Lecture 2 - More Actin

Question 1

Actin-binding proteins function through regulatory mechanisms that assemble/disassemble the actin filaments. Provide examples.

Answer 1

1. Inhibition of the spontaneous polymerisation of G –> F actin.
2. Nucleation of new actin filaments.
3. Control of length.
4. Elongation/shortening of pre-existing actin filaments.

Question 2

Provide two examples of actin-monomer binding proteins that regulate polymerisation.

Answer 2

Thymosin beta 4 and profilin.

Question 3

Thymosin Beta 4.

Answer 3

Sequesters G-actin, preventing polymerisation and nucleotide exchange.

Question 4

Profilin.

Answer 4

Binds to G-actin and increases the rate of nucleotide exchange. Only prevents polymerisation at the (-) end.

Question 5

Provide two examples of actin-monomer binding proteins that regulate nucleation.

Answer 5

Formins and the Arp 2/3 complex.

Question 6

Formins.

Answer 6

Bring 3 monomers together to promote the formation of the tri-actin nucleus.

Question 7

Arp 2/3 complex.

Answer 7

ARP = Actin Related Protein. 5 other subunits involved. As well as nucleation, also promotes the branching of MFs.

Question 8

Actin-filament binding proteins can be capping or severing. Name two examples.

Answer 8

Capping = gelsolin. 
Severing = cofilin.

Question 9

Function of gelsolin?

Answer 9

Binds to F-actin at the (+) end –> uncapped (-) end can disassemble. Has the ability to dissolve an actin filament.

Question 10

Function of cofilin?

Answer 10

Also called ADP-cofilin, which stands for Actin Depolarising Factor. Binds to ADP-actin, either free as G-actin or at the (-) end. If it binds to the (-) end then it can increase polymerisation. If it binds deeper into the filament = severing.

Question 11

Actin-filament binding proteins orchestrate higher order actin architectures. These are cross linking proteins. Name some of the structures and the formation the actin filaments take.

Answer 11

Stress fibres = antiparallel contractile structures that are orchestrated by alpha-actinin.
Cell cortex = branched network orchestrated by filamin.
Filopodium = tight parallel bundles orchestrated by fimbrin.

Question 12

Describe the structure of the protein fimbrin.

Answer 12

Monomeric protein with 2 actin binding domains. Each domain can bind to a filament, giving rise to the tight parallel bundles of filopodia.

Question 13

Describe the structure of the protein alpha actinin.

Answer 13

It is a dimeric protein; 1 actin binding domain. They form a head-to-tail structure which is responsible for the antiparallel nature of the stress fibres.

Question 14

Describe the structure of the protein filamin.

Answer 14

It is a dimeric protein; 1 actin binding domain. Form a ‘V’ shape, which is responsible for organising the filaments into branched structures.

Question 15

Name two specialised cross linkers.

Answer 15

Spectrin and dystrophin.

Question 16

Structure and function of spectrin?

Answer 16

Tetrameric; forms the meshwork that gives RBCs their shape.

Question 17

Structure and function of dystrophin?

Answer 17

Monomeric; part of a multi-protein complex that connects muscle fibre cytoskeleton to the e. matrix. Plays a key role in force generation in muscle.

Question 18

General structure of myosins?

Answer 18

They consist of 1 or 2 heavy chains in a complex with light chains. They have a globular head region that binds actin. ATPase activity generates force to walk.

Question 19

Which myosins interact with the cytoskeletal membrane?

Answer 19

I and V.

Question 20

Which myosin group is involved in muscle contraction?

Answer 20

II.