Wasp undergoes a conformational change that releases the C-terminal acidic activation domain. (A-C-W domains; The acidic A domain activates ARP23 and W domain binds and transfers G-actin to ARP23) Wasp also has a lipid binding domain which binds to the plasma membrane
35
Where is actin branching used ?
1. Listeria bacterium movement around and out of the host cells 2. Endocytosis 3. Phagocytosis
36
How does listeria hijack actin polymerisation machinery?
The cell surface protein of Licteria ActA is an ARP2/3 complex activating protein. so it leads to actin nucleation. Filaments grow at their + ends until capped by CapZ. Actin is recycled through the action of coffin which enhances depolymerisation of the minus end, confining polym. at the back of the bacterium thus propelling it forward
37
How does endocytosis implement actin polymerisation ?
Endocytosis assembly factors on the plasma membrane recruit NPFs(like wasp) to start actin polym. the actin filaments create invaginations which contain the cargo
38
What are the 4 steps of phagocytosis that involve actin production?
1) Opsonisation- Bacterium is coated by specific antibodies to one of its cell surface proteins 2) The leukocyte surface Fc receptor binds the Fc region of the antibody 3) Binding of Fc receptor and antibody activates Arp 2/3 complexes that create an actin network which moves the cell membrane around the opsonised bacterium. When the two flaps of membrane fuse they create a phagosome which is propelled n the cytoplasm 4) Fusion of lysosomes with phagosome degrades bacterium
39
What do forming do?
Assemble unbranched filaments
40
What is common to all forming?
Formin Homology Domains FH1 and FH2
41
What domains do formins contain?
Rho binding domani and FH1 and FH2
42
What is the purpose of FH1?
It is rich in proline which allows it to bind to profilin. FH1 increases the local concentration of ATP-G-actin bound to profilin
43
Where do formins add actin subunits?
To the + end thus preventing it from being capped and promoting filament growth
44
In what cell structure are forming fundamental in ?
Filopodia
45
Describe the structure of a filopodium
Different straight actin filaments produced by Formin push the plasma membrane and are held together in a bundle by proteins called fascins. A filopodia starts retracting when these actin filaments are capped. Filopodia sit on a network of already established filaments
46
What holds formins in an inactive state?
Association of RBD with FH2
47
What causes Formin activation?
Binding of RBD to the active form of Rho GTPase which is bound to the membrane. This exposes FH1 and FH2 leading to actin nucleation
48
What are the two types of cross-linking proteins?
Fimbrin is a single protein with 2 binding sites Alpha-actinin has one binding site but it dimerises Spectrin, which exists as a tetramer creating a big distance between filaments Filamin, which has a highly flexible cross-linker that behaves like a spring enabling them to make stabilising cross-links between filaments
49
Where are fimbrin found?
In bundles of actin filaments with same polarity such as the ones in microvilli, filopodia, focal adhesions
50
Why are alpha-actinic fibres different than the ones made by fimbrin?
the filaments are more spaced out and that gives them flexibility needed in Muscles
51
What does spectrin do?
Creates networks under plasma membrane by cross linking actin filaments
52
What is the network under red-blood cells plasma membrane made of?
14 subunits stabilised by the protein tropomyosin and by the captain tropomodulin
53
How are the short filaments in red blood cells held together?
Via spectrin proteins
54
How are short filaments in red blood cells anchored to the membrane?
1) Through ankyrin to a transmembrane protein called Band 3 | 2) Through the spectrin/F-actin binding protein called band 4.1 to another transmembrane protein called glycophorin c
55
How is sickle cell anemia related to cytoskeleton?
Lack of plasticity in the membrane cytoskeleton due to accumulation of oxidised actin

Cell Biology (3 decks)

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