Cytoskeleton: Actin-based Cell Movements Flashcards
What is G-actin
A globular monomeric actin protein that binds ATP and polymerises into F-actin
What is F-actin
Filamentous actin, a helical polymer of G-actin with 13 monomers per 37nm repeat
What gives actin filaments polarity
Asymmetric binding between subdomains 2-4 and 1-3 during polymerisation
How does actin filament work
ATP-actin binds more readily to the + end
After polymerisation, ATP is hydrolysed to ADP, destabilising the filament
ADP-actin dissociates from the – end
List the 5 main types of actin building proteins and their roles
- Nucleation – Initiate filament formation
- Capping – Prevent addition/loss of monomers
- Severing – Cut filaments
- Sequestering – Bind and hold monomers
- Bundling – Organise filaments into parallel arrays
Are actin filaments stronger in tension or compression
Tension (good for pulling, not pushing)
What are some structural and motility roles of microfilaments
Organelle transport (plants/fungi)
Contractile systems (animals)
Support and movement of growing cell edges
What powers actin-based transport
Myosin, a motor protein with ATPase heads that “walk” along actin
How does myosin movement differ from kinesin
Less coordinated, still uses ATP to generate movement along actin
What causes cytoplasmic streaming in plant cells
Drag from vacuoles + myosin-actin-driven vesicle/organelle movement
What are muscle fibres composed of
Multinucleated cells formed by myoblast fusion, packed with myofibrils
What is a sarcomere
Repeating unit of contraction with actin and myosin filaments
Where are actin plus ends anchored in the sarcomere
Z-discs
What proteins stabilise thick filament structure
Titin (defines length and elasticity) and no-balling
What causes sarcomere shortening during contraction
Myosin pulls actin filaments inward, filament length doesn’t change
What is the myosin cross-bridge cycle
Pi release ➝ strong binding ➝ power stroke (6nm movement, 0.7 pN force)
Heads are detached ~95% of the cycle, maintaining tension via staggering.
Why can’t all myosin heads bind actin at once
Myosin spacing and actin filament arrangement are mismatched
What accessory proteins regulate actin-myosin binding in muscle
Troponin and tropomyosin
What does calcium binding to troponin C do
Causes tropomyosin to shift, exposing myosin binding sites on actin
Where is calcium stored in muscle cells
Sarcoplasmic reticulum, electrically connected to the plasma membrane
What role does actin play in cytokinesis (animal/fungi)
Forms a contractile ring of actin and myosin-II, forming mini-sarcomeres
What are stress fibres
Actin-myosin bundles in non-muscle cells that anchor at the plasma membrane
What structures are involved in cell crawling
Lamellipodium – Branched actin mesh pulling the cell forward
Filopodium – Thin, dynamic actin bundles sensing the environment
Stress fibers – Contract the rear of the cell
What is treadmilling
Net growth at the front (+ end) and net loss at the rear (– end) of an actin filament network, allowing motion without filament movement
Does myosin-II participate in lamellipodium protrusion
No - actin polymerisation alone drives protrusion
How do small GTPases regulate actin organisation
Rho ➝ Stress fiber formation
Rac ➝ Lamellipodia formation
Cdc42 ➝ Filopodia formation
How do GTPases function as molecular switches
Bind GTP to activate and stimulate other proteins; inactivate by hydrolyzing GTP to GDP
