Lecture 3 - Cytoskeleton III

Question 1

Define Intermediate Filaments

Answer 1

Family of Related proteins that share common structural and sequence features

Question 2

How does IF Diversity compare to Actin/Tubulin

Answer 2

Significantly more diverse, as:
* IFs - 67+ genes
* Tubulin - 6a and 6B isoforms
* Actin - 6 Isoforms

Question 3

What is the Basic Structure of IF monomers?

Answer 3

IF monomers typically display:
* a-helical central domain (rod domain)
* Head/Tail domains - non-a-helical, display wide length/ sequence variation across IF protein families

Question 4

What are the general characteristics of Intermediate filaments

Answer 4

• Short Flexibile Rope-like filaments - due to short persistence length (µm)
• Non-polar - due to antiparallel association of dimers (symmetrical)

Question 5

What Dynamics of IFs have been visualised by Fluorescent Labelling of Subunits?

Answer 5

• End-to-End Annealing
• Subunit Exchange

(2 Points)

Question 6

What are the Primary functions of Intermediate Filaments in Cells?

(2 Points)

Answer 6

1. Serve as tension-bearing elements, helping to maintain cell shape and rigidity (provide mechanical stability)
2. Also serve to Anchor organelles in place e.g.;
   * Nucleus
   * Desmosomes/Hemidesmosomes - protein complexes that form intracellular/matrix junctions

Question 7

What are Nuclear Lamins? What are their Functions?

Answer 7

IFs that form meshwork on nuclear side of inner nuclear membrane to:
* Provide Shape/Stability to Nuclear Envelope
* Provide Structural Links between Nuclear Envelope and DNA (Interacts with Chromatin)

Question 8

How do the Nuclear Lamins differ from cytoplasmic intermediate filaments?

Answer 8

• Contain Additional 42aa in rod domains
• Contain Nuclear Localisation Sequence in Tail domain

Question 9

What happens to the Nuclear Lamins:
(i) During Mitosis?
(ii) At end of Mitosis?

(3 Points)

Answer 9

(i) Nuclear Lamins are depolymerised via phosphorylation at specific sites by CDKs, with protein monomers being resorbed by ER
* This causes Inner Nuclear Membrane to Lose Definition

(ii)Nuclear Envelope and Lamina reassemble in stepwise manner

Question 10

What are Laminopathies (Give Examples)?

Answer 10

12+ diseases caused by mutations in nuclear lamin genes, particularly LMNA (encodes Lamin A/C)
* E.g., Lipodystrophy, Progeria, Leukodystrophy

Question 11

(i) What are Keratins?
(ii) What is their Structure?

Answer 11

(i)Type of Intermediate Filament present in epithelial cells, which give mechanical strength to the tissue by providing anchoring at sites of cell-cell/cell-matrix contact

(ii) Keratin Filaments are formed from heterodimers of acidic and basic monomers (equal mixture), which assemble into tetramers

Question 12

Keratin Mutations are Associated with Several Diseases. Provide One Example

Answer 12

Epidermylysis Bullosa Simplex - Skin exhibits blistering upon very minor/slight mechanical stress

Question 13

What is the Role of Neurofilaments in Neurones?

Answer 13

• Essential for growth and structural stability of Myelinated Axons, with the level of expression of Neurofilaments dictating axonal diameter

Question 14

What Diseases are Associated with Neurofilament Defects ?

(3 Examples)

Answer 14

• ALS/MND - associated with accumulation and abnormal assembly of neurofilaments in Motor Neurons
• Alzheimer’s/Parkinson’s Disease - associated with point mutations in NF-M rod domain
• Charcot-Marie Tooth Disease - associated with 14 NF-L mutations