| back 1 - Helical polymers G-actin
- Flexible, linear bundles, 2D
networks
- Concentrated at cortex; beneath cell membrane
- Strength and shape to plasma membrane
- Structures -
microvilli, extremely stable
- Cell migration - lamellipodia
and filopodia
- Motor proteins- myosin (will move on actin
fibers and transport cargo. Allow for sliding of adjacent actin
fibers and is important during muscle contraction)
|
| back 2 - Long and hollow cylinder of tubulin
- Rigid, long, and
straight
- Function - internal framework for organelles (tracks
for vesicular trafficking), structural support and motion (flagella
and cilia), mitotic spindle formation during cellular division
- Motor proteins- kinesin and dynein
|
| back 3 - Rope-like fibers instead of helical structure, stable
- Heterogenous family (tissue specific)- laminin, keratin
- Animal cell nucleus- nuclear lamina
- Flexible and
strong
- Function - create nuclear lamina, mechanical strength
for epithelial cells and layers
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| back 4 - Dynamic framework
- Regulated by accessory proteins
- Cell can regulate length, stability, number, and geometry
- Can bind stabilize disrupt and create junctions of those
networks
- Cell changes cytoskeleton
organization in response to signals
- If migrating, will
adjust inside of cytoskeleton that allows and facilitates movement
of the cell
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front 5 Actin microfilaments and actin binding proteins | back 5 - Microfilaments are assembled into diverse structures (by means
of actin binding proteins), usually associated with the plasma
membrane
- Cross-linked bundles and networks: microvilli, cell
cortex, adherens belt, filopodia, lamellipodium/leading edge, stress
fibers, phagocytosis, moving endocytotic vesicles, contractile
ring
|
| back 6 - Highly conserved proteins across eukarya
- Signifies
that it is extremely important
- Binds to ATP
and ADP
- ATP hydrolysis in F-form
- Slow ATP
hydrolysis
- G-actin in f-actin able to hydrolyze
ATP
- Reversible assembly
into filaments- two helices of actin subunits oriented in the same
direction
- Monomer - G-actin (globular)
- Polymer -
F-actin (filamentous)
- Plus (barbed) and minus (pointed)
polarity
- Nucleotide binding site on minus end, where
the ATP/ADP will bind
- Able to self assemble
to form a longer filament
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front 7 Dynamics of actin filaments | back 7 - Microfilaments are dynamic structures
- Assembly and
disassembly regulated process- actin binding proteins
- Plus
end - fast growing end (highly dynamic, when actin is bound to
ATP it's likely to bind to the filamentous actin on the plus
end. The ATP is hydrolyzed into ADP and eventually dissociates
from actin filament)
- Minus end - slow growing end
- ATP hydrolysis promotes F-actin depolymerization
- Actin bound to ADP less binding strength among monomers
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front 8 Actin nucleation - rate limiting step structures | back 8 In vitro (test tube)
- Nucleation- initial aggregate that allows for elongation
- Polymerization- spontaneous (faster) in the presence of seeds
(preformed filament)
- Critical concentration (Cc) -
concentration of G-actin at steady state (rate of polymerization =
rate dissociation)
- In cells, concentration of free G-actin
much higher than Cc
- Allows for dramatic and fast
changes in the actin cytoskeleton
- Has proteins that
sequester G actin, liberates them when the cell needs them,
allows for reshaping of cytoskeleton
|
| back 9 - Steady state - rate of polymerization =
dissociation(depolymerization)
- F-actin
- T form - ATP
- D form - ADP
- G-actin critical
concentration (Cc)
- High in t form
- Low in D form
- Suggests the plus end is the growing end and the minus
end is the slow growing end
- Intermediate
G-actin concentration- growth in plus end and shrinkage in negative
end
|
| back 10 - At steady state, rate of growth in plus end is equal to
shrinkage in minus end (Net addition of actin subunits at the plus
end while simultaneously losing subunits at minus end)
- Size
of F-filament remains constant
- Allows filament to move
|
| back 11 All are toxic to cells- suggesting dynamic equilibrium G-actin -
F-actin are essential |
| back 12 Depolymerizes, binds actin subunits |
| back 13 Depolymerizes, caps filament plus ends |
| back 14 Stabilizes, binds along filaments |
| back 15 Important in the regulation of this dynamic equilibrium between
G-actin and F-actin
Actin monomers, monomer-sequestering protein, bundling protein,
myosin motor protein, side-binding protein (tropomyosin), capping
(plus end blocking) protein, cross-linking protein, severing protein,
nucleating protein |
front 16 Actin treadmilling- regulation | back 16 Actin turnover and filament length regulated by actin-binding proteins |
front 17 Monomer availability: Thymosin | back 17 Binds G-actin to provide reserve G-actin when it is needed
Sequesters free G-actin (releases it when the cell needs to reshape
actin cytoskeleton) |
front 18 Monomer availability: Profilin | back 18 Enhances the exchange of ADP for ATP on G-actin
Promotes polymerization and F-actin formation on plus end |
front 19 Actin-nucleating proteins | back 19 Key proteins from which an actin filament can grow- straight of
branched filaments |
| back 20 - Straight filaments (promote polymerization of plus end and make
it faster)
- Location- plus end of F-actin
- Long
actin filaments (stress fivers, contractile ring)
|
| back 21 - Branched filaments
- Location- minus end of F-actin
- Act most efficiently on existing filaments (act as nucleation
factor, promote growth and association of new G-actin)
- Branched F-actin networks (ex: leading edge of motile
cells)
|
| back 22 - Actin severing protein- fragments of F-actin
- Fate of
fragments depends on other actin binding proteins
|
| back 23 - Enhances the rate of loss of ADP-actin from the minus end
- Promotes depolymerization on the minus end
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front 24 Filament: Capping proteins | back 24 - Stabilize F-actin (prevent polymerization and
depolymerization)
- If attached to plus end, can't grow any
longer
|
front 25 Higher structural levels- accessory proteins | back 25 - Higher structural levels of actin filaments can be achieved
with accessory proteins
- Crosslink different actin
filaments
- Attach to other proteins, associations with each
other or other types of proteins
|
| back 26 - Motor proteins that move along actin filaments
- Crosslink and slide filaments relative to one another or
transport cargo
- Powered to ATP hydrolysis ->
conformational change
- Move towards plus end of actin
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| back 27 - Two heavy chains + 4 light chains
- Head: motor region
(actin binding and ATP hydrolysis)
- Neck: lever-arm/hinge
region
- Tail: tail-tail interaction and cargo binding
|
| back 28 Assembles into bipolar fragments- muscle contraction |
| back 29 Facilitates binding actin filaments to the membrane -> important
during endocytosis |
| back 30 Transport vesicle on the F-actin |
| back 31 - Myosin converts ATP hydrolysis to mechanical work by amplifying
a small conformational change in the head when bound to F-actin
- ATP hydrolysis- change in conformation- reduced affinity for
actin
- Propels forward (power stroke)
|
| back 32 - Skeletal muscle: contractile myofibirils are composed of
thousands of repeating units called sarcomeres
- Myosin thick
filaments and actin thin filaments
|
front 33 Accessory actin binding proteins | back 33 Stabilize actin filaments |
| |
| |
| back 36 Regulates length of thin filament, surrounds F-actin |
| back 37 Molecular spring
Enables during muscle relaxation back to achieve the normal size of
the a-band |
front 38 Skeletal muscle contraction | back 38 ATP-dependent sliding of myosin thick filaments along actin thin
filaments to shorten the sarcomere and hence the myofibril |