Cytoskeleton

Question 1

What are the functions of the cytoskeleton?(3)

Answer 1

• Shaping of the cell
• Intracellular movement of organelles
• Cell movement

Question 2

How is dynamicity regulated?

Answer 2

Dynamicity is regulated by accessory proteins that regulated site and rate of filament formation, polymerisation/ depolymerisation

Question 3

Describe the structure of actin filaments(3)

Answer 3

• Twisted chain of units (monomers) of the protein actin (G-actin, aprox.43 KDa). This chain constitutes the filamentous form (F-actin).

2. • Thinnest class of the cytoskeleton filaments (7 nm)
3. • Presents structural polarity- The + end where monomers can be added.

Question 4

How is F-actin grown?

Answer 4

1. By addition of actin monomers G at either end.

2. presence of G actin is the determining factor for how fast polymerization occurs.

Question 5

How is G actin filaments controlled?

Answer 5

1. controlled by two groups of actin binding proteins: Profilin- facilitates actin polymerization
2. thymosin beta4: prevents the addition of actin monomers to F-actin.

Question 6

Describe the polarities of the two ends of G-actin(3)

Answer 6

The reaction at the positive end is favourable

2. ATP actin has a higher affinity for the growing end (+ve) and prefer to add onto the plus end
3. they form an ATP cap which promotes further growth.

Question 7

Why isn’t there polymerisation at -ve end of the actin filament?

Answer 7

ADF or actin depolymerizing factor (cofilin) is an actin protein that binds to the minus end of an actin filament associating with ADP actin that promotes depolymerization.

Question 8

How many isoforms of G-actin are there and what are they?

Answer 8

3, – α-actin found mainly in muscle cells

– β-actin and γ-actin in non-muscle cells

Question 9

What are the two actin binding proteins that bind to F actin?(2)

Answer 9

ctin bundling proteins
→ keep F-actin in parallel bundles (as in the microvilli observed in epithelial cells)

Cross linking proteins
→ keep actin in gel like mesh work under membrane

Question 10

How is the length of filaments determined?(2)

Answer 10

1. The availability of actin binding proteins

2. The availability of actin monomers

Question 11

What is the role of F-actin severing proteins?

Answer 11

break F-actin into smaller filaments

Question 12

What is the role of motor proteins(myosin)?

Answer 12

transport of vesicles and/or organelles through actin filaments

Question 13

What are the functions of actin filaments?

Answer 13

1. Cytokinesis- involvement of an actin-myosin ring

2. – Cell migration- The cell pushes out protrusions at its front

Question 14

How do the severing proteins increase the surface area when breaking actin down?

Answer 14

they work on different points like endo peptidases.

Question 15

How is actin arranged in skeletal muscle?

Answer 15

Arranged in a para-crystalline array integrated with different ABPs
→Interaction with Myosin motors allow muscle contraction

Question 16

What is the function of actin in non-muscle cells?

Answer 16

Cell cortex : form a thin sheath beneath the plasma membrane

→Associated with myosin form a purse string ring resulting in cleavage of mitotic cells

Question 17

What is the function of actin in cell migration?(3)

Answer 17

1) Elongation - protrusions (lamellipodia and filopodia) pushed out
2) Adhesion - Integrins link the filaments to the extracellular matrix surrounding the cell
3) Contraction - actin and myosin interaction, contraction and retraction

Question 18

What are the properties of intermediate filaments (5)

Answer 18

1. • Toughest of the cytoskeletal filaments (resistant to detergents, high salt etc).
2. • Ropelike with many long strands twisted together and made up of different subunits.
3. • Intermediate size (8-12nm) between actin and microtubules.
4. • Form a network: Throughout the cytoplasm, joining up to cell-cell junctions (desmosomes).
   • Withstands mechanical stress when cells are stretched.
   – And surrounding nucleus
   • Strengthens the nuclear envelope.

Question 19

What is the structure of IFs?(4)

Answer 19

•	Each unit is made of:
–	N-terminal globular head
–	C-terminal globular tail
–	Central elongated rod-like domain
•	Units form stable dimers
•	Every 2 dimers form a tetramer
•	Tetramers bind to each other and twist to constitute a rope-like filament

Question 20

What are the types of IFs?(4)

Answer 20

CYTOPLASMIC:

1) Keratins (in epithelia, protects from damage/stress)
2) Vimentin/ vimentin related (in connective tissues, muscle cells and neuroglial cells)
3) Neurofilaments (in nerve cells) NUCLEAR:
4) Nuclear lamins (in all nucleated cells)

Question 21

What do the intermediate filament binding proteins do?(2)

Answer 21

1. Mainly linkers of IF structures.

→IFBP stabilize and reinforce IF into 3D networks

Question 22

What are the intermediate filament binding proteins and their functions?(3)

Answer 22

→Fillagrin:
binds keratin filaments into bundles.

→Synamin and Plectin:
bind desmin and vimentin
Link IF to the other cytoskeleton compounds (i.e. actin and microtubules) as well as to cell-cell contact structures (desmosomes).

→Plakins:
Keep the contact between desmosomes of epithelial cells.

Question 23

What are the functions of IFs in the cytoplasm?

Answer 23

1. Tensile strength: this enable the cells to withstand mechanical stress (to stretch!)

→Structural support by:
Creating a deformable 3D structural framework and reinforcing cell shape

Question 24

what are the functions of intermediate filaments in the nucleus?(4)

Answer 24

1. present in all nucleated eukaryotic cells
   →form mesh rather than “rope-like” structure
2. line in the inner face of the nuclear envelope to:
   →strengthen it
3. provide attachment sites for chromatin
4. disassemble and reform at each cell division as nuclear envelope disintegrates i.e. very different from the stable cytoplasmic intermediate filaments
   →process controlled by post-translational modifications (mainly phosphorylation and dephosphorylation)

Question 25

What are the properties of microtubules?(4)

Answer 25

• Hollow tubes made up from the protein tubulin
• Relatively stiff (25nm), is the thicker of the filaments
• Each filament is polarized (i.e. has direction – head/tail or +/-)
• It is a dynamic structure allowed by tubulins

Question 26

What is tubulin?(2)

Answer 26

the protein that polymerizes into long chains or filaments that form microtubules
2. • tubulin in cell is roughly 50:50 as free or in filament

Question 27

What is the MTOC?(2)

Answer 27

Microtubule organizing centre (MTOC) are specialized protein complexes from where assembly of tubulin units starts
2. Centrosome in most cells

Question 28

What constitutes the microtubule and what type of growth happens?

Answer 28

→Heterodimers of α and β tubulin constitute the microtubule.

→It is a polarized growth (i.e. there is an end that grows faster (+end) than the other (- end).

Question 29

What are the functions of microtubules in intracellular transport?(4)

Answer 29

– act like railway tracks on which molecular motors run eg. Dyenin and Kinesin
– Dynein move toward the negative end called retrograde
– directionality of filaments is vital (each motor only moves in one direction)
4. organises position of organelles
hence, provides polarisation of cells

Question 30

What are the functions of microtubules with cilia?

Answer 30

Rhythmic beating of cilia and flagella

→Motile processes, with highly organized microtubule core.

→Bending of cilia & flagella is driven by the motor protein Dynein- slides microtubules relative to one another.

Question 31

Recall the types of IFs keratin(5)

Answer 31

Type II keratins are called hard keratins- hair or nails

2. Type I keratins are abundant in the cytoplasm of epithelial keratins
3. TypeIII intermediate filament proteins are
4. Type IV intermediate filaments consist of alpha internecin, neurofilaments which are found in high concentrations along the axons of vertebrate
5. Type V are the lamins which occur in all nucleated cells