Actin based cell movement

Question 1

How does actin polymerisation driven motility?

Answer 1

Listeria hijacks cell actin machinery to propel bacterium through the cell.
Bacteria expresses protein on its surface that mimics an actin binding protein.
This causes the cells normal machinery to think this is where actin needs to be polymerised.
It recruits Nucleation protein, starts formation of actin nucleus, leads to polymerisation, generates actin filaments, drives it forwards.

Question 2

What are capping proteins for?

Answer 2

Turn off polymerisation.
Tails stay the same length as it moves along, shows it has been turned off.
There is a finite amount of actin, so actin needs recycling.

Question 3

What are the minimum requirements for actin-based motility?

Answer 3

Nucleation of new actin filaments.
Capping of older filaments.
Recycling of monomers from old filaments - depolymerising and re-polymerising.

Question 4

What is nucleation of new filaments?

Answer 4

Formation of dimers and trimers is energetically unfavourable.
Once formed, the trimer rapidly elongates into filaments.
Several protein components nucleate from the fast-growing end of the actin filaments, overcomes unfavourable reaction.

Question 5

What is the Arp 2/3 complex?

Answer 5

Nucleation proteins:
7 protein complex containing 2 actin related proteins.
Look like actin structurally but are not.
Can form a trimer to allow polymerisation of new filaments.

Question 6

What is the significance of Arp 2/3 proteins?

Answer 6

Only 1 more actin is needed to form the trimer, instead of 3.
This overcomes the initial unfavourable reaciton.

Question 7

How does the Arp 2/3 complex nucleate new filaments?

Answer 7

Arp 2/3 complex sits alongside existing actin filament.
Binding to protein complexes gets a nucleus of actin.
VCA domain on proteins bind to the Arp 2/3 complex, brings in actin monomer, and accelerates formation of actin filament.

Question 8

What are conventional capping proteins?

Answer 8

A dimer of alpha and beta subunits.
Found in muscle and non-muscle.
Caps the end of an actin filament to block access to it.

Question 9

What are Gelsolin capping proteins?

Answer 9

An actin binding protein that severs actin filaments and binds to the plus ends, which protects it.

Question 10

What are gCap39 capping proteins?

Answer 10

Non-muscle capping protien.
Acts similiarly to Gelsolin but does not sever actin.

Question 11

What is the function of capping proteins?

Answer 11

Allows the cell to switch on and off polymerisation.
Restricts polymerisation to the new filament barbed ends.
Prevents disassembly.

Question 12

How do capping proteins work in platelets?

Answer 12

Platelets have actin filaments, respond to damage when cut.
When unactivated, filaments are protected by capping proteins.
When we are cut, want quick activation, capping protein can be quickly removed, and allows polymerisation.

Question 13

How are capping proteins restrictors?

Answer 13

Can rapidly uncap filaments at the side to ensure movement of the protein in the right direction.

Question 14

Why are monomers recycled?

Answer 14

There is a fixed amount of actin, more protein can be made, but in a short period of time, need to use existing monomers effectively.
Filaments at one end of the cell can be recycled and moved to another end.

Question 15

What proteins are responsible for recycling?

Answer 15

Actin depolymerising factor (ADF) / Cofilin
Profilin

Question 16

What does ADF do?

Answer 16

Recycles actin monomers:
Binds to actin monomers that are in the ADP form, which destabilises actin by twisting or bending it.
Appears to be required for actin assembly.

Question 17

How does profilin recycle monomers?

Answer 17

Profilin binds to actin monomers in the ADP or ATP form.
When it binds to ADP actin, promotes exchange of ADP for ATP.
Profilin ATP actin can add to the barbed ends of filaments.

Question 18

How do ADF and profilin work together?

Answer 18

Cofilin promotes disassembly of ADP actin monomers.
Profilin competes with cofilin for binding to ADP actin monomers.
Profilin promotes exchange of ADP for ATP in cytoplasm.
ATP-actin and profilin assembles on filament.

Question 19

What are key actin structures?

Answer 19

Filopodia - key for migration process
Lamellipodia - leading edge of the cell.
Stress fibres - generate force and contractile movement.

Question 20

What is filopodia?

Answer 20

Finger-like protrusions from the cell membrane.
Contain bundles of 10-30 actin filaments, the barbed end points towards the tip.
Used in sensing the environment e.g. neuron pathfinding (sensing direction of chemo attractant).

Question 21

What are lamellipodia?

Answer 21

Thin sheet like protrusions of membrane at the front of the cell.
Committed movement of the cell.
Contains a dense network of branched actin filaments, barbed end pointed towards tip.
Actin polymerisation used to push leading edge of cell forwards.

Question 22

How do filopodia and lamellipodia work together?

Answer 22

Both formed by actin polymerisation.
Formed by nucleation of new filaments and prevention of capping.
Often appear together to move the cell forwards.

Question 23

What are stress fibres?

Answer 23

Highly ordered structures containing cross-linked actin filaments and myosin.
Anchored at focal adhesions.
Used to generate tension within the cell.

Question 24

What are the types of stress fibres?

Answer 24

Named for where they appear:
Dorsal and ventral stress fibres, these are the main generators of cellular force.
Transverse arcs move actin towards the centre of the cell.

Question 25

How does cell movement occur?

Answer 25

Filopodia senses the environment to know where to go.
Lamellipodia fills in actin to push cell forward by actin polymerisation.
Forms new adhesions to move cell forwards. Stress fibres form between them.
Generation of force generates tension in cell, translocates contents of cell towards front.
Back of cell is detached by proteases, so rest of cell can move forwards.