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MCBG Exam 3 > Cytoskeleton > Flashcards

Cytoskeleton Flashcards

1
Q

What are the three major protein filaments of the cytoskeleton?

A

1) Actin
2) Microtubules
3) Intermediate Filaments

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2
Q

List the cytoskeletal protein filaments from smallest diameter to biggest diameter.

A

Actin 5-9 nm
Intermediate Filaments 10 nm
Microtubules 25 nm

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3
Q

What are the main functions of actin?

A

Produce cell shape (regulate surface area)
Cell adhesion (cell-cell and cell-matrix)
Polarization (distinguishing apical from basolateral side)
Phagocytosis
Muscle Contractions (sarcomere)
Cell migration

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4
Q

Where in the cell is most actin localized?

A

Just beneath the membrane in the cortex (“cortical actin”)

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5
Q

Describe how actin helps with cellular migration

A

The leading end (“lamellipodium”) of the cell can be extended via actin polymerization. Focal contacts produce traction. Contraction of the rear end of the cell and breakage of focal contacts produces movement forward.

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6
Q

Describe the structure of soluble G-actin

A 
Globular actin (G-actin) units are polar monomers
Tightly bound to ATP
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7
Q

Describe the structure of fibrilar actin

A
  • F-actin is made up of a helix of 2 protofilaments
  • G-actin monomers aggregate head-to-tail to produce a polar protofilament
  • During polymerization, ATP hydrolyzed to ADP
  • F-actin subunits have ADP bound
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8
Q

How do ATP/ADP affect the binding strength between actin monomers?

A

Hydrolysis (ATP –> ADP) decreases the binding strength

- This makes it easier to disassemble filaments after they are formed

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9
Q

Describe the dynamic structure of actin filaments.

A

Polymerizing filaments can look like an ant-trail when subunits are added on one end and removed from the other.
In response to an input signal, cells can rapidly reorganize the cytoskeleton at a new site.

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10
Q

What limits the rate of actin filament assembly?

A
  • Nucleation
  • Without special nucleation proteins, it is not energetically favorable to polymerize long filaments
  • There is a lag phase below a certain concentration where filaments do not assemble
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11
Q

What happens at the steady state of actin filament assembly?

A

There is a balance between subunit addition and removal (Treadmilling)

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12
Q

Where in the cell are actin filaments nucleated?

A

Near the cell membrane

This is why there is so much cortical actin

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13
Q

What is an ARP and what is its function?

A

ARP= Actin related protein complex

  • Nucleates actin at the minus end
  • Can bind to pre-existing filaments at 70° creating a gel
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14
Q

Compare the efficiency of end binding proteins to subunit binding proteins

A

End binding proteins are much more efficient because each actin filament only has 2 ends, but many subunits.

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15
Q

What is cofilin?

A

An actin depolymerization factor that preferentially binds to “aged” subunits that have ADP bound

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16
Q

How do the T and D form of actin subunits impact filament stability?

A

The T-form (ATP bound) stabilizes the growing plus end of the filament
The T-form has a lower critical concentration than the D-form, so subunits are more often lost when in the D-form

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17
Q

What is filamin?

A

A dimer that orients actin filaments at angles to each other leading to mechanically strong gel/web formations

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18
Q

How are actin filaments cross-linked?

A

Proteins with actin binding domains orient actin fibers. The spacing and angle of fibers differs between different binding proteins

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19
Q

What actin bundling protein produces tight parallel bundles?

A

Fimbrin produces tight parallel fiber bundles

Alpha-actinin produces looser anti-parallel bundles

20
Q

What determines if myosin can bind to actin fibers?

A

How tightly packed the actin fibers are, which is determined by which actin binding proteins are present. Alpha-actinin spaces actin far enough apart to allow myosin access

21
Q

Describe the structure of microvilli

A
  • Parallel actin filaments are tightly packed and crosslinked by villin and fimbrin
  • The plus end of all filaments extend away from the cell
  • The lateral filaments are attached to the plasma membrane by myosin I and calmodulin
22
Q

What is the function of the Rho family of GTPases?

A

They trigger global rearrangements of actin filaments in the cell.

  • Without Rho, actin is primarily in the cortex
  • With Rho activated, actin forms stress fibers and focal contacts
  • With Rac activated, lamellipodia and membrane ruffles form
  • With Cdc42 activated, microspikes and filopodia form
23
Q

What is the effect of Phalloidin?

A

It binds and stabilizes actin filaments, preventing disassembly
- Reduces the critical concentration for assembly to zero

24
Q

What is the major motor protein that interacts with actin?

A

Myosin II

25
Q

Describe the structure of myosin II

A
  • Mysoin II is a dimer with two N-terminal globular heads and a coiled-coil C-terminal tail
  • The heads generate the force creating movement towards the plus end of actin
26
Q

Describe the myosin power stroke cycle

A

1) Rigor state: myosin head is bound to actin
2) ATP binds to the myosin head, detaching from actin
3) Hydrolysis of ATP to ADP cocks head forward
4) Release of phosphate increases myosin affinity for actin
5) ADP leaves, POWER STROKE

27
Q

What are the functions of microtubules?

A

Positioning of organelles
Intracellular transport
Cell motility

28
Q

Describe the structure of microtubules

A
  • α and β-tubulin dimers make up the soluble subunit
  • Assemble head to tail into polar protofilaments
  • both α- and β-tubulin bind GTP, in filament form, β is bound to GDP
  • A microtubule consists of a hollow ring of 13 protofilaments
29
Q

Which end of a microtubule is more dynamic?

A

The plus end is more dynamic than the minus end, which is buried within the centrosome

30
Q

What is an MTOC?

A

Microtubule organizing center

  • Spherical structures with γ-Tubulin ring complexes (γ-TuRC) that act as microtubule nucleating sites
  • Centrosomes orient all microtubules with plus end extending away
31
Q

What is dynamic instability?

A

Repetitive cycles of filament growth/shrinkage

  • GTP cap favors growth of filament
  • Catastrophe: when GTP cap is lost, GDP at end results in rapid depolymerization
32
Q

How does the geometry of microtubules change when GTP is bound compared to GDP?

A

When GTP is bound, microtubules are straight with strong interactions between subunits
When GDP is bound, microtubules are curved and unstable, leading to polymerization

33
Q

What are MAPs?

A

Microtubule associated proteins

These proteins regulate filament spacing and stabilization

34
Q

Describe the spacing between microtubules with MAP2 arms compared to spacing with Tau protein arms

A

MAP2 is a long extended MAP, which leads to larger spacing than microtubules with Tau spaces

35
Q

How are microtubule targeting drugs useful for cancer therapy?

A

They can prevent cell division by compromising mitotic spindle fibers

36
Q

What does Taxol cause?

A

Taxol stabilizes microtubules, preventing the formation of mitotic spindles

37
Q

What motor proteins interact with microtubules?

A

Kinesins move toward the plus end of microtubules
- Away from nucleus towards plasma membrane
Dyneins move toward the minus end of microtubules
- Toward nucleus

38
Q

Describe the movement of motor proteins across microtubules

A

Homodimers with 2 globular heads “walk” across microtubule

  • Each head is attached 50% of the time
  • ATP hydrolysis detaches head from microtubule
39
Q

Describe the structure of axonemal dyneins.

A

9+2 organization: 9 microtubule doublets in a ring around 2 microtubule singlets
Dynein arms extend from the doublets and periodically contact adjacent doublets, leading to cliliary movement

40
Q

What are basal bodies?

A

Structures that organize cilia and flagella near the cell surface
- composed of 9 microtubule triplets in a ring (similar to centrioles)

41
Q

What are the major functions of intermediate filaments?

A

Mechanical strength
Cell adhesion
Axon diameter
Nuclear lamina: gives nucleus shape

42
Q

Describe the structure of intermediate filaments

A

1) 2 parallel alpha helical monomers form a coiled coil dimer
2) Dimers form staggered antiparallel tetramers
3) Tetramers pack end to end to form overlapping staggered octamers
4) Eight tetramers twist into ropelike filament (10 nm diameter)

43
Q

What are tonofilaments?

A

Bundled cytokeratins formed by interactions between C-terminal domains and neighboring filaments

44
Q

What accessory proteins are associated with intermediate filaments?

A

Filaggrin: bundles keratin intermediate filaments
Plectin: bundles vimentin intermediate filaments

45
Q

What are the 4 types of intermediate filaments?

A

Nuclear
Vimentin-like
Epithelial
Axonal

MCBG Exam 3 (5 decks)

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