Module 6: The Cytoskeleton (Function and Origin of Cytoskeleton, Actin and Actin-binding Proteins) Flashcards
It allows cells to organize themselves in space, interact mechanically with each other, and engage with their environment.
cytoskeleton
What are the main families of protein filaments in the cytoskeleton? (3)
- Actin filaments
- Microtubules
- Intermediate filaments
shape the cell’s surface, enable whole-cell locomotion, and assist in the pinching of one cell into two during division.
a family of protein filaments in the cytoskeleton
actin filaments
determine the positions of membrane-enclosed organelles, direct intracellular transport, and form the mitotic spindle.
a family of protein filaments in the cytoskeleton
microtubules
provide mechanical strength to the cell.
a family of protein filaments in the cytoskeleton
intermediate filaments
are dynamic and adaptable, allowing them to change or persist according to the cell’s needs.
Cytoskeletal systems
In a cell, this can occur with little extra energy when conditions change.
Structural rearrangements
Where are actin filaments located in animal cells?
plasma membrane
actin filaments provide __ and __ to the thin lipid bilayer of the plasma membrane.
- strength
- shape
What are the cell-surface projections formed by actin filaments? (2)
- lamellipodia (sheet-like projection)
- filopodia (spike-like projection)
Name two structures that are supported by actin filaments. (2)
- Stereocilia
- microvilli
found in a cytoplasmic array that extends to the cell periphery.
a family of protein filaments in the cytoskeleton
microtubules
They form a bipolar mitotic spindle during cell division.
a family of protein filaments in the cytoskeleton
microtubules
motile structures that function as whips or sensory devices.
cilia
Microtubules form tightly aligned bundles that serve as tracks for the transport of materials along __.
neuronal axons
In plants, microtubules help to direct the pattern of __.
cell wall synthesis
In protozoans, microtubules form a __ upon which the entire cell is built.
framework
- line the inner face of the nuclear envelope.
- They act as a protective cage for the cell’s DNA.
a family of protein filaments in the cytoskeleton
intermediate filaments
In the cytosol, intermediate filaments twist into strong cables that help hold __ together.
epithelial cell sheets
help nerve cells extend long and robust axons.
a family of protein filaments in the cytoskeleton
Intermediate filaments
Intermediate filaments form __ such as hair and fingernails.
tough appendages
The role of cytoskeleton essential for cell division.
rapid reorganization
Give an example of a cell type that undergoes rapid cytoskeletal reorganization.
Fibroblast
What structure do interphase microtubules form during cell division?
bipolar mitotic spindle
crawl across the surface of the dish
actin
It forms a belt around the middle of the cell to pinch it into two.
contractile ring
A cell that has protrusive structures filled with newly polymerized actin filaments
neutrophils
What characterizes stable, differentiated morphology in cells?
stable, large-scale structures
What are two examples of specialized epithelial cells that have distinct structures?
- Microvilli in the intestines
- cilia in the lungs
They maintain a constant location, length, and diameter over their entire lifetime.
specialized epithelial cells
Microvilli in the intestines; cilia in the lungs
How long do microvilli on intestinal epithelial cells typically last?
a few days
What is the lifespan of stereocilia on hair cells?
lifetime of the cell
How do these stable structures enable cells to perceive their orientation?
allow cells to differentiate between top and bottom, or front and back.
responsible for establishing large-scale cellular polarity, allowing cells to differentiate between top and bottom, or front and back.
cytoskeleton
- have an apical surface and a basolateral surface.
- maintain strong adhesive contacts with one another, enabling the single layer of cells to function as an effective physical barrier.
Polarized epithelial cells
A cell builds __ by assembling large numbers of small subunits.
filaments
The filament subunits diffuse rapidly in the cytosol, allowing for __.
rapid structural reorganizations
What are the subunits of actin filaments and what energy source do they utilize? (2)
- actin subunits
- ATP hydrolysis
What are the subunits of microtubules and what energy source do they utilize? (2)
- tubulin subunits
- GTP hydrolysis
A subunit that is smaller and symmetrical, forming helical assemblies
intermediate filament subunit
have asymmetrical subunits (head-to-tail orientation) that create polarity (2)
- actin filaments (actin subunits)
- microtubules (tubulin subunits)
A subunit that is symmetrical, meaning they do not have polarity and do not catalyze the hydrolysis of nucleotides.
intermediate filament subunit
built of 13 protofilaments, which are linear strings of subunits joined end-to-end that associate laterally to form a hollow cylinder.
a family of protein filaments in cytoskeleton
microtubules
- The greater energy required to break multiple noncovalent bonds simultaneously allows microtubules to resist .
- The structure allows for rapid subunit __ and loss at the __.
thermal breakage
* addition
* filament ends
Proteins which determine the spatial distribution and dynamic behavior of the filaments by binding to the filaments or their subunits to regulate the sites of new filament assembly.
Accessory proteins
Proteins which bring cytoskeletal structures under the control of extracellular and intracellular signals, maintaining a highly organized yet flexible internal structure.
Accessory proteins
Proteins which bind to polarized cytoskeletal filaments, use ATP hydrolysis for movement along the filaments, and transport “cargo” such as membrane-enclosed organelles.
Motor proteins
Proteins which cause cytoskeletal filaments to exert tension or slide against each other.
Motor proteins
Bacteria have __ of all eukaryotic cytoskeletal filaments.
homologs
A tubulin homolog in bacteria that forms the Z-ring, which contributes to septum formation during cell division and generates a bending force for membrane invagination.
FtsZ
“filamentous temperature-sensitive”
Serves as a site for the localization of enzymes and generates a bending force to drive membrane invagination during cell division.
Z-string
Actin homologs that provide a scaffold to direct the synthesis of the peptidoglycan cell wall. (2)
- MreB
- Mbl
What happens when mutations occur in MreB and Mbl? (2)
- abnormalities in cell shape
- defects in chromosome segregation
a bacterial actin homolog encoded by a gene on certain bacterial plasmids that also carry genes responsible for antibiotic resistance.
ParM
ParM assembles into __ that associate at each end with a copy of the __, and the growth of the __ pushes the replicated __ copies apart.
- filaments
- plasmid
- ParM filament
- plasmid
a homolog of intermediate filaments that influences the crescent shape of Caulobacter crescentus.
Crescentin
uses crescentin to influence its crescent shape.
an organism
Caulobacter crescentus
The actin subunit is called
globular or G-actin
375-amino-acid polypeptide carrying a tightly associated ATP or ADP molecule
actin subunit (globular or G-actin)
G-actin has __ amino acids and carries a tightly associated __ or __ molecule.
- 375
- ATP or ADP
Which type of actin is found in muscle cells?
α-Actin
Which types of actin are found in almost all non-muscle cells? (2)
- β-actin
- γ-actin
subunits which assemble head-to-tail to form a tight, right-handed helix.
actin subunits
What is the diameter of filamentous or F-actin?
8nm wide
What are the two structurally different ends of actin filaments? (2)
- minus end or “pointed end”
- plus end or “barbed end”
Actin filament end that is slower-growing
minus end or “pointed end”
Actin filament end that is faster-growing
plus end or “barbed end”
In which direction is the nucleotide-binding cleft of actin filaments directed?
minus end
It is the minimum length at which random thermal fluctuations are likely to cause the filament to bend.
In actin filaments
persistence length
Why is the regulation of actin filament formation important?
controls the shape and movement of the cell.
the process where subunits assemble into an initial aggregate, or nucleus, stabilized by multiple subunit–subunit contacts, allowing rapid elongation.
Nucleation
In actin filament nucleation, the formation of __ is rate-limiting and is further inhibited by __.
- small actin oligomers
- actin-binding proteins
What are the three phases of in vitro polymerization of G-actin? (3)
- Nucleation
- Elongation
- Steady-state
Lag period where G-actin aggregates into short, unstable oligomers that can act as stable seeds or nucleus once they reach a certain length.
a phase in in vitro polymerization of G-actin
Nucleation
Rapid filament growth by addition of actin monomers to both ends.
a phase in in vitro polymerization of G-actin
Elongation
G-actin monomers exchange with subunits at the filament ends, with no net change in total filament mass.
a phase in in vitro polymerization of G-actin
Steady-state
During the __ phase of G-actin polymerization, G-actin aggregates into short, unstable __. When this reach a certain length, they act as stable __ or __ for further growth.
in vitro polymerization of G-actin
- nucleation
- oligomers
- seeds or nuclei
What happens to G-actin monomer concentration as F-actin filaments grow?
decreases
During steady-state, G-actin monomers exchange with subunits at the __, but there is __ in the total filament mass.
in vitro polymerization of G-actin
- filament ends
- no net change
The concentration of the pool of unassembled subunits when the steady-state phase has been reached, where subunit addition balances subunit dissociation.
actin filament dynamics
critical concentration (Cc)
The ratio of the “on” and “off” rate constant, measuring the concentration of G-actin where subunit addition equals subunit dissociation.
actin filament dynamics
dissociation constant
Actin filaments grow faster at the (1)__ than at the (2)__, with the (1)__ elongating (3)__ as fast as the (2)__.
1) plus end
2) minus end
3) 5–10 times
This is manifested by the different rates at which G-actin adds to the two ends, with the plus end growing faster than the minus end.
polarity of F-actin
__ actin filaments nucleate the polymerization of G-actin, leading to longer newly __ at the plus end compared to the minus end.
- Myosin-decorated
- polymerized (undecorated) actin
What is the length difference of newly polymerized (undecorated) actin at the plus end compared to the minus end?
|actin filament dynamics
5–10 times
What causes the difference in elongation rates at the opposite ends of an actin filament?
difference in critical concentration (Cc) values
When the __ is capped, the filament can only elongate from its __.
|actin filament dynamics
- plus end
- minus end
Elongation takes place only at the __ when the __ of a filament is blocked.
|actin filament dynamics
- plus end
- minus end
How does the critical concentration (Cc) for polymerization compare between the plus and minus ends of an actin filament?
polymerization at the plus end is about six times lower than for addition at the minus end.
1) What happens to filament growth when G-actin concentration is below Cc+?
2) What occurs when G-actin concentrations are between Cc+ and Cc-?
3) What happens to filament growth when G-actin concentration is above Cc-?
1) no filament growth
2) Growth occurs only at the plus end
3) no growth at both ends
G-actin concentrations are intermediate between the Cc values for the (+) and (-) ends, with subunits continuing to be added at the (+) end and lost from the (-) end.
phase in in vitro polymerization of G-actin
steady-state phase
Newly added subunits travel through the filament like they are on a __, until they reach the (-) end, where they dissociate.
|actin filament dynamics
treadmill
Chemical inhibitors of actin (3)
- Lantrunculin
- Cytochalasin B
- Phalloidin
Chemical inhibitors of microtubules (3)
- Taxol (paclitaxel)
- Nocodazole
- Colchicine
What factors control the polymerization of actin? (3)
- concentration
- pH
- concentration of salts and ATP
Actin behavior is regulated by __ that bind actin monomers or filaments.
accessory proteins
measures how long an individual actin monomer spends in a filament as it treadmills.
Filament half-life
What percentage of actin is in filament form and what percentage is in soluble monomer form? (2)
- 50%
- 50%
The cell contains proteins that bind to __, making polymerization much less favorable.
actin dynamics
actin monomers
inhibits actin polymerization; it cannot associate with either the plus or minus ends and does not hydrolyze or exchange its bound nucleotide.
Thymosin
binds to the face of the actin monomer opposite the ATP-binding cleft, blocking the side that would normally associate with the filament minus end, while exposing the site that binds to the plus end.
Profilin
Profilin leaves the actin filament one subunit __ and competes with __.
- longer
- thymosin
Profilin phosphorylation and its binding to __ influence actin dynamics, promoting polymerization and filament growth.
inositol phospholipids
What is a prerequisite for cellular actin polymerization?
Filament nucleation
bring several actin subunits together to form a seed for filament growth.
Actin-nucleating proteins
What are the two main types of actin-nucleating proteins? (2)
- Arp 2/3 complex
- formins
Nucleates actin filament growth from the minus end, allowing rapid elongation at the plus end and can attach to the side of another filament, creating a treelike web.
actin-nucleating protein
Arp 2/3 complex
nucleate the growth of straight, unbranched actin filaments.
actin-nucleating protein
Formins
- capture two actin monomers to nucleate filament growth.
- remain associated with the rapidly growing plus end while allowing the addition of new subunits.
actin-nucleating protein
Formin dimers
strongly enhanced by the association of actin monomers with profilin.
actin-nucleating protein
Formin-dependent actin filament growth
alter filament behavior by binding along the side or to the ends of the filaments.
actin filament-binding proteins
a side-binding protein that stabilizes and stiffens actin filaments by binding simultaneously to six or seven adjacent actin subunits, preventing interaction with other proteins and controlling muscle contraction.
actin filament-binding proteins
Tropomyosin
binds to the plus end of an actin filament to stabilize it, preventing further polymerization or depolymerization
actin filament-binding proteins
Capping protein (CapZ)
caps long-lived actin filaments at the minus end, binding tightly to prevent elongation and depolymerization, especially when stabilized by tropomyosin.
actin filament-binding proteins
Tropomodulin
__ coat the filament completely and are present in high amounts, while __ affect filament dynamics at the ends.
actin filament-binding proteins
- Side-binding proteins
- end-binding proteins
break actin filaments into many smaller fragments, generating new filament ends that can nucleate further filament growth and promoting the depolymerization of older filaments.
Severing proteins
activated by high levels of cytosolic Ca²⁺. It interacts with the side of an actin filament and binds until a thermal fluctuation creates a gap, allowing it to insert itself and break the filament.
severing proteins
Gelsolin superfamily/gelsolin
an actin depolymerizing factor that binds along the length of the filament, causing it to twist more tightly, which weakens the contacts between actin subunits and preferentially targets ADP-containing actin filaments for dismantling.
severing proteins
Coffin
Actin filaments containing __ are resistant to __, making them more stable compared to those with __, which are more readily dismantled by severing proteins like cofilin.
severing proteins
- ATP
- depolymerization
- ADP
What are the types of actin filament arrays? Add short description (3)
- Dendritic networks – Arp 2/3 complex
- Bundles networks – made of the long, straight filaments produced by formins
- Weblike (gel-like) networks – not well-defined
What determines the differences in actin networks?
specialized accessory proteins
Proteins that cross-link actin filaments into a parallel array.
specialized accessory proteins; actin filament types of arrays
bundling proteins
Proteins that hold two actin filaments together at a large angle to each other, forming a looser meshwork.
specialized accessory proteins; actin filament types of arrays
gel-forming proteins
enables stress fiber and other contractile arrays to contract.
actin filament array
Myosin II
allows for the close packing of actin filaments; it is not contractile.
actin filament array
Fimbrin
cross-links oppositely polarized actin filaments into a loose bundle, allowing the binding of myosin and formation of contractile actin bundles.
actin filament array
α-Actinin
tight packing prevents myosin II from entering bundle
actin filament array
parallel bundle
loose packing allows myosin II to enter bundle
actin filament array
contractile bundle
- forms a loose and highly viscous gel by clamping together two actin filaments roughly at right angles, creating actin filament webs or gels.
- connects and coordinates a wide variety of cellular processes with the actin cytoskeleton.
actin-binding protein
filamin
can lead to defects in nerve-cell migration during early embryonic development, resulting in periventricular heterotopia, where cells in the periventricular region of the brain fail to migrate to the cortex and instead form nodules.
a mutation
Filamin A gene mutations
a web-forming, long, flexible protein made out of four elongated polypeptide chains, consisting of two α subunits and two β subunits.
actin-binding protein
Spectrin
In RBCs, __ is concentrated beneath the plasma membrane, forming a two-dimensional weblike network that provides a strong, yet flexible cell cortex, allowing RBCs to spring back to their shape.
actin-binding protein
spectrin
are crucial for regulating actin dynamics, facilitating the assembly and disassembly of actin filaments, which in turn enables motility and force production in various cellular processes.
Accessory proteins
- Bacteria and viruses that use components of the host cell actin cytoskeleton to move through the __.
- To move around in a cell and invade neighboring cells, overcome this problem by recruiting and activating the __ at their surface.
- cytoplasm
- Arp 2/3 complex
- What is an example of a bacterium that recruits the Arp 2/3 complex?
- It recruits and activates Arp2/3 complex; forms a “__.”
- Listeria monocytogenes.
- comet tail
