Cytoskeleton

Question 1

What are the main components of the cytoskeleton?

Answer 1

• Microfilaments (actin)
• Microtubules (tubulin)
• Intermediate filaments

Question 2

Briefly explain what the cytoskeleton is

Answer 2

A network of protein filaments found in the cytoplasm of cells which controls cell shape, maintains intracellular organisation and provides mechanical support essential to cell movement.

Question 3

Describe the structure of microfilaments

Answer 3

Globular actin monomers which form filamentous polymers, diameter of approx. 7nm

Question 4

How are microfilaments formed?

Answer 4

The globular actin subunits exist freely in the cytosol, when they assemble into a polymer, the monomers are added at the plus end, and disassemble from the minus end.

(Dynamic equilibrium, constant assembly and disassembly)

Question 5

Where are actin monomers mostly found in the cell?

Answer 5

Can exist anywhere in the cytosol, but concentrated at the edge (cell cortex) and underneath the plasma membrane.

Question 6

What are the functions of microfilaments?

Answer 6

They act to reinforce the plasma membrane and static membrane structures.
Also involved in generating pulling and pushing forces on the cell membrane.

Question 7

Give an example of microfilaments reinforcing the plasma membrane and the proteins involved

Answer 7

Microvilli:

• Parallel core of actin fibres that extend downwards into the cell as a root
• Actin bundling proteins connect the actin filaments in a stiff core
• Lateral cross links (type of protein) connect actin filaments to plasma membrane
• Spectrin connects roots of the microvilli together (terminal web of spectrin)

Question 8

Give 3 examples of the pulling action of microfilaments

Answer 8

Contractile ring:

• Forms at late stages of mitosis and meiosis
• Leads to cytokinesis
• Ring of actin and myosin

Stress fibres:

• 10-30 actin filaments crosslinked by α-actinin
• Connects ECM to cytoskeleton

Adhesion belt:

• Actin myosin band at top of epithelial cells
• Interconnects cells
• Important in formation of the CNS in early embryos

Question 9

Give 2 examples of the pushing action of microfilaments

Answer 9

Microspikes:

• Long, thin protrusions
• Migrate in direction they need to go

Lamellipodia:

• Leading edge of cell
• Pushes cell membrane outwards and whole cell moves along with it
• Subunits of actin added to plus end at the leading edge
• Known as actin treadmilling

Question 10

Describe the structure of microtubules

Answer 10

Globular subunits of alpha and beta tubulin forming fibres approx. 25nm
Tubulin assembles in a directional manner to form microtubules

Question 11

Describe the assembly of microtubules

Answer 11

Globular monomers assembled as a spiral, however appears as long tubes called protofilaments as all of the subunits line up with eachother.

Microtubules are organised from the centriole (minus end) or basal body (minus end for cilia and flagella)

Question 12

Which protein is also found in association with microtubules?

Answer 12

Microtubule associated protein:

• Either stabilise the microtubule
• Or sets of this protein are used to generate forces along the microtubule

Question 13

Centrioles and basal bodies are known as…

Answer 13

MTOCs

Microtubule-organising centres

Question 14

What are the 3 main functions of microtubules?

Answer 14

• Organisation of organelles in cytoplasm
• Move vesicles, organelles and chromosomes
• Nerve cells; fast axonal transport

Question 15

Describe the role of microtubules in fast axonal transport

Answer 15

• Neurotransmitters packaged into secretory vesicles in the Golgi
• Membrane of vesicles contains kinesin, kinesin binds to microtubules
• Kinesin moves from minus to plus direction, moves along microtubule powered by hydrolysis of ATP
• Recycling vesicles contain dynein, moves from plus to minus direction along microtubule

Question 16

What is the diameter of intermediate filaments?

Answer 16

10nm

Question 17

Describe the structure of intermediate filaments

Answer 17

Made of large fibrous protein subunits which interact with eachother in a non-directional manner (can attach end to end and side to side)

Question 18

Describe the assembly of monomers to form intermediate filaments

Answer 18

2 monomers join to form a dimer, 2 dimers join to form a tetramer, tetramers join to form sheets, sheets coil and roll up to form the filament bundle.

NB: no directionality, cannot be used for driving directional movements

Question 19

Where are intermediate filaments found in a standard cell?

Answer 19

• One set enclosing the outside of the nucleus and radiating out to the plasma membrane
• One set within the nuclear envelope

Question 20

How are microfilaments involved in the nuclear lamina?

Answer 20

The nuclear lamina is composed of a set of nuclear filaments called nuclear lamins.

The nuclear lamina sits just below the nuclear envelope.

Nuclear lamins control structure and assembly of the nucleus and support the structure of the nuclear envelope.

Question 21

Filaments: nerve cells

Answer 21

Neurofilaments: provide axons with mechanical support

Question 22

Filaments: muscle

Answer 22

Desmin: holds contractile units of muscle together

Question 23

Filaments: mesenchyme

Answer 23

Vimentin: mechanically supports cells and is associated with strength

Question 24

Filaments: epithelia

Answer 24

Cytokeratins: provide mechanical strength

Question 25

How is phosphorylation/dephosphorlyation involved in the assembly of microfilaments?

Answer 25

If protein monomers are phosphorylated the filaments will disintegrate.

If the protein monomers are dephosphorylated the filaments will assemble again.

Various phosphorylase enzymes in cytosol control this.

Question 26

Describe a clinical use of knowledge of microfilaments

Answer 26

• Identification of origin of tumours

- Biopsy performed, cytokeratins present are profiled to identify origin