actin Flashcards
how many isoforms of actin are there? what are they?
- two muscle specific a-actin isoforms
- B-actin (cell cortex, leading edge)
- gamma actin (stress fibres)
what is G-actin?
the monomeric form of actin, globular. has an ATP binding site
what is F-actin?
filamentous actin. two parallel helical strands of polymerised G actin monomers
what gives actin its polarity?
the ATP binding site, which is orientated towards the minus end
which end are monomers added to the actin filament?
the plus end
how does ATP hydrolysis regulate actin filament dynamics?
- 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
how does the process of ‘tread milling’ differ from microtubule dynamic instability?
- neither end of an actin filament contains a cap
- there is a constant turnover of subunits
what is capZ?
a capping protein that binds to and blocks the plus end. this prevents addition of subunits
what is tropomodulin?
a capping protein that binds to and blocks the plus end, stabilising muscle actin filaments
why is tropomodulin highly expressed in muscle?
muscle filaments have to be of uniform length. capping proteins regulate this
what is cytochalasin D and what is it used for?
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
where does actin nucleation occur?
cell cortex - below plasma membrane
what are the requirements for actin nucleation?
- 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
how are unbranched actins nucleated?
- nucleated by formins, which are activated when there is binding of Rho GTP
- formins recruit actin monomers to (+) end
wherever unbranched actin filaments located?
in microvilli, contractile muscle and stress fibres
why are unbranched actin filaments found in microvilli?
they form bundles of long, straight filaments that can branch outwards to form projections, increasing the surface area
what is required for branched actin nucleation?
- activation by Rac and Cdc42 GTPases
- Arp2/Arp3 complex - bind to and create branches on existing actin
- WASP/WAVE
what are bundling proteins? give an example
cross-linking proteins that bind unbranched filaments, creating parallel arrays e.g. fimbrin
located in microvilli, filopodia
what are gel-forming proteins? give an example
cross-linking proteins that form a loose-mesh work e.g. spectrin, which forms the cell cortex
what do adaptor proteins do? name some
attach actin filaments to the membrane. e.g. integrins which connect actin cytoskeleton to extracellular matrix for the purpose of cell adhesion and migration
why do cells need to migrate?
- embryonic development
- epithelial cells for wound healing
- white blood cells to sites of infection
- fibroblasts to maintain extracellular matrix
what is the filopodia in a migrating cell?
a fine membrane projection that acts as sensors to co-ordinate direction of movement.
what is the lamellipodium?
a broad membrane extension that pushes the front of the cell forward
what are stress fibres?
contractile bundles that squeeze the rear of the cell forwards
what are the steps of cell migration?
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
why are the GTPases Rho, Rac and Cdc42 important for cell migration?
the localised activation of each GTPase by extracellular signals leads to formation of specific actin structures in different areas of the cell
what does Cdc42 activation at the front of the cell cause?
- microtubule rearrangement and cell polarisation, filopodia formation via WAVE
- Rac activation, leading to lamellipodium formation via WASP
what does Rho activation at the rear of the cell cause?
formin activation and stress fibre formation in unbranched actin
