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Histology/Cell Biology > Cytoskeleton > Flashcards

Cytoskeleton Flashcards

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AP® Biology flashcards Biology 101 flashcards
1
Q

Describe four characteristics of microfilaments.

A
  1. made from actin filaments
  2. 5-7 nm in diameter
  3. double helix
  4. polar (asymmetric)
    - plus end and minus end
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2
Q

What are the subunits of microfilaments?

A

free actin monomers (45kDa globular protein subunit) aka G-actin (globular actin)

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3
Q

Describe the mechanism by which microfilaments are assembled.

A
  • start with building block of actin monomers
  • each monomer can bind ADP or ATP as cofactor for enzymatic activity
  • stacking of subunits causes displacement that creates the appearance of double stranded helix
  • As ATP is hydrolyzed to form polymer, conformation changes
  • ADP bound to subunit is weaker than ATP so subunits fall off at - end
  • higher conc of G-actin required to keep filament growing
  • protein binding at - end to stabilize filament
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4
Q

What role do actin-binding proteins play in regard to microfilament assembly?

A
  • ABP’s help regulate polymerization of actin filaments (and rate, depending on conc of G-actin)
  • responsible for filament organization
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5
Q

Distinguish between the + end and - end of actin filaments/microfilaments.

A

+ end: fast-growing end, where polymerization occurs (requires ATP)
- end: slow-growing end, where ADP is bound to actin (break down of filament)

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6
Q

Different characteristics of actin filaments depend on what proteins?

A

Characteristics of actin filaments depends on what ABP is present.

  • actin-bundling proteins
  • actin filament-severing proteins
  • actin-capping proteins
  • actin cross-linking proteins (i.e. filamin)
  • actin motor proteins hydrolyzes ATP to allow for movement along the actin filament from - end to + end (i.e. myosin)
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7
Q

Describe the mechanism by which profilin stabilizes actin filaments.

A
  • binds to subunit at + end

- helps present subunits for polymerization by catalyzing exchange of ATP & ADP

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8
Q

How does branching occur in actin filaments?

A
  • branching is catalyzed by Arp complex which allows for elongation of “daughter” actin filament off “mother” filament
  • important for phagocytosis (bringing material into cell using plasma membrane), need rapid polymerization of actin
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9
Q

What role does formin play in the growth of microfilaments?

A
  • catalyzes growth of microfilaments
  • recruits new subunits to create rapid growth of filaments
  • polymerizes and depolymerizes under specific conditions
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10
Q

Name 5 functions of microfilaments

A
  • cell division (contractile ring separates dividing cells)
  • endocytosis/exocytosis/transport
  • cell shape change (gives structure to epithelium in cell-cell junctions)
  • cell motility (stress fibers)
  • structural –> microvilli
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11
Q

Explain how stress fibers aid in cell attachment and mobility.

A

aggregates of actin filaments form to create stress fibers which are bound to the cellular matrix

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12
Q

What effect would removing a capping protein from a microfilament have on intercellular junctions such as a zonula adherens?

A
  • capping proteins work to stabilize the - end of a microfilament so that it does not depolymerize
  • removing the caps destabilizes the - end of microfilaments, allow them to disassemble themselves and zonula adherens
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13
Q

What are the differences and similarities between stereocilia and microvilli?

A
  • both stereocilia and microvilli are made of an actin core
  • in the case of stereocilia, they are essentially elongated microvilli that allow for movement
  • the main function of microvilli is to increase surface area for absorption rather than movement
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14
Q

Describe five characteristics of microtubules.

A
  • 24 nm tube
  • 13 rows of subunits (protofilaments)
  • rigid
  • polar/asymmetric (ends are distinguishable from one another)
  • all microtubules originate from same point within cell
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15
Q

What molecules form the subunit of microtubules?

A
  • alpha tubulin and beta tubulin form a heterodimer (110 kDa, 8 nm long)
  • beta end is the + end, has enzymatic activity (binds to both GDP and GTP)
  • alpha subunit always contains inactive GTP
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16
Q

Identify the only place where the heterodimer can interact with proteins in the cytosol.

A
  • alpha unit contains binding site with highly negative charge
  • limits activity within cell
17
Q

What is required for the assembly of microtubules?

A
  • GTP

- originates from centrioles

18
Q

How do tubulin heterodimer subunits add to make microtubules?

A
  • one at a time (protofilaments do not assemble individually

- heterodimers constantly adding on to + end with GTP form on end

19
Q

What causes subunits to fall off microtubules? From where?

A
  • when GTP hydrolyzes to GDP form
  • GDP can’t remain on filament
  • dynamic instability –> forming or depolymerizing very quickly
20
Q

What is the difference between centrioles, MTOC, and centrosomes?

MTOC = microtubule organizing center

A
  • centrioles = paired cylinders built from nine microtubule triplets, arranged perpendicular to each other, found close to the nucleus
  • centrosome (aka MTOC) = two centrioles plus gamma tubulin ring complex & pericentriolar material, disappears when centrioles are not present
  • both can only form at certain places in the cell
  • all ends at centriole/centrosome are - ends
21
Q

What is the difference between centrioles and basal bodies?

A

basal bodies come from centrioles when they have migrated to the apical surface of a cell to form microtubules for cilia formation

22
Q

What is the organization of microtubules that make up cilia and flagella?

A

9+2 configuration

23
Q

What determines the shape of cells?

A
  • shape of cells determined by the contact microtubules make at the surface
  • microtubules disappear and reform until they find something to stabilize them
24
Q

dynein

A
  • acts as motor to make cilia move
  • in 9+2 configuration, each pair of microtubules is bound to two dynein proteins
  • have ATP motor complex which moves it from + to - end during intracellular transport
  • cargo is determined by what it is interacting with at the + end
25
Q

Describe the five major functions of microtubules

A
  1. intracellular vesicle transport (uses dynes and kinesins)
  2. movement of cilia and flagella
  3. attachment of chromosomes to mitotic spindle and chromosome movement during mitosis/meiosis
  4. cell elongation and movement
  5. maintenance of cell shape and asymmetry
26
Q

Describe the role of microtubules in intracellular transport

A
  • information going from nucleus to cytoplasm –> - end to + end
  • kinesins carry info away from nucleus
  • information going from cytoplasm to nucleus –> + end to - end
  • dyneins carry info toward nucleus
  • both have ATP hydrolyzed in head domain
27
Q

Example of cargo of dyneins/kinesins

A

i.e. one pair of microtubules walking along another pair, causing displacement

28
Q

Describe 6 characteristics of intermediate filaments

A
  • no asymmetry
  • very diverse –> all different proteins but have common theme in how they are assembled and disassembled
  • 8-10 nm in diameter
  • no organizing principle
  • no common orientation
  • no common site of synthesis
29
Q

What does the formation of intermediate filaments look like on an EM?

A
  • forms grid-like systems in cell in which protein synthesis & biosynthesis can occur
30
Q

How could you distinguish between different tissues by focusing on intermediate filaments?

A
  • all intermediate filaments are made up of rod-shaped protein subunits but different tissues/cells use different proteins
Keratins (Type I) --> Epithelial cells
Vimentin (Type II) --> Mesencymal
Desmin (Type II) --> Muscle
Glial fibrilary acid protein (Type II) --> glia/astrocytes/Schwann
Neurofilaments (Type III) --> neurons
Nuclear Lamins (Type IV) --> all
  • antibodies to specific proteins would only light up in specific cells with that protein
31
Q

Describe the organizing process of intermediate filaments

A
  1. dynamic structures moderated by phosphorylation and dephosphorylation, form helical monomers (asymmetrical)
  2. helical monomers twist around each other to form coiled-coil dimers (asymmetrical)
  3. dimer of dimers forms when two coiled-coil dimers twist around each other in antiparallel fashion to generate staggered tetramer (symmetrical)
  4. each tetramer is aligned along axis of filament and ends of filament are bound together to form the free ends of filament –> stable, staggered helical array (symmetrical)

Histology/Cell Biology (12 decks)

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