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Cell Biology > Intermediate Filaments and Microtubules > Flashcards

Intermediate Filaments and Microtubules Flashcards

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1
Q

What are the main roles of intermediate filaments?

A

Intermediate Filaments - IMFs - are highly dynamic entities providing structural support, providing a scaffold and buffering against movements.

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2
Q

What is the structure and function of microtubules?

A

Polymers of alpha-beta-tubulin dimers arranged into a tube.

Role in cellular tracks used by microtubule motor proteins; kinesin and dynein, to transport organelles, vesicles, chromosomes within the cell.

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3
Q

What are the 5 major classes of intermediate filaments? (In order)

A
  1. Acidic Keratins
  2. Basic Keratins
  3. Vimentin
  4. Neurofilaments
  5. Lamins
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4
Q

The IMFs have a large diversity in sequence in size relative to microtubules and microfilaments. True or False?

A

True

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5
Q

Describe the intracellular localisation of Keratins

A

Keratins make up cytosolic intermediate filaments that extend to the cell membrane in keratinocytes; skin epithelial cells.

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6
Q

Which class of IMFs support the inner nuclear membrane?

A

Type V - Lamins

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7
Q

In what cell types can Vimentin be found?

A

Endothelial and fibroblastic cells in stromal tissue.

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8
Q

What is the role of Vimentin?

A

To support the morphology, shape and integrity of cells. Keeps nucleus and organelles in position.

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9
Q

What are the features of Basic Keratins?

A
  • Fibrous Proteins
  • Localised in outer epithelia; epidermal keratinocytes.
  • Heterodimers of basic and acidic subunits.
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10
Q

Name the staining technique for localisation of Keratins

A

Red Keratin Staining

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11
Q

Name the staining technique for Lamins

A

Blue Lamin Staining

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12
Q

Describe the staining technique for Vimentin and state what you would see and where.

A

Brown immunostaining shows presence of Vimentin Filaments in endothelial and fibroblastic cells in stromal tissue.

Antibody to Vimentin marking capillaries in villi brown, and the edge of larger blood vessels.

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13
Q

What are Neurofilaments?

A

Type IV Intermediate Filaments

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14
Q

Describe the organisation and role of Neurofilaments.

A

Organised into parallel bundles.

Provide structure to axons, determining axon diameter and hence speed of conduction.

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15
Q

Explain how staining techniques can identify the origination of tumors.

A

Fluorescently tagged antibodies for different specalised intermediate filament proteins can identify whether tumors are of epithelial, mesenchymal or neuronal tissue origin. This is because the expression of IMF proteins are well-characterised.

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16
Q

What are Lamins?

A

Type V intermediate Filaments

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17
Q

What are the roles of Lamins?

A

Fibrous Network supporting inner nuclear membrane.

May organise different types of chromatin, playing a role in DNA transcription.

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18
Q

Describe the conserved structure of IMFs

A

IMF proteins consist of a globular head at N terminus and globular tail at C terminus which are separated by an extended alpha-helical region.

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19
Q

Describe how intermediate filament proteins assemble into higher order structures. (5 points)

A

Two monomers wrap around each other to form a parallel dimer.

Two parallel dimers go head-to-tail to form an antiparallel tetramer.

Antiparallel tetramers stack end-on-end to form protofilaments.

2 protofilaments form a protofibril.

4 protofibrils wrap around each other to form intermediate filaments in diameter.

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20
Q

Which regions of IMF proteins are conserved?

A

The alpha-helical region, the backbone of the filament, is highly conserved, whereas diversity exists in the N and C termini globular regions, which protrude from the filament.

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21
Q

The N and C termini of IMF proteins are both equally important in filament assembly. True or False?

A

False.
The N terminal domain is more important in filament assembly.
The C terminal domain is more important for stability.

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22
Q

What are the roles of the terminal domains in intermediate filament proteins? (3 points)

A

Vital for inter-filament interactions and with other cytoskeletal and cellular components.

Role in filament assembly (N terminus)

Role in filament stability (C terminus)

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23
Q

IMFs are strictly homopolymers and consist of only one subunit type. True or False?

A

False.

Keratins always heterpolymerise of Type I / Type II, whereas Vimentins can homo- or heteropolymerise!

24
Q

What determines whether IMFs can be heteropolymers or homopolymers?

A

Spacer sequences within the alpha-helical core.

25
Q

The assembly of IMFs is not spontaneous and requires energy through ATP/GTP. True or False?

A

False. ATP/GTP is not required for IMF formation.

26
Q

What are the role of Keratins?

A

Keratins link the nucleus to peripheral proteins and cell adhesion structures: desmosomes and hemidesmosomes. These are critical for cell-to-cell and cell-to-substrate interactions to form tissues.

27
Q

Describe the processes involved in migration of basal epidermal stem cells to the stratum corneum.

A
  • Stem cells in basal epidermal layer differentiate and change keratin expression profile during expression to outer layer.
  • Basal Keratinocytes (BKs) express K5/K14 in dermis and assemble K5/K14 heterodimers for IMFs.
  • BKs progress from stratum basale -> str. spinosum -> str. granulosum -> str. lucidum -> str. corneum during which they lose expression of K5/K14 and gain expression of K1 and K10.
28
Q

Describe how mutations in keratins can lead to the condition epidermolysis bullosa simpex (EBS)

A

Mutations in N or C termini of K5/K14 result in inability of K5/K14 heterodimers to form protofilaments.
Cells at the base of epidermis are therefore weakened, the epidermis and dermis are easily separated, leading to EBS.

29
Q

Explain how mutations in LMNA leads to the presentation of Hutchinson-Gilford progeria syndrome.

A

Mutations in the LMNA gene produce an abnormal variant of the Lamin A nuclear filament.
The nuclear envelope becomes unstable and prone to damage, causing phenotypes associated with cellular ageing: osteoporosis, hairloss, cardiovascular disorders, diabetes, muscular atrophy, etc.

30
Q

Describe the structure of microtubules

A

Polymer of globular tubulin dimers (one alpha, one beta) arranged into a tube

31
Q

Both alpha- and beta- tubulin bind GTP. True or false?

A

True

  • alpha binds GTP irreversibly
  • beta bind GTP reversibly
32
Q

Which tubulin subunit catalyses GTP hydrolysis?

A

beta-tubulin

33
Q

What is “non-exchangeable GTP” in the context of tubulin subunits?

A

GTP that cannot be exchanged for another as GTP in alpha-tubulin binds irreversibly and is not hydrolysed.

34
Q

Describe the assembly of alpha-beta tubulin dimers into higher order structures.

A
  • Dimers assemble longitudinally into short protofilanents end-to-end.
  • Dimers associate laterally into curved sheets with increased stability.
  • Longitudinal addition of dimers can occur spontaneously
  • Sheet wraps into a microtubule which grows by addition of more dimers.
35
Q

How are microtubules orientated/polarised?

A

Orientated with beta-tubulin at + end of microtubule and anchored at the - end to an MTOC

36
Q

What is an MTOC?

A

Microtubule Organising Centre. It is the structure to which microtubules are anchored at the - end.

37
Q

In animals cells an MTOC is also called…

A

A centrosome

38
Q

What is a centrosome?

A

MTOC for animal cells.

Located near the nucleus.

39
Q

Describe the structure of centrosomes.

A

Consists of a pair of centrioles, which consist of short microtubules.

40
Q

What are the function of centrosomes?

A

To direct assembly and orientation of microtubules, and to direct vesicle traffic and orientation of organelles.

41
Q

When is a GTP cap produced on + end of microtubules?

A

When the rate of addition of heterodimers to the + end is greater than GTP hydrolysis by beta-tubulin a cap is produced.

42
Q

What is the role of the GTP cap?

A

Stabilises the MT due to higher affinity of tubulin with GTP on beta-subunit for its neighbour

43
Q

The probability that a beta-tubulin subunit has GDP bound increases as you go further toward the - end. True or False?

A

True

44
Q

The GTP cap is characteristic of MTs that are growing relatively slow. True or False?

A

False. GTP cap is produced in fast growing MTs.

45
Q

What is the result on microtubule growth if the dimer concentration is greater than the critical concentration?

A

Microtubules grow and are formed by polymerisation, with preferential addition of tubulin to the + end.

46
Q

What is the result on microtubule growth if the dimer concentration is less than the critical concentration?

A

There is preferential loss of tubulin at the + end

47
Q

When does spontaneous growing/shrinking occur?

A

MTs show dynamic instability either side of the critical concentration - growing and shrinking.

48
Q

Describe how the centrioles of a centrosome anchor microtubules.

A

Centrioles are linked through pericentriolar matrix proteins (gamma-tubulin and pericentrin) to the - end of microtubules allowing the + end to grow freely.

49
Q

What happens to centrioles during mitosis?

A

Centrioles replicate and migrate to opposite poles of dividing cell, where the MTs separate the sister chromatids.

50
Q

What is the Basal Body?

A

An MTOC in cilia/flagella organising MTs to be anchored to the cell membrane.
It has a cylindrical structure similar to centrioles.

51
Q

All MTs are anchored to MTOCs. True or False?

A

False

52
Q

What are the two major classes of microtubule motor proteins?

A

Kinesins and Dyneins

53
Q

Describe the function of kinesins

A

To transport organelles, vesicles and chromosomes along cellular microtubule tracks in an anterograde direction (away from the MTOC)

54
Q

Describe the function of dyneins

A

To transport organelles, vesicles and chromosomes along cellular microtubule tracks in an retrograde direction (towards MTOC)

55
Q

Describe the general structure of kinesins

A

Tetramer: Two head domains with ATPase activity on each of the heavy chains that dock with the microtubules and two light chains that can bind cargo.
Similar structure to myosins.

56
Q

Explain why single vesicles may have both kinesin and myosin motor proteins attached.

A

Regions within a cell which the vesicle must cover can be MT-poor and MF-rich (or vice versa).

Kinesin motors carry through body of cell rich in MTs and a Myosin V motor to attach to cell cortex and reach different regions rich in MFs.

57
Q

What is the pericentriolar matrix?

A

An entity containing gamma-tubulin and pericentrin which anchor microtubules at the - end.

Cell Biology (9 decks)

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