9.2.16 Lecture Flashcards
The cytoskeleton involves a dynamic array of what three interacting filaments?
- Actin (smallest, 5-7nm diameter)
- Microtubules (largest, 25 nm diameter)
- Intermediate filaments (medium, 10 nm diameter)
What are the 6 major functions of actin?
- Dictate cell shape
- Mediate cell adhesion
- Polarization
- Phagocytosis
- Muscle contraction
- Cell migration
Actin is localized below the ___.
Cell membrane
What determine cell shape and surface area?
Cortical actin and actin arrays
What are the two types of cell adhesion mediated by actin?
- Cell-cell, through adherens junctions
2. Cell-matrix through focal contacts
Actin arrays partition polarized cells into ___ and ___ compartments.
Apical; basal
The ___ is an actin-based structure.
Phagosome
Bundles of actin filaments are used by ___ for muscle contraction.
Myosin motors
What are the four general instances needing cell migration?
- Development
- Wound healing
- Immune system
- Cancer metastasis
What is the soluble subunit of actin?
Actin monomer
Describe the structure of the actin monomer.
Globular, contains ATP when in the cytosol, ADP when in the filament, has + and - end (polar)
What is the actin filament made of?
Flexible helix of 2 protofilaments
Actin monomers and filaments are ___. This gives it ___. Which end is more dynamic?
Polar; directionality; +
___ regulate actin filament assembly and disassembly.
Accessory proteins
Small soluble actin subunits are in ___ with large filamentous polymers.
Equilibrium
A signal can lead to disassembly of filaments and rapid ___ of subunits, followed by reassembly at a ___ site.
Diffusion; new
What are the three phases to actin filament assembly?
- Nucleation (lag phase)
- Elongation (growth phase)
- Steady state equilibrium (equilibrium phase)
What is the rate limiting step to actin filament assembly?
Nucleation
What is the critical concentration?
The concentration of actin monomers at steady state
What happens to the kinetics of actin formation if actin “nuclei” are added directly?
Removes the nucleation/lag phase
Actin is nucleated at the ___ end.
Minus
Nucleation occurs preferentially at the ___, which allows for cell surface structures to form.
Cell membrane
Nucleation occurs via what protein(s)?
Actin related proteins (ARP) -> ARP2 and ARP3
ARP binds pre-existing filaments at a ___ angle.
70 degree
What modulates filament growth and localization? What do each of these do?
End binding proteins (ARP and Cap Z) - ARP caps and nucleates the minus end, CapZ binds and stabilizes the + end.
___ modulate filament elongation. These are ___ (less or more) efficient than end binding proteins.
Subunit binding proteins; less
___ modulate filament stability and orientation. This includes what protein?
Filament binding proteins; cofilin (actin depolymerization factor that preferentially binds to ADP subunits in existing actin filaments.)
Filamentous actin contains enzymes to…
…hydrolyze ATP present in subunits.
ATP hydrolysis converts ___ form to ___ form.
T (stabilizes + end of filament); D
Nucleotide hydrolysis results in treadmilling. What is this?
+ end addition is fast and hydrolysis lags behind. - end addition is slow and hydrolysis catches up. This creates a flux of subunits.
A high critical concentration is favorable for ___; a low critical concentration is favorable for ___.
Loss; addition
Critical concentration of ___ is less than that of ___.
T; D
Describe actin protrusion in lamellipodia.
Net filament diassembly occurs behind the leading edge via cofilin binding to the ADP form of actin to cause depolymerization and Cap Z binding. Free actin subunits can then reassemble at the leading edge, extending the cytoplasm of the migrating cell.
What are the 5 types of actin arrays in cells?
- Stress fibers/focal contacts: contractile bundles of anti-parallel actin; link to extracellular matrix via integrins.
- Cell cortex: actin gel-like network beneath plasma membrane
- Filopodia and microvilli: tight parallel bundles of actin
- Lamellipodia: gel-like network of actin
- Adherens junctions: link to other cells via cadherins
___ determines the type of actin array generated.
The geometry of actin binding domains
What are the two types of proteins involved in actin filament cross-linking?
- Bundling proteins
2. Gel forming proteins
What are the two types of bundling proteins?
Fimbrin (monomer, parallel bundle, tight packing, prevents mysoin II entrance) and Alpha-actinin (dimer, anti-parallel contractile bundle, loose packing, allows mysoin II to enter)
What are the two types of gel forming proteins?
Spectrin (tetramer) and filamin (dimer)
___ activation induces polymerization and bundling.
Rho (protein family of GTPases)