Cytoskeleton (Sept. 5-Denning) Flashcards by Andrew Taniguchi

List the 3 types of main elements which compose the cytoskeleton

Actin
Microtubules
Intermediate Filaments

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What is another name for Actin?

Microfilaments

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List the main functions of actin (microfilaments)

Cell Shape (surface area)
Cell adhesion
Polarization
Phagocytosis
Muscle contraction

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Actin can also allow for cell migration. What are the functions it facilitates.

development
wound healing
metastasis

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Describe 3 general features of the structure of actin when monomers assemble into protogilaments

Monomers bind head to tail for form Plus (+) and Minus (-) end of chain
Binding site for ATP/ADP in the center
Aggregate into 2 chains of protofilaments

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What happens when the ATP undergoes hydrolysis to ADP when actin filaments are coalesced together?

Gives rise to the instability of the actin filaments

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What is the “ant-trail” analogy?

If a cell needs to move or respond to a signal it can rapidly depolymerize the actin on one side and reassemble the actin filaments on another side

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The kinetics of actin filament formation:
(A) is spontaneous
(B) involve a nucleation lag phase followed by a rapid elongation phase and steady-state phase (equilibrium phase)
(C) depends on the final critical concentration
(D) Relies on the number of free actin subunits available

A - nucleation lag phase
B - there is a significant energy barrier that is present in the nucleation phase that must be overcome. Actin monomers aggregate into oligomers and then rapidly polymerize.
C - steady state is the final concentration of dynamic actin filaments and does not affect kinetics
D - # of free actin subunits affects final crit. concentrations. # of Nuclei and their addition affects elongation speed.

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Describe actin nucleation

ARP - actin related protein complex - is activated by an “activating factor” - nucleates at minus end & binds pre-existing filaments at 70° angle.

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What is the purpose of CapZ?

End binding protein which prevents loss or growth of additional subunits. Creation of a stable protofilament.

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What is Cofilin and what does it bind?

Binds cofactors which are aged. Causes destabilization of filaments via hydrolysis of ATP -> ADP contained/bounded in protofilaments

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What are the 2 forms of actin subunits

T and D form
T = soluble form
Polymers are a mixture of T and D configurations

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Describe the hydrolysis process competing against polymerization at the (+) and (-) end

+ end is faster growing end – hydrolysis lags behind

- end is slower growing end - hydrolysis catches up (tread-milling)

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What allows the tree like formation/protrution of actin protofilaments in lamellipodia

Actin filaments are able to bind like a tree at 70 degrees to other actin filaments: it becomes a tree or gel of network of actin with plasma membrane covering.
Initiated by ARP 2/3 complex.

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What are the 3 types of actin arrays found in a cell?

contractile bundle (alpha-actinin) - found in stress fibers
gel like network - cell cortex
Right parallel bundle (fimbrin) filopodia

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Actin filaments can be crosslinked using these (4) proteins

Bundling proteins:
Fimbrin monomers - parallel association of protofilaments
a-actinin dimers - antiparallel association
Gel-forming proteins:
Spectrin tetramers
Filamin dimers

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What are the differences between a contractile and parallel bundle?

Contractile -> actin & a-actinin (loose - allows myosin II to enter bundle)
Parallel -> actin & fimbrin (tight - prevents association of myosin II)

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What are microvilli linkers?

The tight parallel structural linking/association of actin protofilaments. Uses fimbrin and villin proteins

Why are actin-targeted drugs unfavorable to be used?

How might bacteria use actin depolymerization in their favor?

Targeting actin to disrupt cell processes is too critical

What is significant about the rho family of GTPases?

They can induce dramatic changes in the active cytoskeleton (microspikes, membrane ruffles, stress fibers)

What are the effects of Phalloidin on actin filaments?

Phalloidin binds actin filaments.
Stabilizes synthesized filaments.
Promotes only active growth of filaments (increase rate of synthesis) - prevents disassembly (no degradation)

Name a molecular motor for the actin family

Myosin

Describe the structure of myosin as a motor protein of actin filaments

2 heavy chains + 2 light chains
Majority of myosins are plus end directed
Form bipolar thick filament
Center area of overlap
Coiled coiled tails – exposed myosin heads – many myosin chains can contact actin molecules

True/False: Myosin spends most of its time in the attached state

False: Myosin spends only a small fraction of its time attached to actin

What are the characterisitcs of myosin molecular mechanism of movement on actin

Myosin “Rowing” on Actin High speed Heads work independently

Describe the involvement of ATP in the interaction between myosin motors and actin filaments

Rigor state – myosin head is touching actin without ATP When ATP is added, the myosin detaches from actin, it is is hydrolyzed and myosin head moves forward one unit and bind to actin Release of ADP from previous head induces the power stroke which moves actin filament. Myosin head is again in rigor state awaiting ATP to bind again.

What are functions of microtubules

``` Positioning of organelles (Golgi and ER) Intracellular transport (mitotic spindles, vesicle movement) Cell motility (flagella, cilia) ```

Describe the general structure of microtubules

Alpha tubulin & Beta tubulin alternating subunits Ring structure made up of 13 protofilaments Plus end is dynamic end (addition and degradation) Minus end is known as the MTOC (centrosome)

Describe the dynamic instability and how it relates to microtubule assembly/disassembly

T form favors assembly D form favors disassembly Catastrophe - rapid degradation due to loss of GTP cap Recovery - regain of GTP cap and regrowth

Which subunit of the microtubule structure contains the GTP/GDP?

Beta form

What are MAPS?

Microtubule associated Proteins

What is the function of MAP2 and tau?

MAPs - responsible for filament cross linking, spacing, and stability, spacer arms determine subunit spacing

What is an implication of microtubule targeted drugs?

These drugs tend to bind to microfilaments – interfere with dynamic instability- detrimental to the cell Important in mitotic spindle formation!

Name the 2 microtubule motor proteins

Kinesins and Dyneins

Describe the movement of kinesins

- move towards plus end (periphery) | - endoplasmic reticulum

Describe the movement of dyneins

- move towards negative end towards center - cytoplasmic (vesicle trafficking and golgi locomotion) - axonemal (cilia/flagella)

Elaborate on the microtubule motor mechanism

High processivity - Allows for stable movement (not as fast, but allows cargo carrying a very long way) - Leading and trailing head - Binding of ATP to leading head allows trailing head (with ADP) to move forward. - Release of ADP locks and resets kinesin - Hydrolysis of ATP allows release of lagging head from microtubule

T/F: Each kinesin head stays attached to the mT 50% of the time.

True

Describe the movement of flagella

Even, wave-like motion (Protozoa and sperm)

Describe the movement of cilia

Whip-like motion (respiratory epithelium)

What is the general function of basal bodies

Anchoring cilia and flagella at cell surface

What are the functions of intermediate filaments

1) Mechanical strength 2) Cell adhesion 3) Axon Diameter 4) Nuclear Lamina

Describe the structure and assembly of intermediate filaments

- High physical strength; maximize subunit interaction - Consist of eight (8) tetramers twisted into a rope like filament - Symmetrical & NOT polar

T/F: Intermediate filaments can form cross-links?

True. C-terminal domains of the associated tetramers can bind neighboring filaments to form a strong tonofilament