actin

Question 1

how many isoforms of actin are there? what are they?

Answer 1

• two muscle specific a-actin isoforms
• B-actin (cell cortex, leading edge)
• gamma actin (stress fibres)

Question 2

what is G-actin?

Answer 2

the monomeric form of actin, globular. has an ATP binding site

Question 3

what is F-actin?

Answer 3

filamentous actin. two parallel helical strands of polymerised G actin monomers

Question 4

what gives actin its polarity?

Answer 4

the ATP binding site, which is orientated towards the minus end

Question 5

which end are monomers added to the actin filament?

Answer 5

the plus end

Question 6

how does ATP hydrolysis regulate actin filament dynamics?

Answer 6

• monomers must be bound to ATP to bind at plus end
• after binding, the ATP is hydrolysed slowly
• the actin molecule undergoes a conformational change
• the minus end contains a greater proportion of ADP, which is unstable
• causing monomers to dissociate at this end

Question 7

how does the process of ‘tread milling’ differ from microtubule dynamic instability?

Answer 7

• neither end of an actin filament contains a cap

- there is a constant turnover of subunits

Question 8

what is capZ?

Answer 8

a capping protein that binds to and blocks the plus end. this prevents addition of subunits

Question 9

what is tropomodulin?

Answer 9

a capping protein that binds to and blocks the plus end, stabilising muscle actin filaments

Question 10

why is tropomodulin highly expressed in muscle?

Answer 10

muscle filaments have to be of uniform length. capping proteins regulate this

Question 11

what is cytochalasin D and what is it used for?

Answer 11

a toxin produced by fungi that blocks the (+) end of the actin, preventing subunit addition, causing all actin in the cell to eventually become depolymerised. can be used for in vivo analysis of the role of actin in cellular processes

Question 12

where does actin nucleation occur?

Answer 12

cell cortex - below plasma membrane

Question 13

what are the requirements for actin nucleation?

Answer 13

• activation of a small GTPase from Rho family: Rho, Rac and Cdc42
• activation at the plasma membrane by a receptor
• recruitment of actin nucleating proteins

Question 14

how are unbranched actins nucleated?

Answer 14

• nucleated by formins, which are activated when there is binding of Rho GTP
• formins recruit actin monomers to (+) end

Question 15

wherever unbranched actin filaments located?

Answer 15

in microvilli, contractile muscle and stress fibres

Question 16

why are unbranched actin filaments found in microvilli?

Answer 16

they form bundles of long, straight filaments that can branch outwards to form projections, increasing the surface area

Question 17

what is required for branched actin nucleation?

Answer 17

• activation by Rac and Cdc42 GTPases
• Arp2/Arp3 complex - bind to and create branches on existing actin
• WASP/WAVE

Question 18

what are bundling proteins? give an example

Answer 18

cross-linking proteins that bind unbranched filaments, creating parallel arrays e.g. fimbrin
located in microvilli, filopodia

Question 19

what are gel-forming proteins? give an example

Answer 19

cross-linking proteins that form a loose-mesh work e.g. spectrin, which forms the cell cortex

Question 20

what do adaptor proteins do? name some

Answer 20

attach actin filaments to the membrane. e.g. integrins which connect actin cytoskeleton to extracellular matrix for the purpose of cell adhesion and migration

Question 21

why do cells need to migrate?

Answer 21

• embryonic development
• epithelial cells for wound healing
• white blood cells to sites of infection
• fibroblasts to maintain extracellular matrix

Question 22

what is the filopodia in a migrating cell?

Answer 22

a fine membrane projection that acts as sensors to co-ordinate direction of movement.

Question 23

what is the lamellipodium?

Answer 23

a broad membrane extension that pushes the front of the cell forward

Question 24

what are stress fibres?

Answer 24

contractile bundles that squeeze the rear of the cell forwards

Question 25

what are the steps of cell migration?

Answer 25

1- the cell extends the plasma membrane forward via the lamellipodium
2- cell re-attaches, forms new adhesion to anchor the lamellipodium in position
3- the rest of the cell moves, the stress fibres contract and the body the cell is pushed forwards
4- the rest of the cell moves as focal adhesions are disassembled

Question 26

why are the GTPases Rho, Rac and Cdc42 important for cell migration?

Answer 26

the localised activation of each GTPase by extracellular signals leads to formation of specific actin structures in different areas of the cell

Question 27

what does Cdc42 activation at the front of the cell cause?

Answer 27

• microtubule rearrangement and cell polarisation, filopodia formation via WAVE
• Rac activation, leading to lamellipodium formation via WASP

Question 28

what does Rho activation at the rear of the cell cause?

Answer 28

formin activation and stress fibre formation in unbranched actin