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MBOD Week 5 > Cytoskeleton II > Flashcards

Cytoskeleton II Flashcards

1
Q

What makes up the structure of microfilaments?

A
  • Actin (main component)

- other proteins that bind and regulate function and activity

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2
Q

In non-muscle cells what is the importance of actin?

A
  • cell structure

- movement of contraction of cells

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3
Q

What generates the force felt by microfilaments?

A

myosin motors

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4
Q

What forms the contractile unit of skeletal muscle?

A

Actin filaments and Myosin

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5
Q

Describe the polymerization of actin

A
  • Single dumbell shaped monomers of actin polymerize head to tail to form a helical filament of actin
  • Head to tail attachment means actin is polar (There is more rapid addition at the + end than at the - end)
  • Actin bound to ATP will add to the growing filament
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6
Q

What are the different isoforms of actin, and what tissues are they present in?

A
  • alpha (muscle tissue)
  • Beta (non-muscle)
  • Gamma (non-muscle)
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7
Q

To which end of the actin filament are monomers more rapidly added?

A

The + end

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8
Q

T or F: The polymerization of actin does not require high energy molecules such as ATP or GTP?

A

False, actin microfilament assembly is ATP-dependent. Free actin monomer bound to ATP is incorporated into polymer faster at the plus end of the filament. Following incorporation, ATP is hydrolyzed promoting depolymerization at the minus end

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9
Q

What promotes actin depolymerization, and from what end does depolymerization occur?

A

ATP hydrolysis causes monomers to break off from the - end

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10
Q

T or F: microfilaments are more stable than microtubules

A

True - This is partly explained by a slower rate of dissociation of monomers under conditions found in the cells and because the critical concentration for assembly is lower relative to the total amount of actin in the cell.

  • generally >50% assembly of actin is in filaments
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11
Q

How do Cytochalasin and Phalloidin affect microfilament polymerization and how do they work?

A

Cytochalasin
- binds to the + end of the microfilament and prevents further polymerization

Phalloidins
- bind tightly along the sides of the actin filament, preventing depolymerization

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12
Q

What do microfilaments look like relative to microtubules other than being thinner?

A
  • They are shorter and more flexible

- Usually they are found in bundles or cross-linked aggregates

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13
Q

How do thymosin and profilin act on actin monomers?

A

Thymosin - binds monomers preventing assembly

Profilin - promotes monomer assembly

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14
Q

What does tropomyosin do?

A

Regulates myosin-dependent force generation

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15
Q

T or F: actin can form many different structures depending on associated proteins

A

True

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16
Q

How does Gelsolin work?

A

converts actin gel into a more fluid state by exposing more actin-ADP which makes actin filaments more likely to disassemble

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17
Q

What are profilin, thymosin, myosin, tropomyosin, and gelosin examples of?

A

Proteins that interact with actin filaments to regulate and change activity on the bases of cellular need

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18
Q

What are 4 common motifs actin can be arranged into and what are their purposes? (I.e. what are the main functions of microfilaments)

A

microvilli - absorption
Stress Fibers - contraction and adhesion
Lamellipodia, Filopodia - Cell Migration
Contractile Ring - cytokinesis

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19
Q

In contractile bundles of stress fibers, what is the alignment of actin?

A

antiparallel

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20
Q

What protein is involved in providing force to epithelial cells in monolayers to undergo shape changes?

A

Actin

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21
Q

What are lamellipodia and filipodia made of and what do they do?

A
  • Parallel bundles of actin filaments
    • end points in the same direction (parallel)

Involved in cell migration

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22
Q

What is the adhesion of cell to the ECM dependent on and how do these structures work?

A

Stress fibers
- anchored in the plasma membrane at integrin depedent focal contacts

-antiparallel microfilament bundles

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23
Q

What is the purpose of Stress fibers, what is their use in the body?

A

-Adhesion of cells to the extracellular matrix is dependent upon stress fibers (anti-parallel microfilament bundles) that are anchored in the plasma membrane at integrin-dependent focal contacts.

  • Exert tension on the matrix surrounding them
  • Essential for wound healing and morphogenesis
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24
Q

What is microfilament organization dependent on?

A

the associated proteins (varies dramatically).

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25
Q

What are the highly branched microfilaments of lamellipodia dependent on?

A

actin-related protein (ARP) complex.

Depolymerizing proteins on the - end of the lamellipodia cause polymerization of actin filaments ‘branches’ and monomers from the ARP complex binding them to the main trunk of the microfilament. These free branches are them transported to the + end of the microfilament trunk where they are re-added and capped. This cycle promotes forward movement.

Some pathogens use actin assembly to propel themselves inside animal cells

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26
Q

T or F: Contractile force provided by actin filaments lead to morphogenetic changes during development

A

True. This causes the invagination of the neural tube during development

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27
Q

What type of protein drives the elongation of actin filament?

A

Formin dimers

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28
Q

What are Rho proteins, what are they activated/deactivated by?

A
  • proteins are responsible for reorganizing different microfilament populations via a cellular signal
  • These run on a GTP/GDP switch with GTP bound state being active and GDP bound state being inactive
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29
Q

What are some members of the Rho family and where are they found?

A

Rho - stress fibers
Rac - lamellipodia
cdc42 - filipodia

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30
Q

What is myosin dependent on for movement, which direction do they move in?

A
  • ATP hydrolysis and actin
  • move to the + end

(activity stimulated 200x by actin)

31
Q

What is the difference in Type I and Type II myosin?

A

Type I

  • single head domain
  • ATP - dependent motor activity
  • tail may hold onto vesicles for transport or cell membrane to provide structural support

Type II

  • 2 head domains and bind each other at their coiled tails
  • form bipolar filaments

Even short bipolar filaments can slide anti-parallel filaments over each other (both in the + direction) resulting in the contraction of the filament bundle

32
Q

What is the sarcomere made of and when is it triggered to contract?

A
  • Type II myosin filaments and microfilaments

- coupled and triggered to contract almost simultaneously

33
Q

What is the role of Ca2+ in the sarcomere?

A

triggers sliding of myosin to the + end of actin filaments that are anchored in the Z-disc resulting in contraction

34
Q

What is the cycle that occurs as myosin walks along actin?

A
  1. Myosin (w/ NO ATP) associates w/ actin in rigor conformation
    • ATP binding reduces myosin aff. for actin
      • ATP is hydrolyzed causing translocation
      • myosin head move ahead by 1 monomer

(myosin now holding ADP and Pi)

    • Weak binding to actin causes Pi release
    • ADP is then released causing myosin Power Stroke
35
Q

What is the job of of troponin and tropomyosin?

A

They regulate the ability of skeletal muscle myoglobin to associate with actin filaments

36
Q

What is the contractile ring and what is it formed by?

A

Ring that cleaves one cell into two daughter cells in mitosis

actin and myosin form the contractile ring

37
Q

T or F: muscle cells are multinucleated and contain multiple myofibrils

A

T

38
Q

What is a myofibril?

A

consists of a chain of identical contractile units called sarcomers

39
Q

What allows several sarcomeres to contract nearly spontaneously?

A

Coupling

40
Q

How does Ca2+ work with the troponin complex to cause muscle contraction?

A

Ca2+ binding to troponin complex triggers a shift in the positioning of tropmyosing along the actin filament, exposing the myosin binding site?

41
Q

When does the sarcoplasmic reticulum release calcium?

A

signal from the nerve terminal results in opening of Ca2+ channel in the sarcoplasmic reticulum releasing the Ca2+ stores

42
Q

What assembly proteins are important in microfilament structure and function?

A

profilin

thymosin

43
Q

What proteins regulate actin filament networks?

A

gelsonin

44
Q

What motor proteins are important to microfilaments?

A

Type I and II myosins

45
Q

What proteins regulate motor activity?

A

Troponins and tropomyosin

46
Q

What proteins anchor microfilaments in the membrane?

A

cadherins

47
Q

What induces cleavage in dividing cells?

A

association of actin filaments with the plasma membrane

48
Q

Where can intermediate filaments be found?

A
  • only in multicellular organisms

- prominent in cells subject to mechanical stress

49
Q

What are some examples of cells exposed to a great deal of mechanical stress and thus abundant in intermediate filaments?

A
  • nerve axons
  • muscle
  • epithelia
50
Q

What does the structure of intermediate filaments look like and consist of?

A
  • ropelike polymers of elongated protein
  • 2 protofilaments that contain 2 coiled dimers
  • NOT POLAR
51
Q

T or F: intermediate filaments require ATP and/or GTP for assembly and disassembly?

A

false - require neither

52
Q

What percentage of I.F. subunits are in tact in a normal cell?

A

95% - very stable, much more so than actin or tubulin

53
Q

T or F: I.F.s are in no way associate with microtubules or the nuclear envolope?

A

False - In cells treated with colchicine to depolymerize microtubules, the network of IF also collapses and IF are found in a perinuclear cap

54
Q

Are microfilaments polar?

A

Yes. Intermediate filaments are not.

55
Q

What is different in IF expression in different cells than something like tubulin and actin?

A

the protein types in each cell are significantly different

56
Q

What is keratin?

A

most diverse class of IF. Intermediate filaments are composed mainly of different proteins that regulate IF function in different cell types

57
Q

What is a clinical implication of specific IFs being in different cell types?

A
  • tissues in tumors will have specific IFs just for that tissue
  • a + stain of Fibrillary acid protein is used to diagnose anencephaly
58
Q

T or F: all multicellular organisms have IFs?

A

False

59
Q

T or F: like actin, the function of IFs is largely determined by its associated proteins

A

False, function is determined by the variable regions of the intermediate filament subunits rather than by associated proteins.

Because specific IFs are identified with specific cell types their presence is used to help diagnose certain diseases (The type of IF filaments found in tumors helps to identify the tissue from which the tumor originated which is important in choosing the best treatment.)

60
Q

What is the major function of IFs?

A

Resists mechanical stress. IF can withstand larger stretching forces than microtubules or microfilaments.

Other functions:
• form major structural components of skin and hair
• give strength and rigidity to nerve axons
• form the nuclear lamina
• stabilize the epithelium by attaching to anchoring junctions at macula adherens(desmosomes: cell-cell; hemidesmosomes: cell/extracellular matrix)

61
Q

How do IFs in epithelial cells stabilize the cell?

A

By attaching to anchoring junctions at macula adherens

  • desmosomes = cell-cell
  • hemidesmosomes = cell - ECM
62
Q

What is Epidemolysis bullosa simplex?

A

autosomal dominant disease causing a deficit in keratin expression in the basal layer of the epidermis causes mechanical sensitivity and blistering

63
Q

What does a plectin deficiency cause?

A
  • plectin links IFs to MTs and MFs,

A deficiency causes mechanical sensitivity and blistering as well as muscular dystrophy and neurodegeneration type effects (and Epidermolysis bullosa simplex)

64
Q

What are nuclear lamins and what do they do?

A

They are a type of IF that assemble to make nuclear lamina that provides structural support the the nuclear membrane

65
Q

What controls the activity of nuclear lamins?

A
  • assembly and disassembly is controlled heavily by phosphorylation in mitosis
66
Q

Why are microfilaments more stable than microtubules?

A

This is partly explained by a slower rate of dissociation of monomers under conditions found in the cells and because the critical concentration for assembly is lower relative to the total amount of actin in the cell.

67
Q

How does fibroblast movement occur?

A

extension of lamellipodia which contain actin filaments.

68
Q

What are myosins?

A

A family of actin associated motor proteins that hydrolyze ATP to provide energy for actin-dependent movement

69
Q

Describe the zones/bands of a sacromere

A

A sarcomere is defined as the segment between two neighbouring Z-lines. Actin filaments are embedded in the Z-lines. The I-band is the zone of thin filaments that is not superimposed by thick filaments. An A-band contains the entire length of a single thick filament. H-band is the zone of the thick filaments that is not superimposed by the thin filaments.

Actin filaments, the thin filaments, are the major component of the I-band and extend into the A-band.
Myosin filaments, the thick filaments, are bipolar and extend throughout the A-band. They are cross-linked at the centre by the M-band

Upon muscle contraction, the A-bands do not change their length, whereas the I-bands and the H-zone shorten. This causes the Z lines to come closer together.

70
Q

Provide a brief explanation of sarcomere contraction initiation.

A

The protein tropomyosin covers the myosin binding sites of the actin molecules in the muscle cell. To allow the muscle cell to contract, tropomyosin must be moved to uncover the binding sites on the actin. Calcium ions bind with troponin-C molecules (which are dispersed throughout the tropomyosin protein) and alter the structure of the tropomyosin, forcing it to reveal the cross-bridge binding site on the actin.

The concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum in the sarcoplasm.

71
Q

Provide a brief explanation of sarcomere relaxation.

A

At rest, the myosin head is bound to an ATP molecule in a low-energy configuration and is unable to access the cross-bridge binding sites on the actin. However, the myosin head can hydrolyze ATP into ADP. A portion of the energy released in this reaction changes the shape of the myosin head and promotes it to a high-energy configuration. Through the process of binding to the actin, the myosin head releases ADP and an inorganic phosphate ion, changing its configuration back to one of low energy. The myosin remains attached to actin in a state known as rigor, until a new ATP binds the myosin head. This binding of ATP binding reduces the affinity of myosin for actin, and ATP hydrolysis causes a conformational change that results in the translocation of the myosin head along the microfilament one actin monomer.

Muscle contraction ends when calcium ions are pumped back into the sarcoplasmic reticulum, allowing the contractile apparatus and, thus, muscle cell to relax.

72
Q

Are IFs found in unicellular organisms?

A

No. IF are found only in multicellular organisms, most, although not all cells in multicellular organisms contain IF.

73
Q

Do microfilaments have structural and kinetic polarity?

A

Yes

74
Q

Association with different IF types/, their associated proteins/, locations found/ and diagnosing power

A

Vitmentin Like IFS:
Vimentin/ Mesenchymal cells/ Lymphomas
Desmin/ Muscle/ Sarcoma
Glial filbrillary acidic protein/ Glial cells/ Gliomas

Keratin IFs:
Type 1 and 2/ Epithelial cells/ Carcinomas

Neuronal IFs:
Neurofilament proteins/Nuerons/ Neuroblastoma

MBOD Week 5 (9 decks)

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